1. CasedHole Seismic
In the early stagesof planning exploration anddevelopment
in a new area, surfaceseismicsurveysare usedextensively
to delineateprospectivestructural or stratigraphictraps. Re-
cent improvementsin digital filtering and processingtech-
niqueshaveled to high-quality resultsunder favorablecon-
ditions. The resolutionof surfaceseismicsurveys,however,
is still fundamentallylimited by low operating frequencies.
When wells aredrilled, opportunitiesexistto improve this
situation through the use of well logs. After editing and
calibrating againstcheck shots, openholesonicand density
logscanbeusedto generatesyntheticseismograms.If open-
hole dataare not available, in many instancesa casedhole
sonic log canbe recordedfor this purpose(seeChapter 3).
Thesesyntheticsare extremely valuablein verifying reflec-
tion eventsin a seismicsectionandrelating seismicfeatures
to geologicalstructures.Velocity anomalies,whichmaycause
exploration wellsto bedrilled off-structure, canberesolved.
A more recentgeophysicalapplicationof wireline logging
measurementsinvolves thepreparationof a vertical seismic
profile (VSP). In this technique,anair gunvibroseis,or other
seismicsourceon the surfacegeneratesthe input signalthat
is detectedby a downhole geophone.As the soundenergy
travels only oncethrough the weatheredsurfacelayers, the
resultantprofile hasmuch better resolution thanthe surface
seismic around the borehole, and, in favorable cases,can
identify reflectors far below the total depth of the well.
Unlike many wireline services, openholeand casedhole
seismicresultsare similar sincethecasingtypically doesnot
affect the seismicsignal. In fact, casedholeseliminate some
of the openholeoperational problems associatedwith poor
hole conditionsandhighly deviatedwells. Also, specialmul-
tisensorarray tools for VSP acquisitioncanbeusedin cased
holes that are not practical for openholeoperations.
CASED HOLE SEISMIC EQUIPMENT
Theequipmentshownin Fig. 9-l consistsof adownholetool
with geophones,the CSU surfacerecording system,offset
shooting equipment, and an air gun system.
The most commonly usedenergy source offshore is the
air gun. Its safety, reliability, cost, broad spectrum, simple
signature,andtransportability maketheair gunaconvenient
seismic source. An array of synchronized air guns can be
usedif a large power output for deeperpenetrationis need-
ed. The air gun firing chambersmay incorporate a wave-
shapingkit that significantly reducesthe bubble effect and
provides aclean signal. The air compressorandair storage
bottles provide anadequateair supply for fast, uninterrupt-
edoperations.Other soundsources,suchasvibroseisunits,
are routinely usedin the field dependingon specific appli-
cations and local conditions.
When using an impulsive sourcesuchasan air gun, the
source signal is recorded at the surfaceby a hydrophone.
This allows a precise determination of the time break and
permits continuous monitoring of the gun signature. The
recorded source signature is used to enhancethe signals
recorded by the geophonein VSP processing.
The dataare recordeddigitally on magnetictapewith the
CSU system. The seismic waveforms can also be stacked
to improve the signal-to-noise ratio.
The downholetools currently in useare theWell Seismic
Tool (WST*), theSeismicAcquisition Tool (SAT*), andthe
Downhole SeismicArray tool (DSA*). The WST tool has
four uniaxially stackedgeophonesthat are primarily sensi-
tive to movementin thevertical direction. The SAT tool has
three mutually orthogonal geophones(which may also be
gimbal-mountedfor usein deviatedwells) for 3dimensional
operation. This arrangementprovides an x, y, z systemof
referencewhere eacharriving ray can be representedby a
vector. Among other applications, the ability to record and
processsignals in three axes allows the recording and in-
terpretationof shearwaves,saltproximity surveys,andlong-
offset VSP surveys.The DSA tool (Fig. 9-2) useseight sen-
sor packages(shuttles)which are positioned along an insu-
latedmulticonductor bridle cableat intervals of up to 50 ft.
The sensorpackagecontainsa vertical geophonefor signal
acquisition, a magneticclamping deviceto securethe pack-
age to the casing, a shakerelementto generatemechanical
vibrations for reference,andelectronic circuitry to transmit
9-l

2. CASED HOLE LOG INTERpRETAT,ON PRINClPLES/APPlKATlONS
Fig. S-l-Well seismic tool hardware

3. Fig. 9-e--Schematic of DSA tool in operation
signals to the cartridge. In the cartridge, the signals pass
through antialiasing filters, sample-holdcircuits, and mul-
tiplexers, andare then digitized andtelemeteredto the sur-
face. The tool canbe combinedwith a casingcollar locator
for depth control.
DIGITAL CHECK-SHOT SURVEY
At eachdepth,theinterval velocity of theformationsbetween
the sourceandtheborehole geophoneis measured.With an
air gun source, the hydrophone monitors the signatureand
timing of the source signal, and the downhole geophone
records the direct and reflected arrivals.
Transit time is measuredfrom thefirst breakof thehydro-
phone(surface)recording to the first break of the geophone
(downhole) recording. Severalshotsareusually madeatthe
CASED HOLE SEISMIC
samelevel andstackedin orderto improvethesignal-to-noise
ratio.
If the hole is deviated or if there is a significant source
offset, the transit times obtained must be converted to true
vertical depth (TVD) transit times. Correction to the seis-
mic referencedatum (SRD) is also necessaryif the source
is aboveor below the seismicdatum.The corrected,stacked
magnetictapedatacanthenbeconvertedto a standardSEG-
Y tape format.
TIME-TO-DEPTH CONVERSION AND
VELOCITY PROFILE
Check-shotsurveysareusedto correctthevelocitiesobtained
by theintegration of the sonicinterval transit times. The ad-
justed sonicmay then be usedfor the translation of surface
seismictime into depth and in the calculation of formation
acoustic impedance necessary for the generation of a
Geogram* syntheticseismogramandfor other applications.
Formation velocities obtainedby the integration of sonic
logs may differ from thoseobtainedby surfaceand check-
shot surveys for the following reasons:
l Becauseof velocity dispersionwith frequency,seismicve-
locities (measuredat roughly 50 Hz) may be asmuch as
6% lower than sonicvelocities (measuredat 20,000 Hz).
l Borehole effects, suchas thosecausedby formation al-
teration, may decreasethe apparentsonic log velocities.
l The sonic transit time measurementis fundamentally
different from the surfaceseismicmeasurement.The sonic
log velocity is measuredin a continuousmanner along-
side the borehole, while the seismicwaves reaching the
geophone(s)takethe mostdirect acoustic(shortest)path.
The long-spaced(LSS) or Array-Sonic tools are required
for casedhole logs andprovide betterdatathanBHC sonics
in openholes. However, all recorded sonic logs should be
edited to correct for borehole effects. To adjust a sonic log
correctly, check shotsare required, Check shotsshouldbe
made at the SRD, at the tops of significant formations, at
the top of the sonic log, and spacednot greater than 500 ft
apart.
Seismictime is normally referencedfrom thecheckshots,
and sonic log measurementsare adjustedaccordingly. The
adjustmentconsistsof computingthe raw drift, selectingthe
drift curve, adjustingthe soniclog, andcheckingthe validi-
ty of the result.
Raw drift is defined asthe correct shottime minus the in-
tegratedsonictime. Theselecteddrift curve is derived from
the raw drift values. The kneesof the selecteddrift curve
are usually locatedat changesin lithology , borehole condi-
tions, soniclog character,andthe drift data. The correction
determinedby the selecteddrift curve is distributed to the
sonic transit times over the interval defined by consecutive
9-3

4. CASED HOLE LOG INTERPRETATION PRINCIPLESIAPPLICAT
knees.The checkof the adjustedsonicis madeby ensuring
that the integrated sonic time and the corrected shot time
agreeat eachshooting level within the accuracyof the shot
time. Obviously, themore checkshots,the moreaccurately
this can be done.
A sectionof a sonic calibration log is shown in Fig. 9-3.
The uncalibrated transit time, calibrated transit time, and
gamma ray curve are displayed along with bulk density,
spontaneouspotential,anddifferential caliperdatafrom open-
holelogs. In theleft track, vertical depth,shotnumbers,cor-
rectedshottimes, uncorrectedl-way integratedsonictimes,
and corrected 2-way integratedsonic times are displayed.
In addition to providing data for sonic calibration, check
shotsallow a time-to-depth conversionto bemadewhen no
sonic log hasbeenrecorded. A similar, related application
is the determination of the weathering correction and the
thickness of the weatheredzone.
GEOGRAM PROCESSING
The seismic waveforms propagating through the earth are
affectedby eachlithologic bedboundary.Specifically, at the
interface of two formations of contrasting acoustic im-
pedances,part of the energy will be transmitted acrossthe
interface andsomewill bereflected. The amountof seismic
energytransmittedandreflecteddependson theacousticim-
pedancecontrastbetweenthetwo formationbeds.The acous-
tic impedanceof a formation, Z,, is given as:
z, = PV 9 (Eq. 9-l)
wherep is theformation densityandv is its interval velocity.
The amountof reflectedenergybetweentwo adjacentbeds
dependson the relative impedancesof the two beds. The
reflection coefficient, R, is defined as:
412 - TX1
% = za2 + z ’
a1
(Eq. 9-2)
where Z% andZal are the acousticimpedancesof layers 2
and 1.
The subsurfacecanbe approximatedfor seismicpurposes
by a seriesof layers having specific acoustic impedances,
which can be usedto produce a seriesof reflection coeffi-
cients at the boundaries(Fig. 9-4). Sincea sonic log mea-
suresacousticvelocity andadensity log measuresbulk den-
sity, the sonic and density logs canbe usedto computethe
reflectivity series,which canthenbeconvolvedwith a suita-
ble wavelet. The result is a Geogram display (synthetic
seismogram).
Geogramprocessingproducesanideal seismictraceonly
if the sonic and density logs have beenproperly recorded,
edited, andadjustedto representthesubsurfaceundisturbed
by drilling. Special programs allow the recomputation of
sonic velocities and bulk densities, taking into accountthe
Fig. Q-3-Sonic calibration log

5. CASED HOLE SEISMIC
Fig. 9-4-Ideal seismic record giving position (in time) of
reflector and value (amplitude) of reflection coefficient
effect of the invaded zone; this is particularly important in
gas-bearingformations.Geogramprocessingenablesqualita-
tive correlations aswell asquantitative evaluationsof seis-
mic data to be made.
The Geogramprocessingsequenceis shownin Fig. 9-5.
The first two steps,which involve editing and sonicadjust-
ments,arc normally madeduring thetime-to-depthconver-
sion. Oncethereflectivity seriesandtransmissionlosseshave
beencomputed,the decision must bemadeon what type of
wavelet to usefor convolution. In order to give the bestap-
proximation of theactualsourcesignature,severalwavelets
arcavailable.TheseincludeRicker minimum- or zero-phase,
Klauder, spikewith Butterworth filter, or otheruser-defined
operators.
The Geogramdisplay canbemadewith or without multi-
ples and/or transmission losses,and with any desired fre-
quency or bandof frequencies.A typical Geogramdisplay
is shown in Fig. 9-6.
The structural dip, asinterpretedfrom adipmetersurvey,
canbe incorporatedinto the presentationto permit theGeo-
gram resultsto be projectedaway from the well (Fig. 9-7).
A Geogramdisplay canhelp in the qualitative evaluation
of the seismic sectionsby providing the following:
l an ideal seismictraceasa referencefor the surfaceseis-
mic data
. time-to-depth conversions
. detection of multiples
. seismic character correlation
* direct correlation with log intervals.
Seismic modeling can also be enhancedand processing
time decreasedby assuminga realistic model basedon the
Geogram computation. The original log data canbe modi-
fied andusedto generatenew syntheticseismictraces.Other
applicationsareinversemodelingandthedesignof thedecon-
volution operator. Furthermore, any log data, raw data, or
Fig. B-5-Geogram processing chain
processeddatacanbepresentedon a time scalefor correla-
tion with the seismic data.
VERTICAL SEISMIC PROFILE
Vertical seismicprotiling is a techniqueof simultaneously
recording theupgoinganddowngoingwavetrains(Fig. 9-8).
This representsa major advantageover theconventionalsur-
facereflection seismictechnique,which recordsonly theup-
going waves.By recording a sufficient number(50 or more)
of fairly regularly spacedlevels in the well, theupgoing and
downgoingwavefieldscanbeseparatedby computerproccss-
ing. An analysisof theupgoing anddowngoing components
permits the detailed study of the change of the seismic
wavetrain with depth. The acousticproperties of the earth
canthenbedirectly linked to andinterpretedin termsof the
subsurfacelithology. The useof downhole sensorsreduces
the signaldistortion causedby the low-velocity shallow lay-
ers since the signal passesonly once through the surface
layers.
The total wavefield recorded at the detector in the bore-
hole consists of signals arriving from above the tool

7. CASED HOLE SEISMIC
Well
2-Way Time
i 4
0.2 0.2
0.6 0.6
0.8 0.8
1.6 1.6
Well
Fig. 9-7-Dip extrapolation: left part of section and right part of Geogram survey were put together and vice versa
Fig. 9-8-A VSP trace contains upgoing and downgoing waves. Multiples can clearly be seen on the display.
Downgoing Multiple
w Direct Wave , Time .
_-__--_-----_----------
2--------------w-
Geophone Position 1
_-__--_-----_----------
--------------w-
__-___-___-_-----------
_--__-_____-___-------
______--------__------
9-7

8. CASEDHOLE LOG INTlZPRETATlON PRINCIPLES/APPLICATIONS
(downgoing) and the signals arriving from below the tool
(upgoing). The downgoing signalsare the direct (first) ar-
rivals andthedowngoingmultiples. Theupgoingsignalscon-
sist of the direct reflections and the upgoing multiples.
Advantages of the vertical seismic profile technique
include:
l recording a real seismictrace in the borehole rather than
relying on a synthetically generatedseismogram
l measuringthe spectralcontentof the downgoing seismic
signal as a function of depth
l establishmentof a precise link betweenthe surfaceseis-
mic results and well logs, since the VSP is a high-
resolution measurement
l the recording of signalswith animproved high-frequency
content,sincetheycrossthehighly absorptivelow-velocity
layers near the surface only once
l improved seismic resolution of subtle stratigraphic fea-
tures around the well, such as faults or pinchouts
l therecordingof deepreflector signalsthatarenotreceived
at surface; this is particularly useful in structurally com-
plex areas
l an excellent record of the band-limited reflection coeffi-
cient seriesthrough deconvolution of the VSP.
‘--yi%$YI Oow”go,“gupgoing
Fig. O-O-Processing of VSP’s involves three major steps:
data editing for optimized shot quality, upgoing and down-
going wavetrain separation, and deconvolution.
1
VSP PROCESSING
TheVSP processingsequence(Fig. 9-9) usually includes
mostof the following steps:
mshotselectionby ananalystto reject the noisy, poor-
quality shots
l consistencycheck of the surface hydrophone signal
mmedian stacking of shots
l checkof coherencebetweena referencelevel andall
others
B monitoring of phaseshifts andacousticimpedanceat
all levels
l bandpassfiltering to eliminate noise and remove
aliasedfrequencies
l filtering to help eliminate tube waves
l true amplitude recovery by atime-variant function to
compensatefor spherical spreading
l velocity filtering to separatethe upgoing and down-
going componentsof the total wavefield
l autocorrelationof the downgoing waveafter filtering
for selectionof the proper deconvolution parameters
using the downgoing wave field as a deterministic
model
l predictive deconvolution to removesourcesignature
effects and to improve resolution
l time-variantfiltering to matchthesurfaceseismicdata
l corridor stacking: summing all the upgoing waves
recorded in a window following the first break.
A vertical seismicprofile display using datafrom the
Downhole SeismicArray tool is shownin Fig. 9-10. The
corridor stackfrom this presentationis shown superim-
posedon the surface seismic section in Fig. 9-11.
OFFSET VERTICAL SEISMIC PROFILE
A normal VSP survey in a vertical boreholewith horizontal
bedding gives very limited lateral information. However,
with dipping reflectors, a normal VSP survey can provide
someinformation on the updip features (Fig. 9-12).
An offset VSP (Fig. 9-13), however, offers the possibili-
ty of large lateral coverage.Lateral coverageof up to one-
half of thesourceoffsetdistancecanbeachievedin thedirec-
tion of the source.Profiling of a feature canbedoneby us-
ing afixed offsetsourcepositionsomedistancefrom thewell
and moving the geophone(s)in the well, or by having the
geophone(s)fixed and moving the source.
WALKAWAY SURVEYS
A walkaway survey provides a 2-dimensional seismicpic-
ture of the formations on either sideandbelow a well. This
9-8

9. CASEDHOLE SEISMIC
TEXAS GULF COAST EXAMPLE
ZERO PHASE PROCESSING - - NORMAL POLARITY
Fig. QlO-VSP using DSA tool
Fig. 9-I 1-VSP corridor stack from Fig. 9-10 superimposed
on surface seismic section
9-9

10. CASED HOLE LOG INTERPRETATION PRlNClPLESlAPPLlCATlONS
by aboatwith theenergysource,moving ataconstantspeed
anddirection alonga 3 km line that passescloseto the well.
Seismic shotswould be generatedwith an air gun at 30 m
intervalsalongthis line andadownholegeophonewould mo-
nitor the arrivals. Eachpassof the boatwould generate100
seismic wave traces at each geophoneposition. Accurate
navigation fixes are obtained for each shot position along
the survey line by placing navigation equipmentonboth the
rig andboat. The seismicwavepatterncreatedby this multi-
source/singlereceiver arrangementis particularly useful in
investigating complex formations.
The following illustrations show the family of borehole
seismicsurveysandthe developmentof awalkaway survey.
The first illustration (Fig. 9-14) showstheresultsof acheck-
shotsurvey. The impedancelog (sonic x density) hasbeen
correctedwith thecheckshottimestomatchtheseismictimes
and a Geogramdisplay computedfrom the data. Both logs
are superimposedon the surface seismic section to allow
correlation of the log datawith the surface seismicevents.
Figure 9-15 illustrates a ZVSP. In the ZVSP survey, the
eventsbeyondthecheckshot’sfirst arrivals arerecordedand
interpreted, providing a time and depth record of upgoing
reflected events. To obtain quality reflected data, a higher
density of receiver positions is usedthan in the check-shot
survey. A corridor stackof the VSP datais shownsuperim-
posedon the surfaceseismicsectionto allow correlation of
depth and time.
Fig. 412-VSP: stationary source, moving receiver
is achievedby using surveytechniquesdevelopedfrom Zero
Offset VSP (ZVSP) and Offset VSPs (OVSP). Walkaway
surveysare unique, however, in that they always employ a
multiple sourceand single receiver arrangement.
A typical offshore walkaway survey would becarried out
Fig. 9-l 3-Offset VSP: moving source, stationary receiver

11. CASED HOLE SEISMIC
1500 1000 500 A 500 1000 1500 m A
Fig. Q-14-An impedance log and Geogram log are shown superimposed on the surface seismic section
1500 1000 500 , 500 1000 1500 m A
Hg. y-15--c;orrluor stack of the VSP is superimposed on the surface seismic section to allOW correla-
tion of depth and time
The samedata from the previous acquisition has been The OVSP is illustrated in display (a) of Fig. 9-17. In this
processedwith 2-dimensionalmodel datato extendthe off- case,the sourceis substantially offset from the well. This
setposition of reflection points on thedipping horizons(Fig. shifts thereflection points away from the well andproduces
9-16). coverage of an extended area around the well. This is
9-11

12. CASED HOLE LOG INTERPRETATION PRINCIPLES/APPLICATIONS
1500 1000 500 P 500 1000 1500 m
Fig. 416-Same data as Fig. 9-15, processed with 2-dimensional model data to extend the offset
position of reflection points on the dipping horizons
500 1000 1500 500 1000 1500 m,
Fig. 417-The (a) display shows the Offset VSP events superimposed on the surface seismic section. The (b) display shows
the combined results of the OVSP and modeled VSP data.
particularly useful for detectionof faults andformation pin-
chouts. The OVSP eventsare shown superimposedon the
appropriatepart of the surfaceseismicdata.The display (b)
showsthe combinedresultsof the OVSP andmodeledVSP
data.
Note that there is no reflector coveragebelow total depth
of the well. A particular attraction of walkaway surveys is
thatthey provide bettercontinuity andmore completecover-
age, especially below the bottom of the well.
9-12

13. CASED HOLE SEISMIC
The use of the Downhole Seismic Array tool to acquire
thedatacansignificantly reducethe acquisitiontime andim-
provetheconsistencyof theseismicsignalfrom levelto level
in the well. Figure 9-18 showsa schematicof theoperation-
al setupfor a walkaway VSP job on land. Two vibrators,
in radiocontactwith theloggingunit, wereusedastheenergy
source. The shotpoints were located at 75 m intervals and
were movedout 3 km from the well in eachof the four or-
thogonaldirections. At eachshotposition, eight levelswere
recorded with the DSA tool.
The east-westwalkaway VSP resultsareshownon theleft
in Fig. 9-19. Faults, nearby andintersectingthe well, were
determinedfrom this seismicsection.The north-southwalka-
way section is shown on the right.
DSA TOOL FOR VSP ACQUISITION
The Downhole SeismicArray tool, with its eight single-axis
geophonearray configuration, provides severaladvantages
over single level tools for VSP acquisition in casedholes:
l time savings.Oneobviousadvantageis the savingsin time
sinceeight levels are recordedat eachfiring of the ener-
gy source.
Fig. 9-18-Schematic of operational setup for a walkaway
VSP job on land
West Offset (ft) East North Offset (ft) South
1000 750 500 250 We”
250 500 250 250 500._ .-_- -_, ----
Fig. 9-19-The east-west walkaway VSP results are shown on the left, and the north-south results on the right.

14. CASED HOLE LOG INTERPRETATION PRINCIPLES/APPLICATIONS
reduction in tube-wave effects. The DSA tool provides
someadvantagesin areaswhere strong tube-wavesmay
contaminate the waveform data. The strong clamping
force, the small cross-sectionalarea, andthe streamlined
shapeof theshuttlehelpto reducetheeffectof tube-waves
on the seismic data.
reduction of navigation errors in walkaway VSP opera-
tions. In offshoremultioffsetoperations,theseismicsource
is movedby aboat ata constantspeed.The DSA tool ac-
quires signalsfrom eight levels while the boat is making
asinglepath,reducingnavigationalerrorsoversinglelevel
tools.
reduction of effectsof sourcesignaturechanges.The ef-
fects of source signature changesdue to suchthings as
changesin gunpressure,gun pit alteration, or tide levels,
arereducedbecausetheeightshuttlesreceivesignalswhich
originate from the sameshot.
accuratetransit times betweenlevels. Sincethe DSA ge-
ophonesareequally spacedon acable, potential distance
errors are eliminated.
PRIMARY USES OF THE VSP SURVEY
Theenhancedresolutionof theVSP makesit possibleto veri-
fy or deny the presenceof reflections that are indistinct or
doubtful on seismicsectionsnearthe well. The VSP is par-
ticularly well suitedto determinetheconditions existing be-
low the well’s total depth. Overpressuredzones,gassands,
and deep reflectors can be verified or recognized.
Since the downgoing wavefield is recorded, multiple
reflections canbe identified andremoved. The samedown-
going waveinformation canbeusedto reprocesssurfaceseis-
mic profiles traversing the vicinity of the well.
Perhapsthe most common usefor the VSP is a link be-
tween reflections observedon a surfaceseismicprofile and
specific petrophysical properties measuredin the borehole.
The correlation role of the VSP is important for reservoir
developmentapplications.
Finally, by positioningthe seismicsourceasignificantdis-
tancefrom thewell, structuralandstratigraphicfeaturesfrom
hundredsto thousandsof feet from the well canbedelineat-
ed and verified againstthe surface seismic.
PROXIMITY SURVEY INTERPRETATION
Proximity surveyshavebeenusedfor many yearsto define
the shapeof salt domes.Now a program is available to en-
tirely mechanizethe interpretation process.After awell has
beendrilled on the flank of a salt dome, a downhole sensor
is lowered into the hole and anchoredat numerousdepths.
An energy sourceis positioned directly over the top of the
structure. A travel time is measuredfrom the sourceto each
of thedownhole sensorlocations. From prior knowledgeof
salt velocities, velocities of formations encounteredin the
well, and at least one salt tie point, distancesfrom salt to
sensorpositions canbecalculated, andthe shapeof the salt
flank determined.
Given the layer velocities andthe sourceandreceiver lo-
cations, the transit times to eachreceiver are measuredby
ray tracing. Next, an initial model is generatedon the com-
putercontainingtheknown sourceandreceiverlocationsand
the layer velocities. The program then calculates, for each
source-receiverpair, all the possibletravel pathsthe acous-
tic energy could havetaken with the total time equalto the
measuredtime. A line through all therefraction points con-
tainsall thepossiblelocationsfor the salt interfacecalculat-
ed from one receiver. The resulting oval is called an apla-
natic surface(Gardner, 1949).Thecomputationcanbeper-
formed for combinations of sourceand receiver, and will
result in a seriesof aplanaticsurfaces.The bestfitting line,
tangentto all the ovals, is the final solution for the location
of the interface.
The techniqueis illustrated in a well in the Gulf of Mexi-
co. Using accurate source-receiver travel times and the
source-receiverpositions, the initial model was generated.
The saltwasaknown distancefrom onereceiver,which ena-
bled theuseof the refraction oval techniquefor the determi-
nation of the salt top. The formation velocities adjacentto
the salt were determinedfrom a vertical check shotandthe
aplanatic surfaceswere generated,as shown in Fig. 9-20.
The salt flank was interpreted asthe common tangentline
illustrated in Fig. 9-21.
The mechanizedproximity interpretationgivesresultscon-
sistent with the interpretation madewith wavefront charts,
but is much less time consuming.
REFERENCES
Anstey, N.A.: SeismicInterpretation: l7zePhysicalAspects,IHRDC, Boston
(1977).
Ausbum, B.E.: “Well Log Editing in Supportof DetailedSeismicStudies,”
Trans., 1977 SPWLA Annual Logging Symposium.
Gardner,L.W.: “SeismographDeterminationof SaltDomeBoundaryDeep
on the Dome Flank,” Geophysics(1949) 46, 268-287.
Goetz, J.F., Dupal, L., and Bowler, J.: An Investigation Into Discrepan-
ciesBetweenSonicLog and SeismicCheck-ShotVelocities, Schlumberger
Technical Services(1977).
Landgren,K.M. andDeri, C.P.:A MechanizedProcessfor Proximity Suwey
Interpretation, SchlumbergerOffshore Services(1986).
McCollum, B. and LaRue, W.W.: “Utilization of Existing Wells in Seis-
mograph Work,” Bull., AAPG (1931) 15, 1409-1417.
Miller, D.E.: “Arrival Times and Envelopes- Inverse Modeling for Sub-
surfaceSeismics,” 53rd Annual International SEG Meeting, Sept., 1983,
Las Vegas, ExpandedAbstracts, 449-467.
Mons, F. andBabour,K.: Vertical SeismicProfiling, SAID QuatriemeCol-
loque Annuel de Diagraphies (Oct., 1981).
Musgrave,A.W., Wooley, WC, andGray, H.: “Outlining of SaltMass-
es by Refraction Methods,” Geophysics(1960) 25, 141-167.
9-14

15. CASED HOLE SEISMIC
m
v = 14,900 ftlsec
v =1Ez:
Gv = 7700
Lv=E
L-
ftlsec
/j - g600ftlsec
Fig. O-PO-Proximity Survey interpretation: refraction ovals Fig. 9-21-Proximity Survey interpretation: final solution
Robinson,E.A. andTreitel, S.: GeophysicalSignalAnalysis, Prentice-Hall
Inc., New Jersey(1980).
Schlmberger Middle East WellEvaluation Review “Borehole Seismics“,
Schlumberger Technical Services, Paris (Autumn 1988).
Schlmberger WestAjica WellEvaluation Conference,SchlumbergerTech-
nical Services, Paris (1983).
9-15