Geophysical exploration

Mr. Selemani Silingi
EARTH SCIENCE INSTITUTE OF SHINYANGA (ESIS)
MODULE NAME:GEOPHYSICAL EXPLORATION
MODULE CODE: AGT-05103
CREDIT NUMBER: 5
COURSE OF STUDY: EXPLORATION AND MINING
GEOLOGY
MODULE INSTRUCTOR: S. SILINGI

MODULE CONTENTS
1. MODULE OUTLINE
 To describe ground magnetic method
 Describe electromagnetic method
 Describe gravity method
 Describe seismic method
 Radiometric method
2. MODULE OBJECTIVES
At the end of this module student must be able to
describe the following;
Ground magnetic survey, electromagnetic, gravity survey
and seismic survey.
Must be able to collect the data in the field using the
methods above.

3. ASSESSMENT
Continuous Assessment-50%
Two (2)test @ 15marks
Two (2) assignment @10
End Semester examination-50%

4. REFERENCES
1. Parasnis, D.S. (1996) principles of applied geophysics,
5th Edition.Chapman and Hall.
2. Telford, W.M, Geldart, L.P, Sheriff, R.E (1990). Applied
Geophysics, 2nd Edition, Cambridge University press.
3. Lillie, R. (1999). Whole Earth geophysics. Prentice Hall
361p.
4. Lowrie, w. (1997), Fundamentals of geophysics.
Cambridge University press. 354p.
5. Surface Geophysical Methods Volume 1, Fall 2004

CONCEPT OF GEOPHYSICAL EXPLORATION
Exploration geophysics
Definition
Is an applied branch of geophysics, which uses physical
methods at the surface of the Earth to measure the
physical properties of the subsurface, along with the
anomalies in those properties.
Geophysical methods include;
Seismic,
Gravitational,
Magnetic,
Electrical
Electromagnetic

CONCEPT OF GEOPHYSICAL EXPL CONT……
These methods identify resources without the need for
sampling.
Usually undertaken with minimal surface disturbance.
The survey used in these methods is called Geophysical
survey.
Geophysical surveys can be conducted either from ;
 The air,
 On the ground or
 Down drill holes.

CONCEPT OF GEOPHYSICAL EXPL CONT….
Airborne geophysical surveys
May comprise of magnetic, radiometric, gravity or
electromagnetic methods.
These surveys provide general geological information for
an area and are often used in the initial stages of
exploration.
These surveys are typically undertaken using low flying
helicopters or light aircraft which fly in a grid pattern.
In airborne survey, instruments may be either mounted on
the aircraft or towed underneath.
The aircraft may fly between 25 and 60m above the
ground, with flight lines spaced between 25 and 200m
apart.

CONCEPT OF GEOPHYSICAL EXPL CONT….
DOWN HOLE SURVEYS
These geophysical surveys involve putting
geophysical equipment down exploration drill holes to
gather magnetic, radiometric or electrical information
from the rocks adjacent to the hole.
Also used to determine the exact path of the drill hole.
Occasionally tools with a small radiometric source
may be used and a detailed risk assessment is required
to ensure the tool is not lost down hole.

magnetic survey,
Seismic surveys,
Magnetic surveys,
Radiometric surveys,
Ground based surveys
There are a number of types of geophysical surveys
which are undertaken on the ground.
These are include;
Gravity surveys,
Induced Polarization
(IP) surveys,
Electromagnetic
(EM) surveys ,
• CONCEPT OF GEOPHYSICAL EXPL CONT….
Note method used for data collection in above survey is called
geophysical methods.

SEISMIC METHOD
The common geophysical method used in Exploration
I. SEISMIC METHOD
Definition
Seismic method refer to method which use seismic
velocities to measure the elastic properties of Earth’s
material.
These techniques provide detailed information about
subsurface layering and subsurface mechanical properties
using seismic waves.
Seismic waves propagate through a rock body at a velocity
which is governed by elastic properties (stress & strain,
Young’s modulus) and density of the geological formation.

SEISMIC METHOD CONT..
A seismic source-such as sledgehammer is used to
generate seismic waves, sensed by receivers deployed
along a present geometry (called receiver array), and
then recorded by a digital device called seismograph.
Diagram showing seismograph and geophone array

Types of seismic waves
There are two groups of seismic waves such as;
i. Body waves and
ii. Surface waves.
• Body waves- Are the waves that can propagate
through the internal volume of an elastic solid
materials.
• Body wave is divided into two types which are;
Compressional waves (Longitudinal, primary or P-
wave)
Shear waves (Transverse, secondary or S-waves).

SEISMIC METHOD CONT…
P-wave travel with a velocity that depends on the
elastic properties of the rock through which they travel.
P-wave has the highest velocity than all seismic waves
and thus will reach all the seismograph first.
Particle motion associated with the passage of a
compressional wave involves oscillation, about a fixed
point, in the direction of wave propagation.
S-waves travel through material by shearing it or
changing its shape in the direction perpendicular to the
direction of travel.
S-waves travel slower than the p-wave as reach at the
seismograph after p-waves.

SEISMIC METHOD CONT….
 Surface waves are the waves which propagate along
the boundary of the solid material on the earth.
 Surface behave like S-waves in that they cause up and
down and side to side movement as they pass, but they
travel slower than S-waves and do not travel through the
body of the Earth.
 These waves are of two types which are;
– Love waves
– Rayleigh waves

SEISMIC METHOD CONT…….
 Rayleigh waves propagate along a free surface, or along
the boundary between two dissimilar solid media, the
associated particle motions being elliptical in a plane
perpendicular to the surface and containing the direction
of propagation surface waves.
 Love waves are polarized shear waves with a particle
motion parallel to the free surface and perpendicular to
the direction of wave propagation.

SEISMIC METHOD CONT…
 Body wave and Surface wave with wavelength.

SEISMIC METHOD CONT……..
 SEISMIC WAVE VELOCITY
 Additional factors which influence compressional and
shear wave velocities are;
Lithology
Extent of fractures
Temperature
Fluid content
Saturation and
Fluid pressure in the subsurface

SEISMIC METHOD CONT……..
• Typical rock velocity
Type of formation P-wave velocity m/s S-wave velocity m/s Density g/cc
Scree, vegetal soil 300-700 100-300 1.7-2.4
Dry sands 400-1200 100-500 1.5-1.7
Wet sands 1500-2000 400-600 1.9-2.1
Saturated shales and clays 1100-2500 200-800 2.0-2.4
Marls 2000-3000 750-1500 2.1-2.6
Porous and saturated sst 2000-3500 800-1800 2.1-2.4
Limestone 3500-6000 2000-3300 2.4-2.7
Chalk 2300-2600 1100-1300 1.8-3.1
Salt 4500-5500 2500-3100 2.1-2.3
Dolomite 3500-6500 1900-3600 2.5-2.9
Granite 4500-6000 2500-3300 2.5-2.7

.
Type of formation P-wave velocity m/s S-wave velocity m/s Density g/cc
Basalt 5000-6000 2800-3400 2.7-3.1
Gneiss 4400-5200 2700-3200 2.7-3.1
Coal 2200-2700 1000-1400 1.3-1.8
Water 1450-1500 - 1.0
Ice 3400-3800 1700-1900 0.9
Oil 1200- 1250 - 0.6-0.9
Note: The Acoustic impendance is defined as the product
of the density and P-wave velocity within a rock unit.
When elastic wave encounters an acoustic impendance
boundary, a portion of the wave energy is Reflected off of
the boundary and a portion is Refracted into the second
medium, according to Snell’s law.

At the critical angle for each interface (energy refracted 90
degrees), the seismic wave will travel along the interface
with a velocity of the underlying layer.
Since P-waves are the fastest portion of the seismic wave,
they represent the first arriving energy at each geophone
(either direct or refracted).

 Seismic Reflection method
 Seismic reflection is the primary geophysical method
used in oil and gas exploration and operated on density
and elastic module of subsurface materiatials.
 The seismic reflection method usually gives better
resolution (i.e., makes it possible to see smaller features)
than other methods, with the exception of measurements
made in close proximity, as with borehole logs.
 Seismic Refraction method
 Seismic refraction is a method to determine the P-wave
velocity structure of the subsurface.

 The refraction method has been used in mineral
investigations to map low-velocity alluvial deposits such
as those that may contain gold, tin or sand and gravel.
 Application in geo-environmental work include
studying the structure, thickness, and hydrology of
tailings and extent of Acid mine drainage around mineral
deposit.
 Note;
 Reflection seismic methods provide fine structural detail
and refraction methods provide precise estimates of
depth to lithologies of different Acoustic impendance.

Seismic data and their acquisition, processing, and
interpretation.
Digital seismic data
 Seismic data are physical observations, measurements,
or estimates about seismic sources, seismic waves, and
their propagating media.
 The form of seismic data varies, and can include analog
graphs, digital time series, maps, text, or even ideas in
some cases.
 We focus on the analysis of data on body waves, mostly
P-waves, in their transmission, reflection, diffraction,
refraction, and turning processes.

 Relationship between data acquisition, processing, and
interpretation.

 Data acquisition
Seismic data acquisition in the energy industry
employs a variety of acquisition geometries.
Example of the typical onshore acquisition geometry
using dynamite or Vibroseis technology as sources
and geophones as receivers.

 Panel (a) shows a split spread, using a shot located in
the middle and many receivers spread around it.
 (b) shows an end-on spread, with a shot located at one
end and all receivers located on one side of the shot.
 Quality control, or QC
 This involves checking the survey geometry, data format,
and consistency between different components of the
dataset, and assuring that the quality and quantity of the
dataset are satisfactory for the study objectives.
 The data QC process is typically part of the pre-
processing.

 Data processing
 To identify and enhance the desired signal by suppress
various kinds of noise in the data.
 The goal of seismic data processing is to help
interpretation, the process of deciphering the useful
information contained in the data.
 Data interpretations and Seismic modeling
 This is based on processed data and seismic imaging
conducted to produce various forms of imagery for the
interpretation process.
 Seismic modeling done after data interpretation and
generate predictions to compare with the real measurements,
and thus verify the interpretation.

 APPLICATION OF SEISMIC METHOD
i. Its employed for hydrocarbon exploration
By providing a high resolution map of acoustic
impedance and petroleum geologists and
geophysicists interpret potential petroleum reservoirs
easily.
ii. Its used in mineral exploration
 By Identifying massive sulphide bodies since massive
sulphide bodies have higher seismic velocities and
densities than their host rocks.
iii. Used to predicts the depth, thickness of geologic strata,
and structural

MAGNETIC METHOD
 MAGNETIC METHOD
 Definition
 Magnetic method is the method used to measure the
variations of the Earth’s magnetic field due to the
presence of magnetic minerals. or
 It is used to investigate subsurface geology on the basis
of anomalies in the Earth’s magnetic field resulting
from the magnetic properties of the underlying rocks.
 Earth's magnetic field refer to magnetic field that
extends from the Earth's interior to where it meets the
solar wind, a stream of charged particles emanating
from the Sun.

 Thus why the Earth’s
magnetic field varies
with time (it is not
constant).
 It believed that the
earth’s magnetic field is
originates from the liquid
outer core of the Earth
containing high
concentration of iron.
 As the Earth rotates, the outer layers of the ionosphere
interact with the solar wind to cause minor fluctuations
in the magnetic field.
• MAGNETIC METHOD CONT…….

MAGNETIC METHOD CONT…….
 If the rocks are magnetic (have high susceptibility) they
become magnetized, and their field adds to that of the
earth. Thus the total magnetic field is stronger over
magnetic rocks.
 Magnetic fields are measured in Nanoteslas (nT), which
used to be called gammas.
 Magnetic rocks contain various combinations of
induced and remanent magnetization that perturb the
Earth's primary field.
 The magnitudes of both induced and remanent
magnetization depend on the quantity, composition,
and size of magnetic-mineral grains.

 In order for something to be magnetic, its dipoles must
be aligned with each other, so that they face the same
direction.
 The direction they face create a North end, while the
opposite end creates a South end.
 Some substances, known as ferromagnetic substances,
have permanently aligned dipoles.
 A magnetic high anomaly is where the measured field
strength is higher than the value predicted by the global
model, and a magnetic low is where the measured field
strength is lower than the value predicted by the global
model.

 Anomalies in the earth's magnetic field are caused by
induced or remanent magnetism.
 This anomaly created when the Earth’s magnetic field is
disturbed by an object that can be magnetized.
 To measure anomalies, magnetometers need a sensitivity of
10 nT or less.
 Induced magnetic anomalies are the result of secondary
magnetization induced in a ferrous body by the earth’s
magnetic field.

 Common causes of magnetic anomalies include dykes,
faults and lava flows.
 Where the rocks have high magnetic susceptibility, the
local magnetic field will be strong; where they have low
magnetic susceptibility, it will be weaker.
 Magnetic gradient anomalies generally give a better
definition of shallow buried features such as buried
tanks and drums, but are less useful for investigating
large geological features.
 Sedimentary rocks generally have a very small
magnetic susceptibility compared with igneous or
metamorphic rocks, which tend to have a much higher
magnetite content.

• INSTRUMENTATION
 A magnetometer is a more complex instrument which
measures both the orientation and strength of a magnetic
field.
 Magnetometer surveys measure small, localised variations
in the Earth's magnetic field.
 Magnetometers are highly accurate instruments, allowing
the local magnetic field to be measured to accuracies of
0.002%.
 There are several types of instruments on the market but
the common ones used for commercial applications are the
Proton precession, Fluxgate, Caesium vapour and
Gradiometer magnetometer systems.

 Planning of magnetic survey
 Once you have considered all the factors as to the type
of magnetometer survey required, then you are ready to
design and lay out a grid to cover the area of interest
 A survey grid usually consists of a base line and one or
several tie lines.
 The base line serves as a zero reference line for the grid,
and the tie lines serve to correct the skewness of the
survey lines.
 From the base line are drawn survey lines
perpendicular to the base line.

 The following figure below illustrates a typical survey
grid, with base lines at 0 and 40 and survey lines
every two metres.

 DATA ACQUISITION
 Ground magnetic measurements are usually made with
portable instruments at regular intervals along more or
less straight and parallel lines which cover the survey
area.
 Often the interval between measurement locations
(stations) along the lines is less than the spacing
between lines.
 To obtain a representative reading, the sensor should be
operated well above the ground.
 In rocky terrain where the rocks have some percentage
of magnetite, sensor heights of up to 4 m have been
used to remove near-surface effects.

 All field readings should be taken twice and the two
readings should differ by no more than 1 nT.
 At each station the location, time and reading must be
recorded, as well as any relevant topographic or
geological information and details of any visible or
suspected magnetic sources.
 Modern instruments can be linked to a DGPS so that
map coordinates are automatically recorded against the
magnetic reading.
 The survey data collected should be corrected at the end
of the survey day or the end of the grid.{see Data
processing in detail}

 DATA PROCESSING
 To make accurate magnetic anomaly maps, temporal
changes in the earth’s field during the period of the
survey must be considered.
 Normal changes during a day, sometimes called
diurnal drift, are a few tens of nT but changes of
hundreds or thousands of nT may occur over a few
hours during magnetic storms.
 Culture noises like buried metal, railroads, pipelines,
bridges and iron fences result error in magnetic
data.
 There are three ways in which you can remove
errors from magnetic data :

i. Use a base station magnetometer to record all the
changes in time and then use this data to remove the
change from the readings in the field magnetometer.
This is the most accurate way of doing it, but also it is
more expensive, as two complete instruments are
required.
 If time is accurately recorded at both base site and
field location, the field data can be corrected by
subtraction of the variations at the base site.

ii. Use a tie-point method while doing the total field
survey.
 This assumes that the field is changing slowly and
evenly between the first time you measured the value
at a station and the next time you check-in to that
station again.
 This method is not as accurate as using a base-
station, but if the field is not changing rapidly, it is
quite adequate to locate an anomaly.
 Generally the data correction is done automatically
while the survey is carried out in the tie-line mode.

iii. Perform a vertical gradient survey.
 Since you are measuring the rate of change between
two sensors, any changes in the background field
will apply to both sensors and you will not see any of
these noise effects.
 This technique is quite effective for near-surface
anomalies.
 Gradiometers measure the magnetic field gradient
rather than total field strength, which allows the
removal of background noise.

 After all corrections have been made, magnetic survey data
are usually displayed as individual profiles (Figure 1) or as
contour maps (Figure 2). Below is a magnetic reading
profile across a dyke

 Magnetic Survey contour map to locate pits containing
buried metallic Containers. Figure 2.

 DATA INTERPRETATION
 Data are usually displayed in the form of a contour map
of the magnetic field, but interpretation is often made
on profiles.
 From these maps and profiles geoscientists can locate
magnetic bodies (even if they are not outcropping at the
surface), interpret the nature of geological boundaries
at depth, find faults etc.
 Like all contoured maps, when the lines are close
together they represent a steep gradient or change in
values.
 When lines are widely spaced they represent shallow
gradient or slow change in value.

 A modern technique is to plot the magnetic data as a
color image (red =high, blue=low and all the shades in
between representing the values in between).
 When interpreting the aeromagnetic image it is useful
to know that magnetite is found in greater
concentrations in igneous and metamorphic rocks.
 Magnetite can also be weathered or leached from rocks
and re-deposited in other locations, such as faults.
 In a geothermal environment, this is a very useful
feature as it may indicate the presence of faults, target
for drilling.

 Example of magnetic map intensity

 In figure above shows a total field magnetic map from
an area.
 The magnetic survey outlined three distinct zones
where the magnetic field ranges from a low of 150 nT
(blue areas) to a high of 500 nT (red areas).
 The central part of the grid (blue area) has the lowest
magnetic expression, whereas the areas to the grid
north and grid south (yellows through reds) are more
magnetic.
 Generally the magnetic maps are interpreted in terms
of geology therefore on correlating with magnetic map
need to have or know rocks present at the area (or
collect samples).

 Magnetic maps are interpreted in terms of geology.
 As low-susceptibility rocks such as limestones show as
areas of low and relatively uniform magnetic fields,
while mafic and ultramafic rocks show as areas of
higher and more variable magnetic fields.
 In geophysical displays, deep blues denote the lowest
values, while reds and purples identify the highest
readings.

 APPLICATION OF MAGNETIC METHOD
 There are two primary applications for magnetic
measurements:
i. Locating and mapping buried ferrous metals, and
Magnetic measurements can be used for locating and
mapping buried ferrous metals (e.g. waste, drums or
underground structures and utilities).
ii. Mapping geologic structures-Magnetic measurements
can be used for geologic mapping by responding to
the magnetic susceptibility of soil and rock.

GRAVITY METHOD
 GRAVITY METHOD
 Definition
 Gravity Survey refer to the survey used to measures the
change of rock density by looking at changes in
gravity.
 Like all matters, the earth generates gravity field that
can be measured by instrumentation called gravimeter.
 The gravimeters are used to precisely measure
variations in the gravity field at different points of the
earth.
 The strength of the gravitational field is directly
proportional to the density of subsurface materials.
 The typical units of gravity field is milligas or gravity

GRAVITY METHOD CONT…….
 Gravity measurements can be obtained either from
airborne (remote) or ground surveys.
 The most sensitive surveys are currently achieved from
the ground.
 The force of gravity is not the same all over the world
(it varies from point to point on the Earth).
 Things like Mountains, Ocean trenches, tidal
movements, even large buildings, Structures, and
Composition of elements within the Earth’s crust all
cause micro-variation in gravity all over the world.

 INSTRUMENTATION
 A gravity meter or gravimeter measures the variations
in the earth's gravitational field.
 Gravity instruments require careful levelling before a
reading is taken.
 This may have to be done manually or it may be
performed by the instrument itself.
 The instrument must be placed on solid ground (or a
specially designed plate) so that it does not move or
sink.
 The common gravimeters on the market are the
Worden gravimeter, the Scintrex and the La Coste
Romberg gravimeter.

 The Worden gravimeter is an entirely mechanical and
optical relying only on an AA battery for illuminating
the crosshairs.
 The Scintrex Autograv is semi-automated, but although
a bit more expensive, it has been shown to have a
higher stability and experience less tares (a sudden
jump in a gravity reading) over long periods of time.
 The Lacoste and Romberg model gravity meter has an
advertised repeatability of 3 microgals (980,000,000
microgals is the Earth's gravitational field) and is one of
the preferred instruments for conducting gravity
surveys in industry.

 DATA ACQUISITION
 Gravity data acquisition is a relatively simple task that
can be performed by one person or two when
determining location (latitude, longitude and elevation)
of the gravity stations.
 Surveys are conducted by taking gravity readings at
regular intervals along a traverse that crosses the
expected location of the target.
 Its expected size will determine the distance between
readings (station spacing), with larger station
separations for large target and small separations for
small ones.

 Obtaining a gravity reading, a horizontal position and
the elevation of the gravity station must be obtained.
 Therefore readings are taken by placing the instrument
on the ground and levelling it and this may be
automatic with some instruments, as it is with the
Scintrex.
 The observed gravity readings obtained from the gravity
survey reflect the gravitational field due to all masses in
the earth and the effect of the earth’s rotation.
 Thus the useful gravity values in detailed surveys is to
determine the Earth tide effect as their gravitational
effects may be greater than the gravity field variations
due to the anomalous features being sought.

 DATA PROCESSING
 The last task of most fieldwork is to determine the
topographic changes and the effects of buildings
surrounding a gravity station.
 Both of these effects will be used later in processing the
gravity data.
 There are a number of techniques to determine the
elevation changes and these usually involve a
combination of recording elevation changes in the field
and computer computations using digital elevation
models (DEM).

 The most common technique is by Hammer where one
records an elevation change in quadrants at set
distances (commonly from 0 to 1000 meters) from the
gravity station.
 To interpret gravity data, one must remove all known
gravitational effects not related to the subsurface
density changes.
 Each reading has to be corrected for elevation, the
influence of tides, latitude and, if significant local
topography exists, a topographic correction.

 Gravity data correction
 Because the “weight” of a mass varies with the square of the
distance to the center of the earth, small variations in
elevation of the survey station can greatly affect the
measured value of gravity.
 As a result, it is necessary to correct for changes in location
and elevation of the survey stations.
i. Latitude correction
Because the earth rotates, it bulges at the equator,
and flattens at the poles.
Thus gravity is stronger at the poles than at the
equator and increases with increasing in Latitude.

Correction is applied ONLY for relative movement in
the N-S direction.
Correction is added as we move toward the equator
and subtracted as the survey station moves
northward.
Latitude correction accounts for rotation and
elliptical shape of the earth.
ii. Free-air correction.
The free-air correction accounts for gravity
variations caused by elevation differences in the
observation locations.
Accounts for the 1/r2 decrease in gravity with
distance from the centre of the Earth.

If a station is above the reference datum, the
correction is added to the reading; if the station is
below the reference datum, then the correction is
subtracted from the reading.
Free air gravity anomaly is given; gfa = gobs - gn +
0.3086h ; Where h is the elevation (in meters)
iii. Bouguer correction
The difference between observed gravity (gobs) and
theoretical gravity at any point on the Earth's surface
after reducing the gravity readings to the geoidal
surface is known as the Bouguer gravity anomaly or
Bouguer gravity.

 This correction accounts for the attraction of material
lying between the station and the reference datum.
 The Bouguer correction is {0.04192 x (density of the
intervening material)} per vertical meter.
 If the density of the intervening material is the average
rock density of 2.67 grams per cubic centimeter (g/cc)
then the Bouguer correction is 0.112 mgal/meter.
 It is applied in the opposite sense to the free air
correction and correction is Subtracted for stations
above the datum and is added for stations below the
datum.

iv. Terrain correction
If the gravity reading is taken on top of a hill, then
there is a deficit of mass on either side of the hill,
compared to a horizontal ground surface.
The figure below need for correction for topography.

 The terrain correction is positive regardless of whether
the local topography consists of a mountain or a valley.
 Terrain corrections are complex, and, in pre-computer
days, involved enormous amounts of hand calculation.
 DATA ANALYSIS AND INTERPRETATION
 The final gravity data are usually plotted and contoured
in the same manner as magnetic data.
 The grid of gravity values, contour maps or the gravity
profiles can be used to determine the lateral location of
any gravity variations and thus quantify the nature
(depth, geometry, density) of the subsurface feature.

 The map generated from gravity data and after
correction is called gravity anomaly map
 A gravity anomaly map looks at the difference between
the value of gravity measured at a particular place and
the predicted value for that place.
 Gravity anomalies are computed by subtracting a
regional field from the measured field, which result in
gravitational anomalies that correlate with source body
density variations.
 Positive gravity anomalies are associated with shallow
high density bodies, whereas gravity lows are associated
with shallow low density bodies.

 Thus, deposits of high-density chromite, hematite, and
barite yield gravity highs, whereas deposits of low-
density halite, weathered kimberlite, and diatomaceous
earth yield gravity lows.
 Uplifts usually bring denser rocks nearer the surface
and thereby create positive gravity anomalies {denser
rock like-Basalt-Granite-Sandstone}.
 Faults that displace rocks of different densities also can
cause gravity anomalies.
 Salt domes generally produce negative anomalies
because salt is less dense than the surrounding rocks.

 Example of positive and Negative anomaly for anticline
(oil) and salt dome

 Contoured data with latitude, free air and Bouguer corrections.
 The figure below shows a contour map of gravity data.
 The data set has had Latitude, Free air and Bouguer
corrections.

 Often gravity results such as these will show a gradual
trend across the area, which will distort local
anomalies.
 The gradual trend in gravity values can arise from deep
seated geological variations, and can be removed from
the data.
 The high gravity at the west end and the low at the east
end both disappear when a linear regional trend is
removed.
 Normally such trend removal is carried out by digital
processing.

 Gravity data of with linear regional trend removed
from data.

 Sulphide bodies are usually more dense than the host
rocks, and thus yield local high-gravity anomalies.
 Graphite, on the other hand, is less dense than
sulphides, and close in density to typical host rocks.
 If the survey area is underlain by rocks of different
densities, then gravity can be used to map the
distribution of each rock type.
 Where positive anomalies indicate rocks with high
density than crustal average.
 Negative anomalies indicate rocks with low density
than the crustal average.

 APPLICATION OF GRAVITY METHOD
 Typical applications for the gravity profiling include:
i. Mapping karst topography or other subsurface
cavities (natural or man-made)
ii. Map regional geologic structures
iii. Map basement topography and sediment thickness
iv. Exploration for geothermal energy including the
location of heat sources.

ELECTROMAGNETIC METHOD
 ELECTROMAGNETIC METHOD
 Definition
 Electromagnetic method provide a means to measure
subsurface electrical conductivity and to identify
subsurface metal objects.
 Electrical conductivity is a function of soil and rock
type, porosity and permeability, as well as the
composition of fluids that fill the pore spaces.
 Electrical conductivity values are given in units of
milliSiemens/meter (mS/m).
 The higher the conductivity, the more current will flow
in the earth for a given electrical field strength.

 The higher the resistivity, the less current will flow for a
given electrical field strength.
 At low frequencies, conductivity and resistivity are
inversely related:
 SYSTEM AND OPERATION OF EM
 The electromagnetic method consists of Transmitter coil
and Receiver coil.
 An AC electric current is applied to a transmitter coil
 This generates a primary electromagnetic (EM) field in
the coil or a large loop on surface.

 This induces small electric currents in the ground,
generating a secondary magnetic field that can be
picked up by a receiver coil. {Please see the illustration
below}.

 The receiver measures two quantities, the in-phase
component and the quadrature component of the secondary
field, expressed as a percentage of the primary field at the
receiver.
 Anomalies from good conductors have large in-phase and
small quadrature components, while weaker conductors
have low in-phase and high quadrature components.
 Normally the primary field is much stronger than the
secondary field.
 In order to detect the secondary field, a small part of the
primary field is sent from the transmitter via cable to
the receiver, and is used to cancel the primary field at
the receiver, leaving only the secondary field to be
detected.

 Frequency domain electromagnetic methods (FDE)
Measure the electrical conductivity of soil and rock
by measuring the magnitude and phase of an
induced electromagnetic current (ASTM D6639-01).
FDE Provides measurements with depths ranging
from a few feet to 200 feet.
The three common frequency domain
electromagnetic instruments are EM31, EM34, and
EM38 as manufactured by Geonics, Ltd.

 Time domain electromagnetic (TDEM) methods
Measure the electrical conductivity of soil and rock
by inducing pulsating currents in the ground with a
transmitter coil and monitoring the decay of the
induced current over time with a separate receiver
coil (ASTMD6820-02).
The EM47 and EM57 systems manufactured by
Geonics, Ltd. are common TDEM systems.
TDEM penetrate at a depth range of approximately
20 to 3,000 feet.

 Very Low Frequency (VLF) measurements
These measurements are made by measuring the
distortions of a VLF wave from a distant transmitter.
Distortions of the VLF wave occur due to a local
increase in electrical conductivity usually found
within fractures.
The increase in electrical conductivity is a function
of the conductive material, such as water, clay or
minerals within the fracture.
NOTE: Measurements are easily and rapidly made
can provide relatively deep measurements (hundred of
feet).

 EM data is typically collected as point readings of
ground conductivity or in-phase taken at regular
intervals along a survey grid that has been set out over
the site area.
 The spacing of the grid-lines and reading stations is
dependent upon the target size.
 Generally smaller targets require closer survey lines
and denser spaced readings.

 PROCESSING & INTERPRETATION
The site data is recorded on a digital data logger for
later downloading to a PC for post-survey processing
and interpretation.
The most commonly used interpretation procedure is
contouring, carried out with specialist interactive
software to produce contour plans.
The contoured data is analyzed in detail by experts to
identify anomalous features relative to the general
background.
Once identified, the anomalies are correlated with
local ground conditions.

APPLICATION OF ELECTROMAGNETIC METHOD
1. Mineral exploration - metallic elements are found in
highly conductive massive sulfide ore bodies.
2. Groundwater investigations - groundwater contaminants
such as salts and acids significantly increase the
groundwater conductivity.
3. Stratigraphy mapping - rock types may have different
conductivities.
4. Geothermal energy - geothermal alteration due to hot
water increases the conductivity of the host rock.
5. Environmental - locate hazards such as drums and tanks.
6. Locating abandoned mineshafts, crown holes & subsidence
features

 RADIOMETRIC METHOD
 Definition
 The radiometric, or gamma-ray spectrometric method
is a geophysical process used to estimate concentrations
of the radioelements potassium, uranium and thorium
by measuring the gamma-rays which the radioactive
isotopes of these elements emit during radioactive
decay.
 Airborne gamma-ray spectrometric surveys estimate
the concentrations of the radioelements at the Earth's
surface by measuring the gamma radiation above the
ground from low-flying aircraft or helicopters.

 ISOTOPES
 Atoms having the same atomic number but different
mass numbers are called isotopes.
 Elements having atomic number (Z >83) are very
unstable.
 Such atoms will disintegrate producing  particles, -
particles -ray radiation.
 Measurement of radioactive isotopes is used in the
dating of geological events and thus tracing the history
of rocks.


 All rocks and soils contain radioactive isotopes, and
almost all the gamma-rays detected near the Earth's
surface are the result of the natural radioactive decay of
potassium, uranium and thorium.
 The gamma-rays are packets of electromagnetic
radiation characterized by their high frequency and
energy.
 They are quite penetrating, and can travel about 35
centimeters through rock and several hundred meters
through the air.

 RBG REPRESENTATION

 Instruments for detecting radioactivity
 There are two principal instruments, the Geiger
counter, and the scintillometer.
 The gamma- ray spectrometer is an extension of the
scintillometer.
 The Geiger-Müller counter is a simple and cheap device
that responds primarily to Beta radiation.
 It must be held close to the outcrop to detect the Beta
rays, and is thus a tool of limited application, seldom
used in modern prospecting.

 The scintillometer works by counting scintillations
produced in a detector by gama radiation.
 The most efficient detector is made by growing natural
crystals of sodium iodide (NaI), treated with thallium
(Tl).
 A logical extension of the scintillometer is a
spectrometer that distinguishes characteristic gamma
rays from 40K, U and Th.
 Therefore gamma radiation from different sources has
different energy levels as illustrated in figure below
(please check next page).

 Typical natural gamma-
ray spectrum recorded at
ground level.
 The peaks due to specific
decay events are
superimposed on a
background of scattered
radiation from cosmic
rays and higher energy
decays.

 APPLICATION OF RADIOMETRIC METHOD
i. Radiometric surveys, both airborne and ground, have
been primary tools in the exploration for uranium.
ii. It’s a key tool in exploration of Rare Earth Elements
(REE) since thorium often is associated with REE
minerals.
iii. K response can be used to identify zones of potassic
alteration, and thus contribute to geological mapping
based on geophysical survey results.

Geophysical exploration

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