diccionario geofisica, sismica, gravimetria, fisica, roca, interpretacion, adquisicion,

1. C
c: 1. Symbol often used for velocity. 2. The velocity of
light in a vacuum, 2.997 925ϫ108
m/s. 3. A seismic
wave reflected at the Earth’s mantle-core boundary.
C¿¿: An object-oriented programming language, a
superset of the C language that was originally devel-
oped for the Unix operating system. Supports data
abstraction and object-oriented and generic program-
ming, and facilitates modularity in programming. See
also Java.
cable: 1. The assembly of electrical conductors used to
connect geophone or hydrophone groups to the
recorder. See also streamer. 2. The assembly of electri-
cal conductors and tensile members used to support a
logging sonde ͑wireline͒, well geophone, or bird.
cablebreak: An arrival in a well-velocity survey caused
by energy travel in the cable that supports the well
geophone. Wave velocity in cables is 2500-3500 m/s.
cable drilling: A method of making a hole in hard rock by
alternately lifting and dropping a tool.
cable strum: Vibration of a marine streamer or ocean-
bottom cable produced by occasional sudden tension,
such as might be caused by pitching of a towing ship,
jerks from a tail buoy, or variations in current. A source
of noise in marine seismic data.
cable tools: Equipment for cable drilling ͑q.v.͒. Occasion-
ally used for seismic shotholes in areas of extremely
hard surface rock. See spudder.
cache: ͑kash͒ A temporary storage place. Computers
include caches at several levels, including cache
memory and disk cache. A cache memory may be on
the same or a separate chip from the microprocessor,
and is faster to access than regular RAM. Disk cache,
either a reserved area of RAM or a special hard-disk
cache, may store recently accessed data for fast access.
RAM can be viewed as a cache. For example, recently
requested Web pages may be stored in a browser’s
cache so that the browser can return to a page from the
cache rather than from the server.
CÕA code: Coarse Acquisition code used to carry informa-
tion for GPS positioning.
CAD: Computer-Aided Design, computer-aided drafting.
cadastral survey: ͑k‫ץ‬ das’ tr‫ץ‬l͒ A survey to determine
boundary lines.
CAEX: Computer-Aided EXploration; interactive inter-
pretation employing seismic, well, and other data.
cage: Perforated steel surrounding a marine energy source
͑small explosive or air gun͒ to attenuate the bubble
effect ͑q.v.͒ by dissipating energy in the turbulent flow
of water in and out of the cage. See Flexotir and Figure
F-13.
Cagniard apparent resistivity: ͑ca¯gn’ yard͒ 1. A resistiv-
ity calculated from orthogonal electric and magnetic
field measurements, used for magnetotelluric measure-
ments in a layered earth. For more complicated struc-
ture, the full tensor impedance must be considered. 2.
In the magnetotelluric method, an apparent resistivity in
ohm-meters at period T obtained from 0.2T ͉Z͉2
,
where Z is the Cagniard impedance ͑q.v.͒.
Cagniard de Hoop technique: An exact method for a
synthetic seismogram calculation that involves the use
of inverse Laplace transforms and iteration in the com-
plex plane. See Aki and Richards ͑1980͒.
Caignard impedance: In the magnetotelluric method, the
ratio of the horizontal electric field component in some
direction, Ex, to the magnetic field, Hy, in a perpen-
dicular direction. For a horizontally layered earth, it is
independent of the choice of x-direction. See also
impedance.
cake: Mud cake ͑q.v.͒.
calculus of variations: The mathematics of finding a
function that will maximize ͑or minimize͒ a definite
integral.
caldera: A large circular depression of volcanic origin.
calibrate: 1. To adjust an apparatus so that it reads values
correctly. 2. To determine the number of units of a
quantity being measured per scale division of the read-
out device. 3. To check equipment readings by measur-
ing a known standard. 4. To adjust values so that the
integrated traveltime agrees with data from surface
sources into a well seismometer in the borehole. 5. To
transform a logging tool signal into a quantified physi-
cal parameter, or the result of this transformation
recorded on a log.
calibration factor: A constant used to compensate for
inaccuracies in electromagnetic field parameters, cur-
rent channeling, and static shifts. For transient electro-
magnetic methods, it depends only on the transmitter-
receiver distance.
calibration resistor: A pure resistance of a known value
used to calibrate a frequency-domain transmitter and
receiver.
calibration tails: Calibration records run before and/or
after a log run and attached to the logs.
caliche: ͑k‫ץ‬ le¯’ che¯͒ A cemented surface layer that forms
in semi-arid temperate zones by the precipitation of
calcium carbonate. Caliche is often a difficult surface
for seismic acquisition.
caliper log: A well log that measures hole diameter. Open-
hole caliper logging tools sometimes have four or more
arms. See Figures M-11 and S-13. Tools for studying
casing or tubing corrosion use many ‘‘fingers.’’
call: An instruction to transfer control to a subprogram.
camera: A recording oscillograph used to produce a vis-
ible pattern representing electrical signals or to make a
visible seismic record on photosensitive paper or film
or by xerography. See also plotter.
camouflet shooting: ͑kam ou fla¯’͒ To generate S-waves
by asymmetry of P-wave sources.
cannel coal: ͑kan’ ‫ץ‬l͒ A sapropelic coal derived from
organic residues by putrefaction under anerobic condi-
tions rather than petrification. High in volatile compo-
nents.
canonical: Simplest form of a generalized relation.
canonical transformation: A transformation from one set
45

2. of coordinates and moments to another set in which the
equations of motion are preserved.
cap: A small explosive designed to be detonated by an
electrical current, which in turn detonates another
explosive. Seismic caps are designed to detonate with
little uncertainty in their time of detonation.
capacitance: The ratio of charge ͑Q in coulombs͒ on a
capacitor to the potential across it ͑V in volts͒ is the
capacitance ͑C in farads͒: CϭQ/V.
capacitive coupling: See coupling.
capacitive reactance: Electrical impedance XC resulting
from capacitance:
XCϭ1/͑2␲fC͒ ohms,
where f is frequency in hertz and C is capacitance in
farads.
capacitivity: ͑k‫ץ‬ pas’ i tiv ‫ץ‬ te¯͒ Permittivity, the property
of a material that enables it to store electrical charge.
Measured in farads per meter. The ratio of the capaci-
tivity of a material to that of free space is the dielectric
constant of the material. Free space has a capacitivity
of 8.854ϫ10Ϫ12
farad/m. See also electric suscepti-
bility.
capex: Capital expenditures.
capillary pressure: The pressure required to displace one
fluid with another with which it is not miscible. The
difference in pressure across the interface separating
imiscible liquids.
cap rock: 1. Impermeable sealing rock above a reservoir
rock. 2. The dense anhydrite and gypsum rock above a
salt dome that results from leaching of the soluble
materials in the salt body.
caprock effect: A sharp positive gravity anomaly pro-
duced by the dense caprock of a salt dome, superim-
posed on a broader negative caused by the salt dome.
Shallow salt is usually denser than surrounding sedi-
ments, so caprock is not essential to produce a positive
gravity anomaly.
capture cross-section: 1. The atomic capture cross-
section for neutrons is the effective area within which a
neutron has to pass to be captured by an atomic
nucleus. It is a probabilistic value dependent on the
nature and energy of the particle as well as the nature of
the capturing nucleus. Atomic capture cross-section is
often measured in barns (1 barnϭ10Ϫ28
m2
). 2. The
macroscopic capture cross-section, ⌺, is the effective
cross-sectional area per unit volume for capture of
neutrons; it depends on the number of atoms present as
well as their atomic capture cross-sections. The unit of
measure for ⌺ is cm2
/cm3
ϭ103
cu, where cu
ϭcapture unit ͑q.v.͒. 3. The rate of absorption of
thermal neutrons with a velocity V is V⌺.
capture unit: A unit of measure of a macroscopic capture
cross-section; 0.1 m2
/m3
. Also called sigma unit and
abbreviated cu or su.
carbonate factory: Conditions of temperature and water
clarity that favor the generation of carbonates. Gener-
ally, this applies to shallow tropical or subtropical seas
away from river mouths so that suspended sediment is
not present.
carbon-dioxide flooding: See miscible recovery methods.
card: 1. A plug-in printed-circuit module. 2. A punch-card
used for data storage.
cardinal theorem: Sampling theorem ͑q.v.͒.
Carpenter electrode array: ͑kar’ p‫ץ‬n t‫ץ‬r͒ Four colinear
equispaced electrodes used in electrical surveying. In
the Carpenter-1 ͑or Wenner͒ array, the two central
electrodes are the potential electrodes; in the
Carpenter-2 array, one end pair are the potential elec-
trodes; in the Carpenter-3 array, the current and poten-
tial electrodes alternate.
carrier: A radio wave having a characteristic ͑e.g., fre-
quency, amplitude, phase͒ that can be varied by modu-
lation.
carry: 1. In arithmetic operations, the transfer of a value
from a lower order position to the next higher order
position as a result of the lower order having equaled or
exceeded the base of the number system. For example,
if subscripts indicate the base,
810ϩ210ϭ0ϩcarry 1010;
12ϩ12ϭ0ϩcarry 22.
2. A computer status bit that indicates whether or not
the last arithmetic operation resulted in a carry from
high-order bit position.
Cartesian coordinates: ͑kar te¯’ zhan͒ Rectangular coor-
dinates where the axes are straight lines and the angles
between them are right angles. Named for R´ ene´ Des-
cartes ͑1596–1650͒, French scientist and philosopher.
cascade: ͑ka ska¯d’͒ To arrange in series.
cascaded filtering: Applying several filters in succession.
cascading menu: A list where entries include subentries,
which in turn include sub-subentries, etc.
cascaded migration: The sequential implementation of a
number of migration processes to improve the accuracy
over a single migration step.
casing: Tubes or pipes used to keep a borehole from
caving in and to prevent fluids within the hole from
entering surrounding formations and prevent formation
fluids from entering the borehole. Shot-hole casing is
usually in 10-ft lengths that screw together, drill casing
in 30-ft lengths.
casing-collar: The coupling between joints of a casing.
casing-collar locator: A magnetic or scratcher device for
locating casing collars, used to correlate cased-hole
logs and serve as reference depths in completion opera-
tions.
casing hanger: Device used to support casing within a
borehole.
casing-head gas: Gas produced along with the oil from an
oil well.
casing point: The lowest depth in a well where casing is
set and cemented.
casing shoe: A short, hollow, heavy, cylindrical steel unit
placed at the bottom of a casing string to protect it as it
is lowered into a borehole.
CAT: Computerized Axial Tomography. See Scudder
͑1978͒.
catalog: 1. Collection of master curves or type curves
͑q.v.͒ for interpreting magnetic, electrical, and electro-
magnetic survey measurements. Catalog lookup
involves using a library of previously calculated and
stored data to determine interpretation parameters. 2. A
list of computer programs; a menu or library.
catalyst: ͑kat’ ‫ץ‬ list͒ A substance that promotes a reaction
and ends up unchanged.
cap 46 catalyst

3. catcher: 1. A device to prevent cap wires from being
blown out of the hole by the shot and endangering
personnel by making contact with high-voltage power
lines. 2. A device to catch a Dinoseis gas gun before it
falls back to the ground after an explosion, thus pre-
venting a second sharp impact that would complicate
the waveform. 3. A device used to retain a core in a core
barrel.
catching samples: Obtaining and studying samples of for-
mations brought to the surface in the drilling mud.
categorical: ͑kat ‫ץ‬ gor’ i k‫ץ‬l͒ Absolute, unqualified,
unconditional.
cathead: A spool-shaped attachment to a winch around
which a rope can be wound for hoisting or pulling. A
breakout cathead helps in unscrewing drill pipe, a
makeup cathead for screwing it together.
cathode: 1. The electrode where reduction occurs and
electrons are taken up. The negative terminal of an
electrolytic cell or the positive terminal of a battery. 2.
The source electrode for electrons inside a vacuum tube
or semiconductor diode device that converts informa-
tion into an electron beam ͑and subsequently to light
energy to provide a visual display͒.
cathode-ray tube: A device for viewing waveforms ͑such
as voltage as a function of time͒ and other information.
Abbreviated CRT or CRO.
cathodic protection: Protection for buried pipelines and
other metallic materials subject to electrochemical cor-
rosion. The pipeline is made electrically negative with
respect to ground.
cation: ͑kat’ ı¯ ‫ץ‬n͒ A positively charged ion. Compare
anion.
cation exchange capacity: Number of negative charge
sites on clay surfaces that can attract sodium ions from
pore fluid. Each sodium ion carries six clay-bound
water molecules.
cationic membrane: A membrane that permits the pas-
sage of cations but not of anions. Shale acts as such a
membrane, allowing sodium ions but not chloride ions
to pass. Important in generating electrochemical SP
͑q.v.͒.
catline: A hoisting or pulling line to move heavy equip-
ment around a drill rig.
cat push: Line-cutting supervisor.
Cauchy criterion: ͑ko¯ she¯’͒ An optimization criterion
involving the minimizing of
͚ ln͕1ϩ͓͑di
Ã
Ϫdi͒/␴͔2
͖,
where di
Ã
are observed and di are model data. Implies
that errors have the probability distribution
͑1/␲␴͕͒1/͕1ϩ͓͑dϪdo͒/␴͔2
͖͖,
where ␴ is the standard deviation about the maximum
likelihood point d0. See Amundsen ͑1991, 2027͒.
Cauchy-type integral: The function at a point z0 within a
closed contour C can be expressed in terms of a line
integral along C if f(z) is a continuous analytic func-
tion:
f͑a͒ϭ͑1/2␲i͒ Ͷf͑z͒/͑zϪz0͒dz.
Named for Augustin Louis Cauchy ͑1789–1857͒,
French mathematician.
causal: ͑kau’ z‫ץ‬l͒ Not existing before some finite starting
time and having finite total energy.
causality: The property of a sequence such that there is
zero energy before some finite starting time. Minimum-
phase wavelets are causal but zero-phase ones are not.
caustic: ͑kau stik’͒ A surface to which rays emanating
from a single source and reflected by a curved reflector
are tangent. A surface formed by adjacent rays.
cave: An immersive environment where an interpreter can
sense 3D effects of a data set, usually by projection
onto three walls, ceiling, and/or floor. The 3D visual-
ization may be provided by stereo effects, rotation of a
display so that it can be seen from different directions,
virtual reality, or other ways.
caving: Collapse or sloughing of the walls of a borehole.
cavitation: ͑kav, i ta¯’ sh‫ץ‬n͒ The formation and collapse of
cavities ͑bubbles͒ in a fluid caused by the static pres-
sure becoming smaller than the fluid vapor pressure.
The collapse generates a shock wave by implosion. The
outward momentum of water and gases from an under-
water explosion ͑and other marine energy sources͒ usu-
ally produces such a low-pressure region, the collapse
of which produces a bubble effect.
C-AVO: Variation of amplitude with offset ͑q.v.͒ for con-
verted waves.
CBL: Cement-Bond Log ͑q.v.͒.
CCD: Charge-Coupled Device.
CCP: Common Conversion Point ͑q.v.͒.
CD: Compact Disk, a computer storage device with large
memory capacity.
CDI: Conductivity-Depth Image ͑q.v.͒.
CDM: Continuous DipMeter; see dipmeter.
CDP: Common-Depth-Point; see common midpoint.
CDR: Compensated Dual-Resistivity tool.
CD-ROM: Compact Disk with Read-Only Memory, to
which one cannot write data.
CDT: Conductivity-Depth Transform. See conductivity-
depth image.
celestial equator: ͑s‫ץ‬ les’ ch‫ץ‬l͒ See equinoctial.
celestial latitude: Angular distance north or south of the
ecliptic. Different than declination.
celestial longitude: Angular distance east of the vernal
equinox measured along the ecliptic. Different from
right ascension.
celestial navigation: See positioning.
celestial pole: The projection of the Earth’s axis onto the
celestial sphere.
celestial sphere: An imaginary sphere of infinite radius
concentric with the Earth on which all celestial bodies
are imagined to be projected.
Celsius: ͑sel’ se¯ ‫ץ‬s͒ A temperature scale where water
freezes at 0 °C and boils at 100 °C at one atmosphere
pressure; formerly called centigrade. If TC, TK, and
TF are Celsius, Kelvin, and Fahrenheit temperatures,
respectively,
TCϭTKϪ293.15ϭ͑5/9͒͑TFϪ32͒.
Named for Anders Celsius ͑1701–1744͒, Swedish
astronomer.
CEM: Crone Electromagnetic Method; see shootback
method.
catcher 47 CEM

4. cement additive: Additives to cement to control the set-
ting time, control the weight, reduce fluid loss, etc.
cementation factor: The exponent m in the Archie For-
mula ͑q.v.͒.
cement-bond log „CBL…: A well log of the amplitude of
the acoustic wave that indicates the degree of bonding
of cement to casing and formations. See Figure C-1.
Also called an amplitude log. The wavetrain may also
be displayed, as in the character log, 3D, microseismo-
gram, VDL, or acoustic signature log.
cement channeling: Uneven movement of a cement
slurry behind casing that then allows free fluid flow
through the channels.
cementing: Applying a liquid slurry of cement and water
to form a protective sheath around casing to secure the
well casing and prevent the migration of undesirable
fluids or for other purposes. In squeeze cementing the
slurry is forced through perforations in the casing to
isolate formations, seal off water, or repair leaks.
center-line fault: Fault pick midway between up- and
down-thrown terminations.
center of gravity: 1. The point in a material body through
which the gravitational attraction acts regardless of the
orientation of the body. The point about which all
weight moments cancel. The moment of a point mass is
the product of the mass and its distance from the
rotation point being considered. 2. Used for other types
of data, e.g., the distribution of CMP points within a
seismic bin.
centigrade: Celsius ͑q.v.͒.
centipoise: A unit of viscosity, equal to 10Ϫ3
kg/(s.m).
The viscosity of water at 20 °C is 1.005 centipoise.
central induction sounding: An electromagnetic tech-
nique in which the vertical magnetic field is measured
as a function of frequency at the center of a large
horizontal transmitting loop.
central limit theorem: A statement about the characteris-
tics of the distribution of the means of random samples.
If we could draw an infinite number of random samples
of a given size where we calculate the mean of each
sample, ͑a͒ the mean of the means of the samples
equals the mean of the population from which the
samples were drawn; ͑b͒ the variance of the sampling
distributions equals the variance of the population
divided by the sample size. ͑c͒ If the original popula-
tion is normally distributed ͑i.e., is bell-shaped͒, the
sampling distribution will also be bell-shaped, and if
the original population is not normally distributed, the
distribution will increasingly approximate a normal dis-
FIG. C-1. Cement-bond application of microseismogram log. (a) Before squeeze; (b) after squeeze. If the casing is
poorly cemented, the energy that travels through the steel casing is strong and little energy travels in the formation; if
the casing is well cemented, the casing signal nearly disappears and the formation signal is strong. (Courtesy Welex.)
cement additive 48 central limit theorem

5. tribution as the sample size increases.
central processing unit „CPU…: The heart of a computer
that controls operations and interprets and executes
programs. Typically includes the arithmetic logic unit
and an instruction processing unit.
CEP: Circle of Equivalent Probability; the radius of a
circle such that half of the measurements fall within the
circle. A circle containing 90% of the measurements
would be 90% CEP.
cepstrum: ͑sep’ strum͒ The Fourier transform of a loga-
rithmic frequency distribution. Let ↔ indicate a Fourier
transform operation. If g(t)↔G(␻), the cepstrum
gˆ (␨) is
gˆ ͑␨͒ϭ͑1/2␲͒͵Ϫϱ
ϱ
͓ln͓G͑␻͔͒ϩj␥͑␻͔͒ej␻␨
d␻.
The transform is usually carried out in three steps:
Gˆ ͑␻͒ϭln͓G͑␻͔͒ϭln͉G͑␻͉͒ϩj␥͑␻͒G͑␻͒
Gˆ ͑␻͒↔gˆ ͑␨͒ϭ͑1/2␲͒͵Ϫϱ
ϱ
͓ln͓G͑␻͔͒ϩj␥͑␻͔͒ej␻␨
d␻.
The inverse transform is
gˆ ͑␨͒↔Gˆ ͑␻͒ϭ͵Ϫϱ
ϱ
g͑␨͒eϪj␻␨
d␨,
G͑␻͒ϭexp͓Gˆ ͑␻͔͒,
g͑t͒↔G͑␻͒.
The variable ␨ is called quefrency and ␥ˆ is called the
saphe ͑being permutations of frequency and phase, as
cepstrum is of spectrum͒. The cepstral domain is often
indicated by the hat. The transform can also be
expressed as z-transforms; see Sheriff and Geldart
͑1995, 298–299; 554–555͒.
cesium clock: See frequency standard.
cesium vapor magnetometer: ͑se¯’ ze¯ ‫ץ‬m͒ A type of opti-
cally pumped magnetometer ͑q.v.͒. Sensitive to 0.001
nT. Also spelled caesium.
CFP: Common Focusing Point, involved in imaging. See
common-image point.
chain: 1. A unit of length equal to 66 ft. 2. To measure
distances directly with a steel tape ͑chain͒ as opposed to
measuring by stadia, electronic distance measurement,
or triangulation. Accuracy of the order of 1:5000 can be
achieved with care. 3. An ordinary ordered group of
computer records.
chain rule: If z(y) and y(t) are differentiable dz/dt
ϭ(dz/dy)dy/dt.
chaining: 1. Measuring distances with a chain. 2. A sys-
tem of storing data records in which each record
belongs to a specific group or chain.
chainman: A surveyor’s assistant, who helps to measure
distances with a chain. Chainmen often work in pairs to
measure out seismic spreads using a steel tape or wire
that is as long as the group length.
chair display: A display combining vertical and horizon-
tal slices through a 3D volume. See Figure T-6.
Challenger expedition: An oceanographic voyage in
1872–1875 by HMS Challenger, led by John Murray.
Among other discoveries, it established the magnitude
of the mid-Atlantic ridge.
Chandler wobble: A precession of the Earth’s rotational
axis about its axis of greatest moment of inertia. Also
called the free nutation of the Earth. The wobble
period is about 435 days and the amplitude is about
0.14 s of arc. Named for Sweth Carlo Chandler ͑1846–
1913͒, American astronomer.
change detection image: An image comparing two
images acquired at different times, sometimes made by
subtracting the earlier from the latter.
channel: 1. A single series of interconnected devices
through which data can flow from source to recorder.
Seismic systems may have thousands of channels
allowing the simultaneous recording of energy from
thousands of geophone groups. 2. A measured or com-
puted quantity that occurs as a sequence of samples
indexed against time, depth, or some other physical
dimension. 3. A localized elongated geologic feature
resulting from drainage or water action. The properties
of the material in-filling a channel often differ from
those of the material into which the channel is cut,
which makes the channel observable and sometimes
bends raypaths creating dip and/or velocity anomalies
in underlying reflections. 4. A feature in seismic data
that is interpreted as indicating erosion and fill by a
river system, such as a curvilinear feature on a 3-D
horizon slice ͑see Figure H-8͒. 5. An allocated portion
of the radio-frequency spectrum. 6. A time gate during
which measurements are made in time-domain electro-
magnetic surveying. Measurements made during sev-
eral time gates following a source pulse yield several
channels of data. 7. A layer whose velocity is such that
seismic energy gets trapped; see channel wave. 8. A
gate with upper and lower energy thresholds followed
by a count rate meter, used in gamma-ray spectrom-
eters. 9. A component of a mainframe processor that
supports and controls I/O.
channel wave: 1. An elastic wave propagated in a layer
where most of the energy is trapped. ͑a͒ The layer may
have lower velocity than those on either side of it ͑so
that total reflection can occur at the boundaries͒, or ͑b͒
a layer boundary may be a free surface ͑so that the
reflectivity is nearly one͒. Instead of having a sharp
interface as a boundary, a velocity gradient that bends
rays that tend to escape back toward the channel may
provide a channel. The SOFAR channel ͑q.v.͒ in the
deep oceans is an example. However, some energy may
escape ͑leaking mode͒, e.g., by converting to another
wave mode. See Figure C-2. A channel is also called a
wave guide and channel waves are also called guided
waves and normal-mode propagation. Coal seams
and a surface water layer often carry channel waves
͑also called seam waves͒. See Sheriff and Geldart
͑1995, 483-487͒.
chaos theory: ͑ka¯’ os͒ Theory dealing with the unpredict-
ability that is intrinsic to nonlinear systems. The effects
of a small ͑perhaps unmeasurable͒ perturbation grow
progressively larger and thus prevent long-term predict-
ability. Chaos in a dynamic system can be viewed in
terms of diverging time-dependent orbits ͑paths͒, which
comprise a finite geometric form that is called the
chaotic attractor ͑or strange attractor͒. The diverg-
ing orbits do not repeat exactly and are confined to
central processing unit „CPU… 49 chaos theory

6. FIG. C-2. Channel waves. (a) The Sofar channel is formed by velocity inversion. The velocity-versus-depth curve
changes with latitude and other factors. (b) Energy from a source in the channel is repeatedly refracted or reflected back
toward the velocity minimum and so undergoes less divergence than normal. (From Ewing et al., 1948.) (c) Phase and
group velocity versus normalized frequency for a liquid layer on an elastic substratum. (From Ewing et al., 1957.) (d)
First-mode wavetrain from a source 4 km distant where the ocean constitutes the channel. (e) The high-frequency
portion of (d), the water wave; its arrival is used in refraction work to determine the range. (From Clay and Medwin,
1977.)
chaos theory 50 chaos theory

7. finite-phase space; the orbits stretch and fold repeat-
edly. Chaotic attractors are fractals ͑q.v.͒. Before a
system becomes chaotic, its parameter typically under-
goes a cascade of bifurcations in a geometric series.
character: 1. The recognizable aspect of a seismic event;
the waveform that distinguishes it from other events.
Usually a frequency or phasing effect, often not defined
precisely and hence dependent upon subjective judg-
ment. 2. The recognizable aspect of a graph or picture
that identifies it with some situation. 3. A single letter,
numeral, or special symbol in a computer system. See
also byte, with which it is sometimes used interchange-
ably.
characteristic: 1. A distinctive feature of a set, object, or
attribute. Characteristics of a set are its name and type;
of an object is its name; of an attribute are its label,
count, representation code, units, and value. See also
character. 2. The integer part of a common logarithm.
3. The exponent number for a quantity expressed in
scientific notation. 4. As an adjective or prefix, often
written in the German, eigen, as in eigencurrent, eigen-
frequency, eigenfunction, eigenstate, eigenvalue, eigen-
vector ͑see individual entries͒.
characteristic curve: An anomaly graph whose shape is
distinctive of a particular model shape. See type curve.
characteristic root: Eigenvalue; see eigenfunction.
character log: A sonic log that displays the full wave-
form.
characteristic thermal diffusion distance: ͌(␬␶),
where ␬ is thermal diffusivity and ␶ is the time for a
temperature change to occur. See Fowler ͑1990, 229–
230͒.
charge: 1. An explosive such as used as a seismic source,
The ‘‘charge’’ is often the weight of the explosive. 2. To
increase the stored energy.
chargeability „M͒: One of several units of induced polar-
ization in the time domain. 1. The ratio of initial decay
voltage ͑or secondary voltage͒ to primary voltage. 2.
The dimensionless induced-polarization parameter of a
material in which there is an induced-current dipole
moment per unit volume P energized by a current
density J: PϭϪMJ. 3. The fractional change in resis-
tance measured on a decay curve, as a function of time:
Mϭ␦␳(t)/␳. 4. The integrated area under an IP decay
curve between times t1 and t2, normalized by the
primary voltage Vp. Units are millivolt-seconds per
volt.
Mϭ͑1/V␳͒͵Vtdt.
For standardization, on-time and off-time may be indi-
cated by subscripts, that is, 33
M1 or M331 means ‘‘cur-
rent on for three seconds and decay is measured for the
first second of a three-seconds off time.’’ Field mea-
surements of chargeability are usually calibrated to the
M331 standard, which differs by a factor of about 1000
from the value of M given in definitions 1 and 2 above.
5. The quantity described above multiplied by the con-
ductivity, often expressed in millifarads/unit length.
Called specific capacity when measured in farads per
meter. 6. Chargeability can be related to frequency
effect ͑q.v.͒.
charge anchor: A device fastened to an explosive charge
to hold the charge in a fixed position in the shothole and
prevent it from floating or moving.
charge-coupled device „CCD…: An electronic medium in
which data is encoded as the presence of positive or
negative charges.
charged: 1. Filled with hydrocarbons 2. A borehole
loaded with explosives or air guns ready for firing.
Chebyshev array: ͑cheb’ ‫ץ‬ sh‫ץ‬v͒ A frequency filter or
uniformly spaced linear array in which elements are
weighted according to Chebyshev polynomials. Such
weighting equalizes the amplitude of minor lobes and
gives a sharp cutoff. Called equal-ripple filter. See
Figure C-3. Also spelled Tchebyscheff array. Named
for Fafnutiy Lvorich Chebyshev ͑1821–1894͒, Russian
mathematician.
check: A test of the reliability of data. Check problems
with known results are run through computers to verify
correct functioning. ͑a͒ A summation check consists of
adding a set of figures and using the sum ͑check sum͒
to verify accuracy and completeness. ͑b͒ A duplication
check requires that an operation be repeated and yield
identical results when repeated. ͑c͒ An echo check is
used to verify transmitted data; the data are sent back
͑echoed͒ to the source station for comparison with the
original data. An echo check is also a technique
whereby a computer prints out values of variables and
data used in a program. ͑d͒ Error-detecting checks
search data for forbidden combinations. A parity check
is such a check; the number of one bits plus a parity bit
must add up to an odd number for odd parity or to an
even number for even parity. A longitudinal parity
check ͑LPC͒, written at the end of a record, does for
each track what the parity bit does for each byte. ͑e͒ A
validity check is a verification that a figure lies within
a certain permissible range.
check bit: Parity bit; see check.
check digit: An extra character attached to a number as a
means of checking the validity of the number.
check shots: Direct measurement of traveltime between
the surface and a given depth. Usually sources on the
surface are recorded by a well geophone ͑see well
shooting͒ to determine time-to-depth relationships at
the well location.
check sum: A summation check; see check.
FIG. C-3. The Chebyshev array response has all minor
lobes of equal height. The response shown is for a 6-ele-
ment array.
character 51 check sum

8. check valve: A valve that allows flow in only one direc-
tion.
chemically enhanced recovery: An enhanced oil recov-
ery ͑q.v.͒ method that involves injecting polymers, sur-
factants, or alkaline substances to improve oil produc-
tion. Polymers increase the viscosity of injected water
͑thickens the water͒ so that the water matches that of
the reservoir oil as close as possible, thus making fluid
flow more uniform. Surfactants break down the surface
tension of oil that has collected in droplets so that it can
be produced. Usually a small amount of surfactants are
injected and are followed by polymer flooding and
finally by untreated water. Alkalyne flooding is similar
to surfactant flooding except that the surfactants are
generated in the reservoir by reacting with petroleum
acids.
chemical remanent magnetism „CRM…: See remanent
magnetism.
chemisorption: ͑chem, ‫ץ‬ sorp’ sh‫ץ‬n͒ Adsorption caused
by chemical rather than simply electrostatic causes.
Cheops pyramid: ͑ke¯’ ops͒ The surface given by the
double-square-root equation ͑q.v.͒ that describes the
traveltime surface of a point diffractor in offset space
where the velocity is constant; see Figure D-20.
cherry pick: To select the easiest.
chessboard technique: 1. A grid arrangement used in
solving the acoustic wave equation. See Claerbout
͑1976, Fig. 10-1͒. 2. A potential-field continuation
method for converting level-flown data to drape-flown
and vice-versa. Level-flown data are continued to a
series of levels that span the drape range, giving a set
like stacked 3D chessboards; interpolation is carried out
vertically to give drape profiles.
child, children: A node pointed to by an element in a tree
structure. A child can access all of parent’s memory but
does not affect other childs ͑children͒. A child can
become a parent. Also called daughter.
chimney: See gas chimney.
chip: 1. A solid-state circuit or circuit element; an inte-
grated circuit or IC. 2. The minimum time interval for
a one or a zero in a binary pulse code.
chirp: Signal of continuously varying frequency. Often
implies a linear change of frequency with time. A
vibroseis-like technique for shallow penetration gener-
ally using higher frequencies.
chi-square: ͑kı¯͒ A quantity distributed as
␹1
2
ϩ␹2
2
ϩ¯ϩ␹k
2
,
where ␹1,␹2,...,␹k are independent, Gaussian, with
zero mean, and with unit variance. The chi-square test
is a statistical test of data distribution. If fi are
observed, frequencies of occurrence and ei are the
expected number of occurrences, the goodness of fit y
is
yϭ⌺͑ fiϪei͒2
/ei.
The goodness of fit can be related ͑by the use of tables͒
to the probability that the observed data fit the model
with the assumed number of degrees of freedom.
chlorine log: A log based on the counting rate of gamma
rays produced by capture of thermal neutrons by chlo-
rine in the formation. By limiting the count to a certain
energy range, the tool is made more sensitive to chlo-
rine and relatively insensitive to formation porosity.
The main replacement for the chlorine log has been the
neutron-lifetime log and thermal-decay-time log.
choke: An orifice that restricts the flow of fluids.
chopper: 1. An electrical switching device sometimes
including an oscillator, used to interrupt a dc or low-
frequency ac voltage so that it can be measured by an
ac voltmeter or amplified by an ac amplifier. 2. A
helicopter.
chopping filter: A high-pass filter ͑q.v.͒.
Christmas tree: The casing-head assembly of pipes,
valves, etc. that controls the flow of fluids from a
producing well.
Christoffel equation: ͑kris’ tof ‫ץ‬l͒ A form of the wave
equation for plane waves used for calculating velocity
and slowness surfaces:
␬2
͑␥C␥T
͒uϭ␳␻2
u,
where ␬ is wavenumber, ␥ is the direction cosine
matrix, C is the stiffness tensor, u the particle displace-
ment vector, ␳ the density, and ␻ the angular frequency.
The quantity in parentheses is called the Christoffel
matrix.
chromatograph: ͑kro¯ mad’ ‫ץ‬ graf͒ See partition gas chro-
matograph.
chron: The time of a reversal of the Earth’s magnetic
polarity; a period of reverse polarity would be marked
by a chron at the top and another at the bottom.
chronographic chart: See chronostratigraphic chart.
chronostratigraphic chart: ͑kro¯n, ‫ץ‬ strat’ ‫ץ‬ graf, ik͒ A
diagram that summarizes conclusions from seismic-
sequence analysis. Geologic time increases upward and
the geographic distribution of units is plotted horizon-
tally. See Figures C-4 and S-5. Also called chrono-
graphic chart.
chronostratigraphy: Classification of sedimentary rock
strata according to time of deposition. Seismic reflec-
tions generally indicate constant times of deposition.
The study of the age of strata and their time relation-
ships, contrasted with lithostratigraphy, which is based
on the mineralogic composition and grain size. Com-
pare lithostratigraphy and see Figure S-5.
CI: Contour Interval ͑q.v.͒.
CIP: Common Image Point ͑q.v.͒.
circle shooting: A marine technique for acquiring 3D data
over the flanks of a salt dome or similar structure,
wherein the boat track forms a spiral centered on the
dome. This technique avoids time lost in changing from
one line to another. It also lessens the CMP smearing.
circular error probability „CEP…: Circular error prob-
ability is the radius of a circle such that half of the
measurements fall within the circle. A circle containing
90% of the measurements would be 90% CEP. Where
the probability is anisotropic, it becomes an ellipse.
circular frequency: Angular frequency ␻ϭ2␲f, where
fϭordinary frequency.
circular patch: Receivers arranged approximately in a
circle.
circulation: 1. The integral along a closed loop, ͐V•‫,␭ץ‬
where V is fluid velocity and ‫␭ץ‬ is a segment of a loop
that moves with the fluid. 2. The movement of drilling
fluid from a mud pit through pump, drill pipe, annular
space in the hole, and back into the mud pit.
check valve 52 circulation

9. circulation loss: Loss of drilling fluid into a porous for-
mation.
circumferential wave: Seismic wave that travels parallel
to the Earth’s surface.
cis ␪: Euler’s identity:
cis ␪ϭcos ␪ϩi sin ␪ϭei␪
.
C-language: A general purpose programming language
that combines powerful capabilities with structured
syntax. See also Cϩϩ. Bell Laboratories tradename.
Clarke ellipsoid: ͑clark͒ The basis for the North Ameri-
can geodetic datum, the reference datum in most of the
Western Hemisphere; the Clarke 1866 ellipsoid. ͑There
FIG. C-4. Chronostratigraphic chart. (a) Cross-section interpreted from seismic line with unconformities interpreted as
sequence boundaries. (b) Chronostratigraphic chart corresponding to (a). (c) Coastal onlap chart (called relative sea
level in early papers) interpreted from (a). (After Vail et al., 1977.)
circulation loss 53 Clarke ellipsoid

10. is also a Clarke 1880 ellipsoid that is used in Africa.͒
See Figure G-2. Named for Alexander Ross Clarke
͑1828–1914͒, British geodesist.
classification: Assigning data to categories on the basis of
measurements of several quantities. Supervised classi-
fication uses foreknowledge of the actual identification
of certain areas to classify similar areas. Used to train
neural networks.
class 1, 2, 3, 4 reservoirs: A classification of reservoir
sands according to their normal-incidence impedance
contrasts with surrounding rocks ͑and their AVO
responses͒. Also written with Roman numerals: class I,
II, III, IV. See amplitude variation with angleÕoffset
„AVAÕAVO…, Figure A-12, and Rutherford and Will-
iams ͑1989͒.
clastic rock: A rock composed of fragments derived from
other rocks.
clathrate: ͑klath’ ra¯t͒ Gas hydrate ͑q.v.͒.
clay smear potential: A ranking of the probability that
clay will be smeared out sufficiently along a fault
surface to prevent the flow of fluids across the fault.
clean: 1. Containing no appreciable amount of clay or
shale. Applied to sandstones and carbonates. Antonym:
dirty ͑q.v.͒. 2. To remove soft magnetization so that
hard remanent magnetization may be studied. See
degaussing.
click: 1. Activating some operation of the matrix element
corresponding to the location of a cursor on a video
display. 2. A kilometer or kilometer per second.
client: A program or process that does data processing or
computing.
clinoform: Inclined deltaic foreset or fan deposition. Cli-
noforms ͑progradational patterns͒ may produce dis-
tinctive seismic reflection patterns. See Figure D-6.
clip filter: A space domain filter that sets values that
exceed some threshold to the threshold value or, more
often, to zero.
clipped: Distorted because amplitude exceeded a maxi-
mum permitted amount. Clipping in analog systems
usually occurs because of saturation of some element of
the system, resulting in distortion of the waveform; see
Figure C-5. Digital clipping ͑loss of a bit͒ may have
variable effects depending on what the lost bit repre-
sents ͑i.e., whether it is the sign, the most significant
bit, a complement code, etc.͒; the clipped signal some-
times bears little resemblance to the unclipped signal.
clock: A generator of the basic timing signal pulses to
which system operations are synchronized.
clock rate: The time between clock pulses. Used in elec-
trical work to synchronize transmitter and receiver.
closed data set: Data whose values are subject to a con-
straint so that the parameters are not all independent
and there are forced correlations between parameters.
An example of a closed data set would be values
expressed as percentages of a whole.
closure: 1. The property of a structure whereby it has a
closing contour. Vertical closure is the vertical distance
from the apex to the lowest closing contour; areal
closure is the area contained within the lowest closing
contour. Compare trap. 2. The cumulative error around
a survey loop; mis-tie ͑q.v.͒. 3. The condition where
radioactive daughters are created and preserved, or lost
at a known rate, so that counting them can be used for
age determination. Closure occurs at different tempera-
tures for different minerals.
cluster: 1. Natural grouping of data. 2. A computer made
of a number of interconnected computer processors
͑often pentium or alpha chips͒ for massive parallel
processing, such as a Beowulf cluster.
cluster analysis: Analysis to see if data sort out into
separate regions defined by combinations of variables.
clustering: Nonuniform distribution of fold, offsets, or
azimuths in a 3D survey.
clutter: 1. Coherent interference, in contrast to incoherent
nonrepeatable interference. 2. Noise on a radar screen
resulting from unwanted echoes or scattering such as
from a rough sea or from rain.
cmos: ͑se¯’ mos͒ Complementary metallic oxide semicon-
ductor ͑MOS͒. A technology that employs both nega-
tive and positive MOS to minimize power and cooling
requirements.
CNL: Compensated Neutron Log.
coagulation: Grouping of data that have values close to
each other. Especially used prior to making a color
display in which different colors are assigned to differ-
ent groups.
coarsening: Increasing in particle size. The direction of
coarsening tends to be characteristic of different strati-
graphic situations. The portions of gamma-ray or SP
borehole-log curves that indicate coarsening upward
are called funnel shaped; see Figure C-6.
coastal onlap chart: A chart made in sequence strati-
graphic analysis that qualitatively indicates eustatic
changes; see Figure C-4.
coastline effect: Distortions in electromagnetic ground
waves and natural magnetic fields produced by the
contrast in electrical conductivity between land and
highly conductive ocean water.
COBOL: ͑ko¯’ bol͒ Common Business-Oriented Lan-
guage, one of the first languages designed for commer-
cial data processing incorporating commonly used
English nouns, verbs, and connectives.
COCORP: COnsortium for COntinental Reflection Pro-
filing, a program of seismic work to study the Earth’s
deep crust and upper mantle by low-frequency,
common-midpoint techniques.
coda: 1. The concluding portion of an earthquake seismic
record after identifable waves have passed, presumably
caused by scattering, etc. 2. The trailing waveshape that
follows the onset of a wave arrival.
code: 1. A system of characters and rules for representing
information in a language capable of being understood
by a computer. See source program and object pro-
gram. 2. A set of program instructions.
coding: 1. Formatting; see format. 2. The process of writ-
ing a computer program.
FIG. C-5. Clipping. Loss of a digital bit can cause various
effects depending on how the data are formatted (e.g., the
sign bit might be lost).
classification 54 coding

11. coefficient of anisotropy: See anisotropy (seismic).
coefficient of coherence: See coherence.
coefficient of variation: Ratio of standard deviation to the
mean. See statistical measures.
coercive force: ͑ko¯ ur’ siv͒ The magnetic field intensity
required to reduce the magnetization of a sample to
zero. Generally associated with remanent magnetiza-
tion. See Figure H-14, hysteresis, and coercivity.
coercivity: The demagnetizing field intensity required to
reduce the induction of magnetic material from satura-
tion to zero. Coercivity is used as a figure of merit for
the magnetic hardness of a material, particularly in
reference to the distribution of coercive forces among a
magnetic system’s components. The latter might be
different mineral phases, grain sizes, etc.
coeval: Developing at the same time as.
cofactor: See matrix.
cogeneration: The simultaneous generation of electricity
and process steam or heat.
coherence: ͑ko¯ he¯r’ ‫ץ‬ns͒ 1. The property of two
wavetrains having a well-defined phase relationship,
i.e., being in-phase. 2. Measures of the similarity
among more than two functions. For example, seismic
reflection events are coherent in a linear way with
respect to dip, in a hyperbolic way with respect to
normal moveout, and in a nonanalytic though system-
atic way with respect to geophone locations. The prin-
cipal evidence for a separate seismic event is coherence
among the members of a set of seismic traces over a
short time interval ͑of the order of a 1 1/2 or so cycles
of the dominant frequency͒ compared with less coher-
ence elsewhere. Coherence is used qualitatively in
record picking and quantitative measures of coherence
are used in automatic picking schemes. Time slices and
horizon slices often display a coherence measure ͑see
Figure A-24d͒. See semblance and Sheriff and Geldart
͑1995, 145–146; 288–289͒. 3. A measure of the simi-
larity of two functions or portions of functions. If the
functions have power spectra Pii and Pjj and cross-
power spectra Pij ͑which may be complex͒, their coher-
ence is Pij/(PiiPjj)1/2
. Also called coefficient of
coherence. Coherence is a frequency-domain concept
analogous to correlation in the time domain.
coherence analysis: Correlating small portions of a trace
with respect to surrounding traces. Abrupt changes in
the system of correlation, that is, incoherencies, are
displayed in a variable-density form to emphasize
FIG. C-6. Coarsening-upward often produces a funnel-shaped pattern with gamma-ray or SP logs. Resistivity logs
(right trace), commonly plotted to the right of the depth scale, tend to produce the mirror response. Fining-upward
(dirtying-upward) produces the opposite pattern. (After Emery and Myers, 1996, 69)
coefficient of anisotropy 55 coherence analysis

12. FIG. C-7. (a) The color aspects of hue, saturation, and density shown on a color cube. The choices available are usually
displayed on a color pallette at a work station. (b) The selection of colors can enhance or obscure features. The channel
indicated by dark red on this horizon slice is nearly obscurred by use of too many colors. (c) Color compared with
variable area permits visualization of both positive and negative amplitudes in like manner to avoid the polarity bias
associated with variable-area displays. Bright spot and flat spot are shown here. Color enlarges the effective dynamic
range. (From Brown 1999, 28, 29, 40, 41, 34.)
coherence analysis 56 coherence analysis

13. FIG. C-7(b) and (c).
coherence analysis 57 coherence analysis

14. changes in correlation, perhaps because of faulting,
stratigraphic change, etc. Common coherence measures
include cross-correlation, semblance, and eigenvector
analysis; see Figure A-24d. Coherence cube is an
Amoco tradename.
coherence filtering: 1. A multichannel filter that empha-
sizes coherent events. 2. A method of noise suppression
that filters out the coherent portion of two signals.
coherency inversion: Identifying interval velocity of a
layer by ray tracing through a velocity model of arbi-
trarily complicated overlying layers. Constructing a
velocity-depth model from picked CMP-stacked data.
Generally, normal-incidence rays are traced through
overlying layers and traveltimes computed, and then the
velocity of the overlying layer is adjusted to minimize
errors. See also stacking velocity inversion ͑q.v.͒.
coherent: Said of waveforms that have the same phase
relationship.
coherent detection: Coherence filtering ͑q.v.͒.
coherent noise: Noise wavetrains that bear a systematic
phase relation ͑coherence͒ between adjacent traces.
Most source-generated seismic noise ͑ground roll, shal-
low refractions, multiples, etc.͒ is coherent. The distinc-
tion between random and coherent noise is usually a
matter of sampling ͑trace spacing͒, that is, very closely
spaced geophones observe as coherent what phones
more widely spaced ͑often Ͼ1–2 m͒ perceive as ran-
dom. See Sheriff and Geldart ͑1995, 183–184͒.
coincident time line: A line ͑surface͒ where two different
waves have the same arrival time. The intersection of a
coincident time line with the surface gives the cross-
over point for refraction data. See Figure H-3.
cokriging: A geostatistical method of interpolating spa-
tially organized data based on a sparse set of data that is
considered as exact and other sets that are dense but
inexact. Often involves interpolating between well data
locations using seismic data as the dense but inexact
data. It is a method of bivariate least-squares prediction
filtering that is essentially a bivariate version of Wiener
filtering. See also kriging and Journel and Huijbregts
͑1978͒.
colatitude: Angular distance from the North Pole, the
complement of the latitude, or 90° minus the latitude in
the Northern Hemisphere, or 90° plus the latitude in the
Southern Hemisphere.
Cole-Cole plot: A plotting convention for in-phase versus
quadrature measurements in which frequency relax-
ation appears as a semicircle. Useful for dielectric
relaxation, seismic velocity, and induced polarization
measurements. See Argand diagram.
Cole-Cole relaxation model: ͑ko¯l ko¯l͒ A model for
induced polarization ͑IP͒ effects in which the effective
impedance varies as a power of the frequency. The
complex resistivity ␳͑␻͒ is given by
␳͑␻͒ϭRo͕1Ϫm͓1Ϫ1/͑1ϩi␻␶͒c
͔͖,
where Ro is the dc resistivity, ␶ is the time constant, and
c is the frequency dependence. Typical ranges for
parameters are mϭ0.1 to 1.0, depending on the min-
eral content, ␶ϭ10Ϫ4
to 104
depending on grain size,
and cϭ0.2 to 0.6 depending on the particle size dis-
tribution. Compare Warburg impedance, for which the
value of c is 0.5.
collar: A coupling device to connect two lengths of pipe.
collar log: Casing-collar locator ͑q.v.͒.
collateral data: ͑k‫ץ‬ lat’ ‫ץ‬r ‫ץ‬l͒ Secondary data pertaining to
an area of interest.
collet: ͑kal’ ‫ץ‬t͒ A cone-shaped sleeve used to hold a rod-
like object.
collimated: Nondivergent, parallel.
collocated: ͑ko¯ lo¯ ka¯ t‫ץ‬d͒ Located at the same place.
cologarithm „colog…: ͑ko¯, log ‫ץ‬ rith’ ‫ץ‬m͒ The logarithm of
the reciprocal of a number:
colog͑x͒ϭlog͑1/x͒.
color: See primary colors, hue, saturation, density and
Figure C-7.
color bar: A display of the values represented by different
colors.
color display: A display of measurements that have been
color encoded. See also attribute and Figure C-7.
colored: Having different values at different frequencies.
Compare white.
colored sweep: A vibroseis sweep that is not linear with
frequency so that its frequency spectrum is not flat.
Also called a nonlinear sweep.
colorimetry: Estimation of intensity or hue of color in a
colored solution by visual comparison or by measuring
the absorption of the complementary color as light is
transmitted through the solution.
color mimicry: A technique for correlating the response
to different types of measurement for features in two-
dimensions. A set of photos, maps, or other displays are
projected through primary color filters and the images
superimposed. See Grossling ͑1969͒.
colorÕopacity editor: The function that changes the color
or opacity of portions of a data set in a visualization.
color palette: ͑col’ ‫ץ‬r pal’ it͒ The colors available for a
display at a work station or by a plotter.
color processing: Encoding a set of measurements as a set
of colors.
color wheel: Distribution of colors at different azimuths
such that complimentary colors are 180° opposite each
other. Thus the color sequence is red, orange, yellow,
green, blue, violet, and back to red.
column vector: See matrix.
comb: An infinite sequence of impulses ␦(tϪn⌬t)
occurring at time intervals ⌬t:
comb͑t͒ϭ ͚nϭϪϱ
ϱ
␦͑tϪn⌬t͒,
comb͑t͒↔comb͑ f ͒,
where frequency fϭ1/t if t is time and ↔ indicates a
Fourier transform operation. See Figures C-8 and F-19.
If the impulses are spaced T apart,
comb͑t/T͒↔comb͑Tf ͒.
Combisweep: A vibroseis sweep technique using several
sequential linear sweeps separated by short listening
coherence filtering 58 Combisweep

15. periods used to reduce correlation ghosts. A Prakla-
Seismos tradename.
commercial: See resource.
common-azimuth migration: Migration of data obtained
within a narrow angular range of source-to-receiver
directions.
common-conversion point „CCP…: The common reflect-
ing point where mode conversion from P- to S-waves or
vice-versa occurred for a set of data. Reciprocity ͑the
conversion point being the same when source and
receiver are interchanged͒ does not apply ͑unless
source and receiver modes are also interchanged͒. See
Figure C-13.
common-depth point „CDP…: The reflecting point that is
common to a subset of data. The use of this term for
common-midpoint ͑q.v.͒ is discouraged because the
reflecting point is not in common if reflectors dip. See
Figure C-9.
common-depth-point gather: Common-midpoint gather
͑q.v.͒.
common-depth-point stack: Common-midpoint stack
͑q.v.͒.
common-focus point: Common-imaging gather ͑q.v.͒.
common-geophone gather: A set of seismic traces having
the same geophone location. Also called a common-
receiver gather. See Figure C-9.
common-image gather: A gather of traces from various
offsets or angles, adjusted for traveltime differences
from zero offset, in time or depth. If the events are not
flat in time or depth, the resulting stacked image will
not be optimum.
common-imaging-point gathers: The set of data that are
reflected at the same point after prestack migration.
common-imaging point „CIP…: See common-imaging
gather. See Figure C-9g.
common-midpoint „CMP…: Having the same midpoint
between source and detector. See Figure C-9. Some-
times erroneously called common-depth-point or
common-reflection-point.
common-midpoint gather: The set of traces that have a
common midpoint ͑q.v.͒.
common-midpoint „CMP… method: A recording-
processing method where each source is recorded at a
number of geophone locations and each geophone loca-
tion is used to record from a number of source loca-
tions. After correcting these data for statics, normal
moveout, and DMO ͑q.v.͒ ͑since dipping reflections do
not involve a common reflecting point͒, they are com-
bined ͑stacked͒ to provide a common-midpoint sec-
tion that approximates the traces that would be
recorded by a coincident source and receiver at each
location, but with improved discrimination against
noise. The objective is to attenuate random effects and
events whose dependence on offset is different from
that of primary reflections. See Figure C-9.
common-midpoint stack: A stack ͑q.v.͒ of common-
midpoint gather traces.
common mode: Having signals that are identical in phase.
For example, signals that appear simultaneously at both
input terminals of a differential amplifier.
common-mode rejection: A differential amplifier that
ignores a signal that appears simultaneously at both
input terminals.
common-offset gather: 1. A side-by-side display of traces
that have the same source-to-geophone distance ͑off-
set͒; see Figure C-9f. 2. A side-by-side display of
crosswell traces in a receiver well that have the same
elevation difference with respect to source elevations.
common-offset stack „COS…: A stack of traces that have
the same offset and which are located within a limited
range of midpoint locations.
common-range gather: Common-offset gather ͑q.v.͒.
common-receiver gather: Traces from different source
locations into a single receiver; see Figure C-9e. Used
in borehole-to-borehole measurements.
common-reflection point: 1. A point in the subsurface
where energy is reflected for certain reflectors and
certain source-receiver pairs; see Figure C-9c. 2. Often
used erroneously for common-midpoint ͑q.v.͒ because
CMP reflection points are not common where there is
dip.
common-reflection-point gather, common-depth-point
gather: A collection of the seismic traces that all
reflect from the same point, often corrected for normal
moveout ͑velocity͒. After correcting for various pos-
sible velocities, events on the gather are horizontal
when the velocities are correct. Same as depth gather.
Differs from common-midpoint gather ͑q.v.͒ if there is
dip or lateral velocity changes.
common-scatterpoint „CSP… gather: A collection of
prestack migrated traces that involve the same scatter-
point. The maximum offset is limited by the migration
aperture and the fold may be large. The gathers, formed
with an inaccurate velocity model usually are analyzed
by conventional techniques ͑such as semblance analy-
sis͒ to yield a more accurate velocity model. conven-
tional NMO corrections, scaling, and stacking of a CSP
gather is the same as Kirchhoff prestack migration.
common-shot gather: See common-source gather.
common-source gather: 1. A side-by-side display of
traces that have the same source location; such as a field
record; see Figure C-9d. 2. A side-by-side display of
crosswell traces at different depths in the receiver well
commercial 59 common-source gather

16. that have the same source location.
communication: 1. Different locations connected so that
fluids can flow freely between the locations. 2. The
ability to interchange data, as when two computers
‘‘have communication with each other.’’
commutate: ͑kom’ y‫ץ‬ ta¯t,͒ To periodically reverse the
direction of an electric current ͑for example, by peri-
odically changing connections͒ so that it becomes uni-
directional; e.g., to reverse every other cycle of an
alternating current.
commutative: Operations that yield the same results
regardless of the sequence in which they are performed.
compaction: Loss of porosity with increasing differential
pressure, usually in a nonelastic way, e.g., by grain
deformation, repacking, recrystalization, etc. The nor-
FIG. C-9. Common-midpoint method. (a) In six-fold shooting with 24-geophone groups and the source point moved
two group intervals between successive shots; the same subsurface is sampled six times (A⇒23, B⇒21, C⇒19, D⇒17,
E⇒15, F⇒13). (b) A reflector that dips does not have a common reflecting point and common-midpoint stacking results
in reflection-point smearing unless DMO (q.v.) processing or migration precedes stacking. (c) To achieve a common-
reflection point in the case of dip requires unequal surface spacing. (d) A common-source gather is a collection of
traces having the same source; (e) common-receiver gather; (f) common-offset gather. (g) If there are horizontal
velocity variations, prestack migration is required to form a common-imaging-point gather. Compare Figure C-13. All
diagrams assume constant velocity.
communication 60 compaction

17. mal porosity ␾ of shales given by the Hubbert-Rubey
law is
␾ϭ␾oeϪcP
,
where ␾o is the initial porosity ͑the Atterberg limits,
q.v.͒, c is a constant, and P is the differential pressure,
the difference between the overburden pressure and the
formation fluid pressure. This leads to a pressure-with-
depth curve that is roughly logarithmic:
Pϭ͑1/c͒ln͑␾/␾o͒.
Compaction is nearly irreversible, i.e., relieving the
pressure that compacted a rock does not uncompact it,
but releaving the pressure is apt to produce a small
change ͑unloading effect͒.
compaction correction: An empirical correction applied
to porosity values ͑such as derived from the sonic log͒
in uncompacted formations. Undercompaction is indi-
cated by low velocities in adjacent shales. See also
differential compaction.
comparative interpretation: The comparison of survey
data with type curves that have been calculated for
bodies of assumed contrasts and geometry.
comparator: A circuit that compares two signals and indi-
cates the result of the comparison.
compass: Magnetic compasses are located at strategic
points along streamers to help determine the streamer
position. Gyrocompasses are used to determine vessel
orientation.
compensated log: A well log made with a sonde designed
to correct unwanted effects or one that has been cor-
rected. The compensated density log uses the signal
from a secondary detector to correct for the effect of
mud cake and small irregularities in the borehole wall.
The BHC compensated sonic log uses dual transmit-
ters and receivers to reduce the effects of irregularities
in borehole size and sonde tilt.
compensation test: Aircraft maneuvers to derive compen-
sation coefficients to correct for the effects of the air-
craft on magnetometer measurements. At high altitude
the aircraft flies in different directions ͑heading test͒
and undergoes oscillating pitch, roll, and yaw of 5–10°
to determine a figure of merit, which should be Ͻ1–2
nT. A new figure of merit has to be determined with
each change of aircraft configuration. After proper com-
pensation, the magnetic effects of the aircraft should
cancel out regardless of the aircraft’s flight direction.
See also lag test.
competent: ͑kom’ pi t‫ץ‬nt͒ A bed that retains its strati-
graphic thickness under stress. It folds or breaks under
stress, in contrast to an incompetent bed that tends to
flow.
compiler: ͑k‫ץ‬m pı¯’ l‫ץ‬r͒ A program for converting a source
program in a high-level language to an object program
in machine language prior to loading and running. A
program for converting concurrently with running the
program is called an interpreter.
complement: ͑kam’ ple¯ m‫ץ‬nt͒ The difference between a
particular value and full scale. In the decimal system,
the complement of x is (10␮Ϫx), where ␮ is a fixed
number; and in binary the complement of x is (2␮
Ϫx). Adding the complement of x is equivalent to
subtracting x, and computers often find it easier to
generate the complement and add than to subtract. See
also one’s complement and two’s complement.
complementary error function „erfc…:
erfc͑t͒ϭ1Ϫerf͑t͒,
where erf(t) is the error function ͑q.v.͒.
complementary filter: A filter that complements another
filter. For example, a 2000-m low-pass filter can be
complemented by a 2000-m high-pass filter to show the
high frequencies that the low-pass filter rejected.
complement of chargeability: An IP time-domain mea-
surement of the area under a decay curve; integrating
over the interval between 0.45 and 1.75 s on a
Newmont-type receiver.
completion: See well completion.
complex delta function: See impulse.
complex frequency: A damped wave can be expressed as
the product of an absorption factor eϪ␣t
and a periodic
factor ej␻t
:
AeϪ␣t
ej␻t
ϭAej͑␻ϩj␣͒t
,
where (␻ϩj␣) is the complex frequency.
complex impulse: The complex delta function, ␦*(t)
ϭ␦(t)ϩ(j/␲)t, where ␦(t) is an impulse ͑q.v.͒.
complex number: A number with both real and imaginary
parts, such as
zϭxϩjyϭAej␪
,
where jϭ͌Ϫ1. ͓The symbol i is also used to indicate
͌Ϫ1.͔ The modulus or magnitude of the above com-
plex number is Aϭ(x2
ϩy2
)1/2
and the angle indicat-
ing its direction with respect to the real axis is
␪ϭtanϪ1
͑y/x͒.
A graph of a complex function or quantity ͑such as a
frequency spectrum͒ is shown in Figure C-10.
complex ratio: See Turam method.
complex relative permittivity: See dielectric constant.
complex resistance: Impedance ͑q.v.͒.
complex resistivity: Representation of apparent resistivity
as having real and imaginary parts. Complex resistivity
is the proportionality between voltage and current
where the two are not in-phase. It is used to accomodate
variations in resistivity with frequency as observed in
FIG. C-10. A complex function (such as a frequency
spectrum) requires 3-D representation. The distance from
the frequency axis gives the modulus, and the angle with
the real plane is the phase.
compaction correction 61 complex resistivity

18. induced-polarization surveys.
complex spectrum: See Fourier transform.
complex-trace analysis: Finding the complex number
representation F(t) of a real time-series f(t):
F͑t͒ϭf͑t͒ϩjfЌ͑t͒ϭA͑t͒ej␥͑t͒
,
where fЌ(t) is the quadrature series, A(t) is the
amplitude of the envelope of the trace ͑also called
reflection strength͒, and ␥(t) is the instantaneous
phase. Displays of instantaneous phase ͑or cosine of
the instantaneous phase͒ show the continuity of an
event. Instantaneous frequency is d␥(t)/dt. Instan-
taneous frequency can be thought of as the frequency of
the complex sinusoid that locally best fits a complex
trace. Used to determine seismic attributes. In the space
domain, ‘‘local’’ is sometimes used instead of ‘‘instan-
taneous.’’ See Figure C-11 and Taner et al. ͑1979͒.
Complex-trace analysis often involves the Hilbert
transform.
compliance: ͑k‫ץ‬m plı¯’ ‫ץ‬ns͒ The relationship of strain to
stress. Compliance is a tensor of rank 4, but it is also
expressible as a 6ϫ6 matrix that is the inverse of the
stiffness matrix. See elastic constants. Compliance is
the mechanical or acoustical equivalent of electrical
capacitance.
composite: ͑k‫ץ‬m’ paz ‫ץ‬t͒ 1. Made up from two or more
different elements, as a composite decay curve or com-
posite time-distance curve ͑q.v.͒. 2. To mix or combine
the energy of different seismic channels without first
applying static and normal-moveout corrections.
composite amplitude: A sum of the absolute amplitude of
reflections attributed to the top and base of a reservoir.
composite decay curve: A time-domain decay curve con-
taining more than one component, usually with differ-
ent time constants or even a combination of positive
and negative decay curves. See Figure D-2.
composite displays: See Figure D-17.
composite reflection: A wavetrain composed of two or
more overlapping reflections.
composite time-distance curve: A refraction traveltime
versus offset-distance graph synthesized from data
obtained by several sources and short spreads at various
locations, as opposed to the graph that would have been
obtained from a single source into an extensive spread
of geophones. See Figure T-8d.
compositor: A device for mixing ͑q.v.͒.
compressed section: A record section with the horizontal
scale compressed. Associated with considerable vertical
exaggeration. Used to present regional and stratigraphic
aspects of seismic data. Also called a squash plot.
compressibility: Change of density with pressure; the
reciprocal of bulk modulus. The total compressibility is
the sum of the compressibilities of the various parts.
The compressibility of water is 3ϫ10Ϫ6
/psi, of oil 3
to 10ϫ10Ϫ6
/psi, of pore spaces 3 to 100
ϫ10Ϫ6
/psi. See elastic constants.
compressional wave: A P-wave ͑q.v.͒.
Compton scattering: ͑k‫ץ‬m’ t‫ץ‬n͒ The principal interaction
mechanism for gamma photons in the 0.4 to 3.0 MeV
range for media in the intermediate atomic number
range. The incident photon changes direction and is
available for repeated scattering, so that the probability
of total dissipation increases with the volume of the
scatterer. Named for Arthur Holly Compton ͑1892–
1962͒, American physicist.
computational error: See error.
computer: 1. One who computes corrections for geophys-
ical data; a computor. 2. A machine capable of accept-
ing information, applying prescribed processes to the
data ͑a sequence of arithmetic and logical operations as
requested by program instructions͒, and supplying the
results to some output device. A simple computer pro-
gram is shown in Figure F-14. 3. An analog computer
uses a physical analogy of position, electric current,
flow, temperature, etc. to solve relationships. 4. A digi-
tal computer applies numerical processes to sets of
discrete numbers.
computer generations: Historical levels of computer
hardware technology: first generation, vacuum tubes;
second generation, transistors; third generation, inte-
grated circuits; fourth generation, very large scale
integration ͑VLSI͒; fifth generation, massively parallel
processors.
computer graphics: The visual display of data stored in a
computer.
computerized tomography „CT…: Examination of cores
by x-ray scans ͑with medical-type equipment͒ and
tomographic analysis to show the distribution and con-
nectivity of porosity.
computer language: The form in which program instruc-
tions can be supplied to a computer for translation into
a machine language program that then can be executed.
Such languages include Algol, Cobol, Fortran, PL-1,
APL, Pascal, Cϩϩ, Basic, and others.
computer modeling: See numerical modeling.
concatenate: ͑kon kat’ ‫ץ‬ na¯t,͒ To unite in a series; to
connect together; to chain. The action by a computer
program of relating data in some organized manner so
as to treat multiple data sets or files as one.
concentration cell: See electrochemical SP.
concentration overvoltage: See overvoltage.
concentric fold: A flexural-slip fold. See folding.
concession: The right to search for oil or gas or minerals
and to remove any that are discovered, in return for
payments and royalties ͑portions of the minerals recov-
ered͒. See also lease.
concordance: ͑kon kord’ ‫ץ‬ns͒ Parallelism of reflections to
sequence boundaries.
concordia diagram: The time graph of 206
Pb/238
U vs.
207
Pb/235
U, assuming a closed system.
concertina display: Simultaneous display of a number of
plots having a major feature in common, such as dis-
play of a number of parallel seismic lines or of different
attribute displays of the same section or of the same
map.
condensate: A light hydrocarbon liquid consisting of vari-
ous proportions of butane, propane, pentane, and
heavier fractions with little or no ethane or methane,
obtained by condensation of hydrocarbon vapors. Con-
densate is usually gaseous in the subsurface.
condensate well: See gas-oil ratio (GOR).
condensed section: A relatively thin, continuous strati-
graphic succession that encompasses a considerable
time span and has reasonable geographic extent. Usu-
ally a deep marine section of hemipelagic and pelagic
sediments deposited far from the coastline ͑distal͒.
Generally, it is very fine grain and rich in both number
complex spectrum 62 condensed section

19. and diversity of fossils, and hence is useful for age
dating. Often the distal part of a maximum flooding
surface ͑q.v.͒ and the top of a transgressive system tract
͑q.v.͒. See also Figure S-32.
condition: 1. Treating drilling mud with additives and
circulating mud through a borehole to make the mud
uniform throughout the system. 2. A restriction or con-
straint, as an initial condition or boundary condition
͑q.v.͒. 3. See condition number.
conditional jump: An instruction causing a transfer to an
instruction other than the next sequential instruction
only if a specific condition is satisfied. The next sequen-
tial instruction is executed if the condition is not satis-
fied. See branch.
FIG. C-11. Complex-trace analysis. Real (a) and quadrature (b) traces for a portion of a seismic trace. The envelope
is shown as the dotted line in (a) and (b). Instantaneous phase is plotted in (c), instantaneous frequency in (d), and
weighted average frequency as the dotted curve in (d). (e) Isometric diagram of a complex trace. (From Taner et al.,
1979.)
condition 63 conditional jump

20. conditional probability: P(E͉Ci)ϭprobability of E if
Ci has already occurred. Bayes’s theorem gives the a
posteria probability:
P͑Ci͉E͒ϭP͑Ci͒P͑E͉Ci͒/͓⌺P͑Cj͒P͑E͉Cj͔͒,
where P(Ci)ϭa priori probability of Ci.
conditional simulation: A geostatistical method of creat-
ing equally probable images of a regionalized variable
based on spatial data. By relaxing some of the kriging
constraints, conditional simulation reproduces the vari-
ance of the control data so that the result captures the
heterogeneity and connectivity most likely present. It is
used to produce measures of uncertainty such as prob-
ability and uncertainty maps.
conditioning: Preparing data for processing or interpreta-
tion by removing bad data, smoothing, resampling, or
applying other reduction operations.
condition number: The value given when the largest ele-
ment of a diagonal matrix is divided by the smallest
element. For a nondiagonal matrix, the condition num-
ber is the ratio of the largest to smallest eigenvalues.
The condition number is infinity if an element is zero. A
too large condition number means that the matrix is
ill-conditioned and therefore unstable.
conductance: The product of conductivity and thickness;
with direct current, the reciprocal of resistance. With
alternating current, the resistance divided by the imped-
ance squared; the real part of admittance. Measured in
siemens ͑ϭmhoϭinverse ohm͒.
conductance referencing: A correction procedure where
the total conductance is kept fixed as the thicknesses
and resistivities are adjusted. Usually the resistivity is
selected to be the average resistivity and the thicknesses
are adjusted.
conduction angle: The number of degrees in a half-cycle
ac wave during which a silicon-controlled rectifier is
turned on. If ␾ is the phase control angle, the conduc-
tion angle is ␲Ϫ␾.
conduction current: Electrical current resulting from the
motion of free charges under the influence of an electric
field. The density of conduction current J at a point in
an isotropic medium is Jϭ␴E, where ␴ is the conduc-
tivity and E is the electric field. Conduction currents
usually are more important than displacement currents
in electromagnetic prospecting, depending on the fre-
quency range. Since in general J and E are vectors, the
conductivity ␴ is a symmetric tensor given in Cartesian
coordinates by
ͯ␴xx ␴xy ␴xz
␴yx ␴yy ␴yz
␴xz ␴yz ␴zz
ͯ.
If one of the coordinates is in the direction of maximum
conductivity and one in the direction of minimum con-
ductivity, nondiagonal elements vanish. In isotropic
materials ␴xxϭ␴yyϭ␴zz.
conductivity: The ability of a material to conduct electri-
cal current. In isotropic material, the reciprocal of resis-
tivity. Sometimes called specific conductance. Units
are siemens per meter.
conductivity-depth image: A diagram showing conduc-
tivity as a function of distance and depth, derived from
profiles of surface or airborne EM data. See also para-
section.
conductivity log: Induction log ͑q.v.͒.
conductivity tomography: Using a transmitter and
receiver in different wells to construct a tomographic
picture. See Figure T-10.
conductor: A body within which electrical current can
flow readily. Often, the ‘‘target’’ of an electromagnetic
survey. An electronic conductor conducts electricity
primarily by electron mobility and an ionic conductor
primarily by ion mobility. Electrolytes are ionic con-
ductors. ResistivityϽ10Ϫ5
⍀.m.
conductor casing: A second casing string with the annular
space filled with cement to protect fresh-water sands.
conductor pipe: A short string of large-diameter casing
used on offshore and marsh wells and some other
conditions to contain the mud upwelling from the annu-
lus so that it flows into the slush pit.
cone bit: A roller bit having conical cutters with teeth for
cutting into formations. See Figure D-26b.
cone penetrometer: Device used by civil engineers to
determine the shear strength of shallow sediments.
confidence bounds: The probability that an interval con-
tains an element with given characteristics; the limits
between which a specified percentage of measurements
are expected to lie. A measure of the uncertainty in
values. Often taken as the standard deviation ͑q.v.͒.
Generally calculated by tracing the estimates of the
uncertainties in the various measurements that contrib-
ute to determining a value, but measurement uncertain-
ties are not always the principal contributors to uncer-
tainty. A rule for calculating uncertainty in
electromagnetic applications is to use confidence levels
of 95% for data with high signal-to-noise ratios to 68%
for noisy data. See Raiche et al. ͑1985͒.
configuration: 1. Arrangement, as of geophones in a
group; array ͑q.v.͒. 2. The hardware and/or software
making up a computer system, and how it is put
together.
conformability: See conformal mapping and map projec-
tion.
conformable: ͑k‫ץ‬n for’ ma b‫ץ‬l͒ Two adjacent parallel
beds separated by a surface of original deposition,
where no disturbance or denudation occurred during
their deposition.
conformal mapping: Mapping one area into another
when there is a continuous one-to-one correspondence
of points and where angles are preserved.
conic: A curve defined by a quadratic equation. See Figure
C-12.
conical wave: 1. A head wave ͑q.v.͒. Also called Mach
waves. 2. A head wave generated by tube waves.
coning: ͑ko¯n’ ‫ץ‬ng͒ The upward encroachment of reservoir
water into the oil column at a well because of a high
rate of production, or the downward encroachment of
gas into the oil column.
conjugate: ͑kon’ j‫ץ‬ ga¯t,͒ The conjugate of a complex
number is the number with the sign of the imaginary
part reversed. Often designated by a superscript asterisk
or superscript bar.
If ZϭaϩjbϭAej␪
, then Z*ϭZ¯ ϭaϪjbϭAeϪj␪
.
conjugate function: Any 2D function F(z) may be writ-
conditional probability 64 conjugate function

21. ten in terms of orthogonal conjugate functions ␾(x,y)
and ␺(x,y):
F͑z͒ϭF͑xϩiy͒ϭ␾͑x,y͒ϩi␺͑x,y͒.
conjugate gradient method: A method for approaching a
minimum for a multidimensional function. Steepest
descent ͑q.v.͒ is such a method.
conjugate points: Pairs of object and image points that
correspond to each other. Object and image within any
pair can be interchanged.
conjunction: ͑k‫ץ‬n junk’ sh‫ץ‬n͒ 1. The condition for which
an AND-gate is used; intersection. Each of two ͑or
more͒ situations must occur. Often written AപB or
A•B ͑read as ‘‘A and B’’͒. See Figure B-5. 2. Align-
ment of celestial objects at the same azimuth and eleva-
tion.
connate water: ͑kon’ a¯t͒ Water trapped in the interstices
of the sediments at the time of deposition, as opposed to
water that migrated into the formations after deposition.
See interstitial water.
connectivity mapping: Determining whether two reser-
voirs are connected hydraulically, e.g., by the ability to
transmit channel waves from one to the other.
Conrad discontinuity: ͑k‫ץ‬n’ rad͒ A sharp increase in the
P-wave velocity in the crust, commonly at a depth of
17–20 km. The velocity below the Conrad discontinu-
ity is of the order of 5.5–6.7 ͑often 6.0–6.7͒ km/s
͑compared to around 8.1 km/s for the upper mantle
immediately below the Moho͒. The Conrad discontinu-
ity is not observed everywhere.
console: The computer operator’s control panel. Generally
includes start-stop keys, keyboard for entering instruc-
tions or data, and display. Also, a work station.
constant separation traversing: Electric profiling ͑q.v.͒.
constant velocity stack „CVS…: Velocity analysis
involves making trial stacks assuming several trial
velocities that are constant in time and space, to deter-
mine the stacking velocities that produce the best
results. See velocity panels and Figure V-7.
constraint: An auxiliary restriction, usually expressed as
an inequality ͑for example, xрb͒. See Figure L-5.
constructive interference: See interference.
contact: A boundary ͑often a fault͒ between two blocks of
different lithologies involving different physical prop-
erties.
contact log: A microresistivity log ͑q.v.͒ in which the
sonde is held against the borehole wall.
contact resistance: The resistance observed ͑a͒ between a
grounded electrode and the ground, ͑b͒ between an
electrode and a rock specimen, or ͑c͒ between electrical
contacts.
continental crust: The 30–60-km thick layer of sedimen-
tary and felsic rock lying above the Mohorovicic dis-
continuity that forms the continents and continental
shelves. See Figure P-5.
continental shelf: The zone adjacent to a continent where
the ocean floor dips about 0.1° that lies between the
shoreline and is less than about 200 m deep, where
there generally is a marked increase in water depth
͑shelf break͒. The significantly greater slope beyond
this point is called the continental slope; it terminates
at an oceanic trench or where the slope again becomes
more gentle ͑continental rise͒, which extends to the
abyssal plain. The continental margin is the part
between the shoreline and 4000 m depth.
continuation: Determining a field over one surface from
measurements of the field over another surface ͑specifi-
cally, at another elevation͒. The field at the elevation z,
F(x,y,z), can be found from the field on the surface,
F(xЈ,yЈ,0). Where the surfaces are horizontal and no
sources intervene, the upward-continuation relation
͑an application of Green’s theorem͒ is
F͑x,y,z͒ϭ
͉z͉
2␲ ͵ ͵ F͑xЈ,yЈ,0͒
R3 dxЈdyЈ.
An interchange of the two fields in this equation gives
the downward-continuation relation. See downward
continuation and Peters ͑1949͒, Telford et al. ͑1990,
§2.6.7 and 3.7.5͒, and Pawlowski ͑1995͒.
continuity: Condition of an unbroken electric circuit.
continuity equation: Where Vϭvelocity and ‫ץ‬␳/‫ץ‬t is the
time rate of change of density, the relation Ϫٌ•V
ϭ‫ץ‬␳/‫ץ‬t.
continuous profiling: A seismic method in which geo-
phone groups are placed uniformly along the length of
the line and sources are so spaced that continuous
͑usually 100%͒ subsurface coverage is obtained along
the line. Continuous profiling can be accomplished with
a variety of spread arrangements. Refraction continuous
profiling requires continuous control on the refractor
being mapped; this may require irregular surface lay-
outs.
continuous-velocity log „CVL…: A sonic log ͑q.v.͒.FIG. C-12. Conic section equations.
conjugate gradient method 65 continuous-velocity log „CVL…

22. contour: ͑kon’ toor͒ A line separating points whose values
are higher than the contour value from points lower,
representing the locus of a constant value on a map or
diagram. Allowance has to be made for uncertainties so
that noise is not contoured, and a contour value of Ci
can be thought of as marking the middle of a zone that
contains values CiϮN, where N is the mean noise
level. Often the smallest meaningful contour interval is
about twice the noise level so that uncertainty ranges do
not overlap. See Sheriff ͑1989; 13–21͒.
contour interval „CI…: The difference in value between
two adjacent contour lines.
contourite: A mound-like or lenticular sediment mass
deposited ‘‘along a contour’’ by along-slope bottom
currents, in contrast with turbidity deposits that are
deposited by currents flowing down slope.
contrast: The difference in value or tone between high-
light and shadow in an image.
contrast stretching: Increasing the contrast to use the full
range of a display by expanding the original range of
values or tones.
control: 1. Accurately known data that can be used to
check the validity or accuracy of a series of measure-
ments. 2. The data on which a map or section is based.
Posting the control on a map or section allows one to
evaluate the interpretation as to what happens between
control points. 3. The section of computer code that is
currently executing.
control character: A nonprinting character that controls
an operating function.
control lines: Survey lines usually orthogonal to the
traverse lines; commonly acquired with larger spacing,
to control line-to-line instrument variations. Also called
tie lines.
controlled mosaic: A composite aerial photograph made
by rephotographing component vertical photographs to
compensate for scale variations resulting from tilt and
variations in flight altitude.
controlled rectifier: An electronic circuit element consist-
ing of a controlled diode or solid-state switch. The
diode is usually turned ‘‘on’’ by a small voltage from an
external circuit and turned ‘‘off’’ when the voltage is
reversed. Used to switch large currents in IP transmit-
ters. When the semiconductor is silicon, a controlled
rectifier is called SCR. A GTO ͑gate turn-off switch͒
controlled rectifier can be turned on and off indepen-
dently of the current through the diode.
controlled-source electromagnetics „CSEM…: An elec-
tromagnetic sounding or prospecting system that uses
artificially generated fields with prescribed characteris-
tics rather than natural fields. Includes all electromag-
netic techniques that use their own transmitters.
controller: 1. See depth controller. 2. A computer periph-
eral that handles multiple devices of the same kind
͑e.g., a tape controller or a disk controller͒.
control station: A point whose position ͑horizontally
and/or vertically͒ is used as a base for a dependent
survey or as control for adjusting survey errors.
control total: A sum obtained by adding the numbers in a
field; used for checking to avoid dropout.
convection: Transfer of energy or mass by motion of the
medium. A transport phenomenon where the field varies
in time with the aid of motion of the medium. Examples
include heat transport because of nonuniform heating
and chemical concentrations in an unstable liquid. Con-
trast with diffusion ͑q.v.͒.
convergence: 1. The condition when calculated values
approach finite-limiting values as the number of terms
or iterations used increases. 2. In iterative operations,
such as modeling, the condition when calculated values
become sufficiently close to observed values. 3. The
effect of computing a survey on a curved surface as if
the surface were plane. Applying convergence correc-
tion changes from assumed rectangular coordinates to
geodetic coordinates.
convergence correction: See convergence.
convergent margin: Active margin ͑q.v.͒.
conversational mode: An interactive procedure in which
each entry from a terminal elicits a response from the
computer and vice-versa.
conversion point: The reflecting point where P-waves
convert to S-waves or vice-versa. The set of data that
have the same conversion point constitute a common-
conversion-point ͑CCP͒ gather; after NMO correction,
FIG. C-13. Conversion point. (a) Where P-waves convert to S-waves upon reflection (i.e., C-waves), reflecting angles
r are smaller than incident angles i, and interchanging source and receiver changes the reflecting point. (b) For common
source and receiver points, the conversion point generally moves toward the receiver for shallower reflectors.
contour 66 conversion point

23. they can be stacked together to display converted
waves. The conversion point is moved when the source
and receiver are interchanged. See Figure C-13.
converted wave: Seismic energy that has traveled partly
as a P-wave and partly as an S-wave, being converted
from one to the other upon reflection or refraction at
oblique incidence on an interface. Since mode conver-
sion is small for small incident angles, converted waves
become more prominent as the angle of incidence ͑and
usually offset͒ increases.
converter: A device to perform digital-to-analog ͑D/A͒ or
analog-to-digital ͑A/D͒ conversion.
convolution: ͑kon, v‫ץ‬ loo’ sh‫ץ‬n͒ Change in waveshape as
a result of passing through a linear filter. 1. A math-
ematical operation between two functions, g(t) and
f(t), often symbolized by an asterisk:
g͑t͒*f͑t͒ϭ͵␶
g͑␶͒f͑tϪ␶͒d␶.
Convolution is not restricted to one dimension. For
example, in two dimensions,
g͑x,y͒*f͑x,y͒ϭ͵␣
͵␤
g͑␣,␤͒f͑xϪ␣,yϪ␤͒d␣d␤.
2. Linear filtering. If a waveform g(t) is passed into a
linear filter with the impulse response f(t), then the
output is given by the convolution of g with f. In
discrete form where the input is a series of impulses of
varying size, each will generate an f(t) of proportional
amplitude and the output will be the superposition of
these. This can be expressed as
gt*ftϭ͚kϭ0
L
gk ftϪk.
This expresses that the output of a linear filter at the
instant t is a weighted linear combination of the inputs.
L is the convolution operator length and (Lϩ1) is the
number of points in the operator. ͑A simple computer
program is shown in Figure F-14.͒ The frequency-
domain operation equivalent to time-domain convolu-
tion consists of multiplying frequency-amplitude curves
and adding frequency-phase curves. Convolution is
sometimes done by ͑a͒ replacing each spike of the
input with a proportionately scaled version of the
impulse response and superposition forms the output;
͑b͒ folding where the impulse response of the filter is
reversed in time and slid past the input, the output for
each position of the impulse response being the sum of
the products of input and folded impulse response for
corresponding points; ͑c͒ multiplying z-transforms of
the input and of the impulse response to give the
z-transform of the output; or ͑d͒ multiplying Fourier
or Laplace transforms to give the Fourier or Laplace
transform of the output. See Sheriff and Geldart ͑1995,
279–81 and 540–2͒. Well logs may be thought of as the
convolution of the response of the earth adjacent to the
borehole with the logging sonde impulse response. 3.
Convolution in two dimensions is used with gravity,
magnetic, and other data to produce grid residual, sec-
ond derivative, continuation maps, etc.; see Fuller
͑1967͒.
convolutional model: The concept that a seismic trace
f(t) can be represented by the convolution of an
embedded ͑equivalent͒ wavelet w(t) with a reflectivity
function r(t) plus random noise n(t):
f͑t͒ϭw͑t͒*r͑t͒ϩn͑t͒.
This model is a consequence of the concept that each
reflected wave causes its own effect at each geophone
͑or hydrophone͒ independent of what other waves are
affecting the geophone and that the geophone response
is simply the sum ͑linear superposition͒ of the effects of
all the waves. The model can be modified to allow for
propagation effects such as absorption. The convolu-
tional model is implied in most seismic processing and
interpretation.
convolution theorem: The Fourier transform of the con-
volution of two functions is equal to the product of their
individual transforms ͑or multiplying their amplitude
spectra and summing their phase spectra͒. See Figures
F-20 and F-22.
cookbook: A method that is prescribed step-by-step.
cool colors: The blue and green colors as opposed to the
red and yellow colors.
Cooley-Tukey method: A Fourier-analysis algorithm that
considerably reduces computing time; see fast Fourier
transform and Cooley and Tukey ͑1965͒.
cooperative inversion: See joint inversion.
coordinate transform: See Figure C-14.
copy: The degree to which events or traces look alike.
core: 1. A rock sample cut from a borehole or retrieved
from the sea floor. See core analysis. 2. The Earth’s
core is the central portion bounded by the Gutenberg
discontinuity ͑Ϸ2900 km deep͒ that separates it from
the mantle. The core’s radius is Ϸ3500 km; it is pre-
sumed to be composed predominantly of iron and sili-
cates. See Figure E-1. Divided into outer core ͑which
will not transmit S-waves͒ and inner core, the radius of
the inner core being about 1220 km. The inner core was
discovered by Inge Lehmann in 1936. 3. A material of
high magnetic permeability placed in the center of a
coil of wire. Used in the flux-gate magnetometer for
measuring magnetic fields. 4. A magnetic core ͑q.v.͒ is
a device that used to be used in rapid-access memories.
core analysis: Cores from boreholes are analyzed for
porosity, permeability, fluid content, and fluid identifi-
cation ͑water and/or oil saturation, residual oil͒, lithol-
ogy, and structure ͑fractures, cross bedding, etc.͒.
Results are often illustrated on a log or graphed against
depth.
coregionalization: ͑ko¯’ re¯ g‫ץ‬n ‫ץ‬l ‫ץ‬ za¯. shon͒ The mutual
spatial behavior between two or more regionalized vari-
ables.
corer: A device for obtaining a solid sample of rock from
a borehole or from the ocean bottom. A core barrel is
a hollow cylinder attached to a special bit, used to
obtain a continuous core section from the bottom of a
borehole. Cores are obtained from the bottom of a
borehole with a wireline corer, the core barrel being
retrievable without having to trip out of the hole. A
sidewall corer obtains a sample from the borehole wall
by firing a hollow cylindrical bullet from a tool sus-
pended in the borehole. A core slicer using diamond-
edged blades cuts a triangular core about 1 inch on a
converted wave 67 corer

24. side and up to 3 ft-long from the side of a smooth
borehole. A box corer usually penetrates less than 3 ft
into the sea floor and has a spade-like device that
retains a sample. A gravity corer penetrates the ocean
floor solely by its own weight. The piston in a piston
corer retracts as the cylinder penetrates the sediments.
The jaws of a grab sampler seize a portion of the sea
bottom for retrieval.
Coriolis acceleration: ͑kor, e¯ o¯’ lis͒ 1. A velocity-
dependent acceleration in a reference frame that is
moving with respect to an inertial reference frame.
Specifically, the acceleration of a body in motion with
respect to the Earth resulting from the rotation of the
Earth, as seen by an observer on the Earth. The Coriolis
acceleration on a body moving on the surface of the
Earth with a velocity V is 2␻V sin ␾ where ␻
ϭangular rotation of the Earth and ␾ϭlatitude. A
Coriolis acceleration of a moving gravimeter is
involved in the Eo¨ tvo¨ s effect ͑q.v.͒. Named for
Gustave Gaspard Coriolis ͑1792–1843͒, French math-
ematician.
corner frequency: ͑a͒ The frequency at which a graph of
the logarithm of the frequency response has an abrupt
change of slope. Also called break frequency. ͑b͒ For
a seismic wave generated by an earthquake, the fre-
quency at which the spectral field begins to decrease. It
is related to the dimensions of the source.
corner reflector: A radar reflector made of sheets of metal
or metal screen at right angles to each other. It reflects
like a mirror at normal incidence no matter from which
FIG. C-14. Coordinate transforms. Unit vectors in x, y, z, ␪, ␾ directions are indicated by i, j, k, ␪, ␾ (a) Rectangular-
cylindrical conversion; (b) rectangular-spherical conversion. (c) Vector operations in rectangular, cylindrical, and
spherical coordinates.
Coriolis acceleration 68 corner reflector