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Suez Canal University Faculty of Science
INTRODUCTION OF
GEOPHYSICS
Prepared By Dr. El-Arabi H. Shendi
Professor of Applied & Environmental Geophysics 2007
٢
:Definitions
Geophysics is the application of physics to study of
the solid earth. It occupies an important position in
earth sciences. It has been taught principally in Earth
sciences departments of university. There is an
obvious need for it to introduce to engineers and
archeologists much more widely than at present.
Geophysics developed from the disciplines of physics
and geology and has no sharp boundaries that
distinguish it from either.
The use of physics to study the interior of the
Earth, from land surface to the inner core is known
as solid earth Geophysics
Solid Earth Geophysics can be subdivided into
Global Geophysics or pure Geophysics and Applied
Geophysics.
Global Geophysics is the study of the whole or
substantial parts of the planet. Geophysical methods
may be applied to a wide range of investigations from
٣
studies of the entire earth to exploration of a localized
region of the upper crust, such as plate tectonics, heat
flow and paleomagnetism.
Applied Geophysics is the study of the Earth's crust
and near surface to achieve an economic aim, or
making and interpreting measurements of physical
properties of the earth to determine subsurface
conditions usually with an economic objectives ( e.g.
discovery of fuel or mineral deposities).
Applied Geophysics
Comprises the following subjects:
1- Determination of the thickness of the crust (which
is important in hydrocarbon exploration.
2- Study of shallow structures for engineering site
investigations.
3- Exploration for ground water and for minerals and
other economic resources.
4- Trying to locate narrow mine shafts or other forms
of buried cavities.
5- The mapping of archaeological remains.
٤
6- Locating buried piper and cables
Engineering Geophysics: the application of
geophysical methods to the investigation of near-
surface physico-chemical phenomena which are likely
to have (significant) for the management of the local
environment.
The discipline of environmental geophysics needs
to bring to the attention of policy- makers and
planners.
The principal distinction between engineering and
environmental geophysics is that the former is
concerned with structures and types of materials,
whereas the latter can also include mapping
variations in pore fluid conductivities to indicate
pollution plumes within ground water, for examples:
- Geophysics can be used to investigate
contaminated land to locate polluted areas prior
to direct observations using trail pits and
boreholes. Large areas can be surveyed quickly
at relatively low cost.
٥
- The alternative and more usual approach is to
use a statistical sampling techniques, the
geophysical survey is used to locate anomalous
areas and there will be a higher certainly that
the constructed trail pits and boreholes will
yields useful results.
- Geophysics is also being used much more
extensively over landfills and other waste
repositories.
- Geophysics can be used to locate a corroded steel
drum containing toxic chemicals. To probe for it
poses the real risk of puncturing it and creating a
much more significant pollution incident.
- By using modern geomagnetic surveying methods,
the drum's position can be isolated and a careful
excavation investigated to remove the offending
(hurt) object without damage. Such approach is
cost effective and environmentally safer.
- Geophysics investing of the interior of the earth
involve taking measurements at or near the
٦
earth's surface that are influenced by the
internal distribution of physical properties.
- Analysis of these measurements can reveal how
the physical properties of the earth's interior
vary vertically and laterally.
- Exploration geophysics developed from the
methods used in global geophysics
Example:
- Exploration seismology used mainly in oil
exploration, have been used in academic studies
relating to the structure of the earth's crust and
upper mantle.
- Geophysics measurement within geographically
restricted areas are used to determine the
distributions of physical properties at depth that
reflect the local subsurface geology.
- An alternative method of geophysical investing
subsurface geology is, of course, by drilling
borehole, but these are expensive and provide
information only at discrete locations.
- Geophysical surveying provides a relatively rapid and effective means of deriving
distributed information on subsurface geology.
Solid Earth Geophysics
Global or pure Geophysics Applied Geophysics
Hydro-Geophysics Mining Geophysics Engineering Exploration Environmental Glacio-geophysics
( Geophysics in ( geophysics for Geophysics Geophysics Geophysics (geophysics in
Water investigation) mineral glaciology)
Exploration) Archaeo-
Geophysics
(in archaeology)
History of Geophysics
The beginning of geophysics has been started since:
a- Gilbert's discovery which stated that the earth
behaves as a great and irregular magnet.
b- Newton 's theory of gravitation .
* The initial step in the application of geophysics to
the search for minerals was taken in 1843 by von
warde which used the magnetic theodolite of Lamont
to discover magnetic ore bodies.
* in 1879 a book by Robert Thalen was published
entitled" on the examination of iron ore deposits by
magnetic methods".
* At that time, the first magnetometer called Thalen-
Tiberg magnetometer was manufactured in Sweden.
* During the past seventy years, geophysics was used
greatly in oil and gas exploration and many
geophysical techniques have been developed for the
٩
detection and mapping of unseen deposits and
structures.
* Advances have been rapid during the past decade
because of the development of new electronic devices
for field equipment and the widespread applications
of the digital computer in the interpretation of
geophysical data.
* Several of the devices now used by geophysicists
were developed from methods used for locating guns,
submarines and aircraft during the two world wars
Examples:
1- Guns were located in France during the First
World War by measuring the arrival times of the
elastic waves generated in the earth by the recoil
of the guns. This lead to the refraction methd of
seismic prospecting
2- Submarines were located by transmitting sound
pulses underwater and measuring the interval
١٠
between the emission and return of the pulses;
knowing the velocity of sound in water, the
distance to the reflecting object could be
determined.
3- Radar, which was developed during the Second
World War, utilizes radio pulses in similar
manner. A modified from of radar has been
widely used for navigation purposes in marine and
airborne geophysical surveys.
4- Ships, submarines and mines were also detected in
both wars by their magnetic properties.
١١
:nd GeophysicsRelation between Geology a
* GEOLOGY
It involves the study of the earth by direct
observations on rocks either from surface exposures
or from boreholes and the deduction of its structures,
composition and historical evolution by analysis of
such observations.
GEOPHYSICS
It involves the study of the inaccessible earth by
means of physical measurements, usually on or above
the ground surface. It also includes interpretation of
the measurements in terms of subsurface structures
and phenomena.
* Geophysical studies are quantitative and tangible,
whereas geological studies are qualitative and descriptive.
Example(1)
* In exploration Geophysics for oil, the petroleum
geologists extract quantitative information from
١٢
Geophysical data (such as seismic records, well
logs,…).
* On the other hand, Geophysicists who are
concerned with measurements of physical
phenomena are incorporating more geology in
order to increase the reliability of the conclusions.
Example(2)
* The information gained about the sea floor
spreading and plate tectonics is due to integrating
geophysical and geological information.
* Every earth scientist, especially the geologist, should
be familiar with the methods of geophysics. This
familiarity should enable one to know:
a – which of the geophysical methods can (or
cannot) be of help in a given geological
situation.
b- The limitations of the geophysical methods.
١٣
* The incorporation of the available geophysical in-
formation in interpretation of geophysical
measurements is very important
PHYSICAL PROPERTIES OF ROCKS
* The physical properties of rocks that are most
commonly utilized in geophysical investigations are:
- Density
- Magnetic susceptibility
- Elasticity
- Electrical resistively or conductivity
- Radioactivity
- Thermal conductivity
* These properties have been used to devise geophysical
methods, which are:
- Gravity method
- Magnetic method
- Seismic method
- Electrical and electromagnetic methods
- Radiometric method
- Geothermal method
١٤
1- Rock Densities:
* Any Geologic condition that result in a horizontal
variation in density will cause a horizontal variation in
gravity or a gravity anomaly.
• It is therefore the significant parameter in gravity
Exploration (i. e. the anomaly source is a local
variation in density).
* Two problems are faced in connection with this
parameter (i. e. density).
1- The maximum density variation between
different rocks and between rocks and minerals is
approximately (2). This is a very small change
compared to the range of magnetic susceptibility
(≈105) and electrical conductivity (≈1010 ).
Examples:
1.6
2.3
Sand
Dolomite
1.95
2.11
Gravels
limestone
1.7
2.24
Clay
sandstone
١٥
2- It is not possible to measure density in situ. A
density borehole logger has been used to a limited
extent in oil exploration
* It is necessary to make density measurements in the
laboratory on small samples of outcrops or drill cores.
• In this case, the laboratory results do not necessarily
give the true bulk density of the formation, since the
samples may be weathered, fragmented or dehydrated.
* Sedimentary rocks have lower densities than igneous
and metamorphic rocks.
• Their densities depend on: composition, porosity and
pore fluids, their age and depth below surface (i.e. the
density increases with depth and time because the rock
becomes compacted and consolidated).
• For that, the laboratory density measurements should
be made, if possible, with the sample in the same
١٦
conditions as those prevailing in the formation from
which it was removed.
* Igneous rocks are denser than sedimentary rocks.
* Basic igneous rocks have larger densities than acidic
forms.
* Porosity is of minor significance in igneous and
metamorphic rocks, unless they are highly fractured.
Examples:
Rock Density Rock Density
Granite 2.64 Basalt 2.99
Gabbro 3.03 Acidic igneous rocks 2.61
Basic igneous rocks 2.79
* Density of metamorphic rocks increases with the degree of
metamorphism since this process tend to fill pore spaces and
recrystallize the rock in a denser form.
Density Metamorphic
form
Density Rock
2.75
2.79
2.6
2.8
Marble
Slate
Quartzite
Gneiss
2.55
2.4
2.35
2.64
Limestone
Shale
Sandstone
Granite
١٧
* The density of metamorphic rock increases as the
acidity decreases.
* Non – metallic minerals are of lower density than the
average of rocks.
*Metallic minerals are heavier than this average.
Copper 8.7 silver 10.5 Galena
7.5
Laboratory estimation of density :
* Density can be determined by direct measurements on
rock samples as follows:
-The sample is weighted in air and in water. The
difference in weights gives the volume of the sample.
- Dry density = weight / volume
* The density of a rock is quite variable. For that, it is
necessary to measure several tens of samples of each rock
in order to obtain a reliable mean density.
١٨
2- Magnetic susceptibility of rock and minerals (K)
* When a magnetizable body is subjected to an external
magnetizing field (H), it acquires a magnetization that is
lost when the applied field (H) is removed.
* Such a magnetization ( ji ) is said to be induced by the
applied magnetizing field (H).
* ( Ji ) is parallel and proportional to the applied field (H)
Ji = KH
* (K) is called the magnetic susceptibility.
* A substance is called Diamagnetic if the (K) is negative;
it is called Paramagnetic and Ferromagnetic if (K) is
positive.
• Magnetic susceptibility is the significant variable in
magnetic, playing the same role as density in gravity.
• Sedimentary rocks have the lowest average
susceptibility and basic igneous rocks have the highest.
١٩
Examples:
Rock K Rock K
Limestone 10x 106 shale 50x106
Granite 200x106 Quartzite 350x106
• The susceptibility depends upon the amount of
ferromagnetic minerals present (i.e. magnetite, ilmenite
or pyrrhotite).
Laboratory determination of ( k) :
• The simplest method involves a comparison of the
sample with a standard by using a laboratory instrument.
• It is to compare the deflection produced on a
tangent magnetometer by a prepared sample (a drill core
or powered rock, Ks ) with that of a standard sample of
magnetic (Kstd, FeCI3 powder in a test tube). "K" is then
given by the following equation:
Ks= Kstd (ds/d std)
Where ds is the deflection of the sample, dstd is the
deflection of the standard.
٢٠
3- Elastic properties of rocks ( Elasticity):
* The seismic method utilizes the propagation of waves
through the earth. This propagation depends upon the
elastic properties of rocks.
* The size and shape of a solid body can be changed by
applying forces (stresses) to the external surface of the
body.
* These external forces (stresses) are opposed by internal
forces (stain) which resist the changes in size and shape.
* As a result the body tends to return to its original
condition when the external forces are removed. This
property is called Elasticity,
• The theory of elasticity relates the forces which are
applied to the external surface of a body to the resulting
changes in size and shapes.
• The relations between the applied forces and the
deformations are expressed in terms of Stress and Strain
• Stress is a measure of the forces (F) per unit area
across a surface element (A) within the material.
٢١
S=F/A
• when (F) is perpendicular to the area element, the
stress is called Normal Stress
• Normal stress can be classified into Tensile stress if
the force is directed away from the material or
Compressive stress if the force is directed into the
material .
Compressive Stress Tensile Stress
* When (F) is tangential to the area element, the stress is
a Shearing stress
٢٢
* Strain is a measure of the relative deformation
(expressed per unit length or per unit volume) of a body
when it is subjected to a stress.
* It is the change in size or shape.
* A change in shape with no change in volume is called a
Shear strain or distortion
* A change in volume without change in shape is called
a dilatation or contraction .
* Strains that are associated with relative change in
length in the direction of stresses are called Normal
strains.
Elastic properties of materials (Elastic or elastic
constants)
* The elastic properties of a material are described by
certain elastic moduli or elastic constants which specify
the relationships between different types of stress and
strain.
٢٣
1- Young's Modulus (E):
* If a load (w) is hung on the end of a wire of length (L)
and cross- sectional area (A), the wire is elongated by a
small length (∆L) in vertical direction (Z).
* Young's Modulus (E) represents the tensile stress (Pz)
tensile strain (ez) proportionality constant:
Pz α ez
Pz = E ez
E= pz/ ez = ( W/A) / ( ∆L/L)
)K(Bulk Modulus -2
* If a body of volume (V) is subjected to a uniform
compression stress (P), its volume will be decreased by
an amount (∆v).
* Bulk modulus (K) is defined as the ratio of the
pressure to the fractional change in volume.
K= P/ (∆V/V)
٢٤
):µ, Rigidity( Shear modulus -3
* It is a measure of the stress/ strain ratio in the case
of a simple tangential stress (Shear), without change
of volume.
Example:
* A pile of cards can be sheared without affecting
total volume of cards.
µ= (F/A) / ф
* The strain in this case is expressed by the angle of
deformation.
4- Poisson's ratio ( σ ):
* It is a measure of the geometrical change in the
shape of an elastic body.
Example:
* A cylinder of a length (L) and diameter (D) when
subjected to a tensile stress parallel to (L), the length
٢٥
will be elongated by (∆L) and the diameter will be
shortened by (∆D). The opposite will occur if it is
subjected to a compressional stress, the length will be
shortened by (∆L) and the diameter will be increased
by (∆D). In either case,
σ = (∆D/D)/ (∆L/L)
* For most rock, the value of (σ) is about 0.25, for
liquids the value of (σ) attains its maximum possible
value of (0.5) as the liquids have no rigidity (µ = 0).
* The relations between the elastic moduli are given
by the following formulas:
E = 9 µ K / (µ + 3K)
K= E/3 (1-2σ)
µ = E/2 ( 1+σ)
σ = (3K-2µ) / (6K+2 µ)
٢٦
4- Electrical properties of rock :
* Several electrical properties of rock are significant
in electrical prospecting which are:
A- Natural electrical potential.
B- Electrical conductivity (or the inverse, electrical
resistivity).
C- Dielectric constant.
* Electrical conductivity is the most important while
the others are of minor significance.
A- Natural ( Spontaneous) potentials:
* These potential are associated with:
- weathering of sulphide mineral bodies.
- Variation in mineral content at geologic contacts.
- Bioelectric activity of organic material (i.e. in
plant roots).
- Metal corrosion of underground pipes, cables, …..
- Thermal gradient in underground fluids.
٢٧
* There are four principal mechanisms producing
these potentials.
a- Electrokinetic or streaming potential :
* It is of mechanical origin, observed when a solution
of electrical resistively (ρ) and viscosity (η) is forced
through a porous medium.
b- Liquid – Junction (diffusion) potential:
* It is of a chemical origin, due to the difference in
nobilities of various ions in solutions of different
concentrations.
* When two identical metal electrodes are immersed
in a homogeneous solution, There is no potential
difference between them, If the concentrations at the
two electrodes are different, There is a potential
difference
٢٨
C –Shale potential :
* It is of chemical origin, occurring when the
concentrations at the two electrodes are different.
* The combined diffusion and shale potentials are
know as the electrochemical or static self potential.
d- Mineralization potential
* It is of a chemical origin, occurring when two
dissimilar metal electrodes are immersed in a
homogeneous solution.
* These potentials are especially pronounced in zones
containing sulphides, graphite and magnetite and
have larger values than the other potentials.
* The presence of metallic conductors in appreciable
concentrations is necessary to produce mineralization
potential
٢٩
B: Electrical conductivity (or the inverse,
electrical resistivities)
* Electrical current is propagated in rocks and minerals
depending on the electrical resistivities of these
materials.
* The electrical resistance of a material is expressed in
terms of its resistivity.
Example:
If the resistance between opposite faces of a
conducting cylinder of length (L) and cross sectional
area (A) is (R), the resistively (ρ) is given by:
ρ = R A / I
If "A" is in meters2 "L" in meter, "R" in ohm, "ρ"
will be in ohm- meter.
• If these dimensions are in cm ,"ρ" will be ohm –
centimeter, where: 1 Ωm - 100 Ω cm.
• 'R' is given in terms of the voltage (V) applied
across the ends of the cylinder and the resultant
current (I) flowing through it , by ohm's law:
٣٠
R= V/I
"R" in ohm "v" in volt and "I" in ampere.
• The conductivity (σ ) is the reciprocal of
resistivity:
σ = I / ρ = L/RA= (I/A) / (V/L) = J/E mhos/m or
mhos/ cm, where J = current density
(ampere/m2), E= electric field (volt/m).
• Most mineral grains are insulators except
metallic ores and clay minerals.
Sample
∆ V
I
A
٣١
• Electric conduction in these mineral grains being
through interstitial water in pores and fissures.
• The conductivity of a porous rock varies with the
volume and arrangement of the pores and the
conductivity and amount of contained water.
• Hard rocks are bad conductors of electricity, but
conduction may take place along cracks and
fissures.
• In porous sedimentary formations, the degree of
saturation and the nature of the pore electrolytes
govern the resistivity.
5 – Radioactivity of rocks:
* Radioactivity of rocks and minerals are attributed
to traces of uranium, thorium and the isotope of
potassium (K40) and their radioactive decay
products.
٣٢
* Among the earth's rocks, granites and shale show
the largest radioactivity.
* In general, the radioactivity in sedimentary rocks
and metamorphosed sediments is higher than that
in igneous and other metamorphic types, with the
exception of potassium – rich granites.
6- Thermal properties of rocks:
* It is a fact that the temperature increases with
depth. Therefore, heat must be flowing upward in
the earth.
* The amount of heat flow depends on the thermal
conductivity of the rocks.
* The thermal conductivity is a measure of how easily
heat flows through a material.
٣٣
GENERAL REVIEW OF GEOPHYSICAL
METHODS
* The physical properties of rocks have been used to
devise geophysical methods that are essential in the
search for minerals, oil and gas and other
geological and environmental problems.
* These methods are:
1- Gravity method
3 - Seismic method
5- Electromagnetic method
7- Geothermal method
2- Magnetic method
4- Electrical method
6- Radiometric method
* Geophysical methods respond to the physical
properties of the subsurface media (rocks,
sediments, water, voids, etc.. ) and can be used
Successfully when one region differs sufficiently
from another in some physical property.
• These methods can be classified into two distinct
types:
٣٤
1- Passive methods:
Which detect variations within the natural fields
associated with the earth, like the gravitational and
magnetic fields, such as gravit, magnetic, some
electric and some electromagnetic methods,
radioactive and geothermal methods.
2- Active motheds:
* These artificially generated signals transmitted into
the ground and then modify the received signals in
ways that are characteristic of the materials
through which they travel. Examples of these
methods are seismic and some electrical methods.
* Generally, natural field methods (passive methods)
can provide information on earth properties to
greater depths and are simpler to carry out than
artificial source methods (active methods).
Moreover, the artificial source methods are
capable of producing a more detailed and better
resolved picture of the subsurface geology.
٣٥
* Geophysical methods may from part of a larger
survey and thus geophysicists must be in contact
with the whole survey team and particularly to the
client.
* Few, if any geophysical methods provide a unique
solution to a particular geological situation. It is
possible to obtain a very large number of
geophysical solutions to some problems, some of
which may be geologically non-sensical. It is
necessary, therefore, always to ask the question:
"Is the geophysical model geologically plausible?.
If it is not, then the geophysical model has to be
rejected and a new one developed which does
provide a reasonable geological solution.
• Conversely, if the geological model proves to be
inconsistent with the geophysical interpretation,
then it may require the geological information to
be re-evaluated.
٣٦
• It is important that geophysical data are
interpreted within geological framework.
• The various geophysical methods depend on
different physical properties. For example:
gravity methods are sensitive to density contrasts
within the sub-surface geology and so are ideal
for exploring for major sedimentary basins
where there is a large density contrast between
the lighter sediments and the denser underlying
rocks.
• It would be inappropriate to use gravity methods
to search for ground water where there is a
negligible density contrast between the saturated
and unsaturated rocks.
• If the physical principles upon which a method is
based are understood, then it is less likely that
the technique will be misapplied or the resultant
data misinterpreted.
٣٧
• The basic geophysical methods are listed below
with the physical properties to which they relate
and their main uses.
Geophysical methods and their main applications
Applications
1 2 3 4 5 6 7 8 9 10
Physical property Method
s s s s p p Density Gravity ـــ ـــ s ــ
s s p p Susceptibility Magnetic ــm ـــ p p ـ
s s p mp p Elastic moduli, density Seismic refraction ـــ ـــ ــ ــ
ms s mp p Elastic moduli, density Seismic reflection ـــ ـــ ــ ــ
mp s p p p p p mmResistivity Resistivity
ــ m m mp mp - - Potential differences Spontaneous ــ
potential
mmm m ms mp mmResistivity Induced
polarization
mp p p p p p p p s Conductance, inductance Electromagnetic
mm s s s mp mmConductance, Inductance EM - VLF ــ
p p p s p p p m- - Conductivity EM–ground radar
mmp p s Resistivity Magneto - telluric ــ ـــ ـــ ــ ــ
P = primary method S = secondary method
m= may be used but not necessarily the best
approach, or has not been developed for this
application, - = unsuitable
٣٨
Applications :-
1-Hydrocarbon exploration (coal, gas, oil)
2-Regional geological studies (over areas of 100s of
km2 )
3- Exploration of mineral deposits.
4- Engineering site investigation.
5- Hydrogeological investigation .
6- Detection of subsurface cavities .
7- Mapping of leachate and contaminant plumes.
8- Location and definition of buried metallic objects.
9- Archaeo-geophysics .
10- Forensic geophysics .
* Several geophysical surveying methods can be used
at sea ( marine geophysics ) or in the air (aero-
geophysics )
* Reconnaissance surveys are often carried out from
the air because of the high speed of operation.
• In such cases the electrical or seismic methods are
not applicable, since these require physical
٣٩
contact with the ground for the direct input of
energy.
• Geophysical methods are often used in
combination.
Example: The search for metalliferous mineral
deposits often utilizes airborne magnetic and
electromagnetic surveying.
- prospecting for oil usually includes gravity,
magnetic and seismic surveying
• The importance of such combination appears in
the interpretation stage, ambiguity arising from
the results of one survey method may be
removed by consideration of results from a
second survey method.
٤٠
Airborne versus ground geophysical methods:
• Airborne geophysical methods are used in
reconnaissance work, but the ground methods
are used in more detailed investigations.
• They are fast and are relatively inexpensive per
unit area.
• Several kinds of surveys can be done at once.
• They can provide a more objective coverage than
ground surveys in many kinds of terrains.
• For example: several hundred line kilometers of
airborne electromagnetic surveying can be done
in a day compared with three to five line
kilometers per crew in a ground EM survey .
• The cost of an airborne electromagnetic survey,
with magnetic and radiometric data included is
likely to be 1/4 to 1/5 the cost of an equivalent
ground EM survey
٤١
• Airborne survey patterns are reasonably
uniform and complete because they do not have
the access and traverse problems of ground
survey in swamps, dense brush and rugged
topography.
• An airborne survey will give more accuracy than
a ground survey in some areas, but it will seldom
provide such detail or such sharp signals as a
ground survey .
1- Gravity method:
• It is mainly used for oil exploration. Sometimes in
mineral and ground water prospecting.
• Gravity prospecting involves the measurement of
variations in the gravitational field of the earth
(i.e. minute variations in the pull of gravity from
rock within the first few miles of the earth's
surface).
٤٢
• Different types of rock have different densities and
the denser rocks have the greater gravitational
attraction.
• If the higher–density formations are arched
upward in a structural high, such as an anticline,
the earth's gravitational field will be greater over
the axis of the structure than along its flanks.
Gravity anomaly over an anticline
Gravity
Anticline
٤٣
* A salt dome which is generally less dense than the
rock into which it is intruded, can be detected
from the low value of gravity recorded gravity
recorded above it compared with that measured
on either side.
Gravity Anomaly Over a salt dome
* Anomalies in gravity which are sought in oil
exploration may represent only one - millionth or
even one - ten - millionth of the earth's total field.
Salt dome
٤٤
* For this reason, gravity instruments are designed to
measure variations in the force of gravity from one
place to another than the absolute force itself.
* The gravity method is useful wherever the
formations of interest have densities which are
appreciably different from those of surrounding
formations.
* Gravity is an effective means of mapping
sedimentary basins where the basement rocks
have a higher density than the sediments.
* Gravity is also suitable for locating and mapping
salt bodies because of the low density of salt
compared with that of surrounding formations.
* Gravity can be used for direct detection of heavy
minerals such as chromite
٤٥
Magnetic method:
* Magnetic method deals with variations in the
magnetic field of the earth which are related to
changes of structures or magnetic susceptibility in
certain near surface rocks.
* Magnetic surveys are designed to map structure on
or inside the basement rocks or to detect magnetic
mineral directly.
* In mining exploration, magnetic methods are
employed for direct location of ores containing
magnetic minerals such as magnetite.
* Intrusive bodies such as dikes can often be
distinguished on the basis of magnetic observations
alone.
Electrical methods:
* Electrical prospecting uses many techniques, based
on different electrical properties of the earth's
materials such as:
٤٦
- The resistively method is designed to give
information about the electrical conductivity of the
earth's rocks.
- In resistivity method the current is driven through
the ground using a pair of electrodes and the
resulting distribution of the potential in the ground
is mapped by using another pair of electrodes
connected to a sensitive voltmeter.
- The resistivity method has been used to map
boundaries between layers having different
conductivities.
- It is employed in engineering geophysics to map
bedrock.
- It is used in groundwater studies to determine
salinity.
- The induced polarization (IP) makes use ionic
exchanges on the surfaces of metallic grains
(disseminated sulphides).
٤٧
- Telluric current and magneto-telluric methods use
natural earth currents and anomalies are sought in
the passage of such currents through earth
materials.
- The self potential method is used to detect the
presence of certain minerals which react with
electrolytes in the earth to generate electrochemical
potentials.
- Electromagnetic methods detect anomalies in the
inductive properties of the earth's subsurface
rocks.
- The method involves the propagation of time
varying, low frequency electromagnetic fields in
and over the earth.
- An alternating voltage is introduced into the earth
by induction from transmitting coils and the
amplitude and phase shift of the induced potential
٤٨
generated in the subsurface are measured by
detecting coils and recorded.
- Electromagnetic methods are used to detect metallic
ore bodies.
Seismic methods:
* There are two main seismic methods, reflection and
refraction:
1- seismic reflection method :
* This method is used to map the structure of
subsurface formations by measuring the times
required for a seismic wave, generated in the earth
by a near surface exploration of dynamite,
mechanical impact or vibration, to return to the
surface after reflection from interface between
formations having different physical properties.
٤٩
* The reflections are recorded by detecting
interments which are called geophones responsive
to ground motion.
* Variations in the reflection times from place to
place on the surface indicate structural features in
the strata below.
* Depths to reflecting can be determined from the
times using seismic velocity information.
* Reflections from depths as great as 20,000 feet can
be observed from a single explosion, so that in most
areas, geologic structures can be determined
throughout the sedimentary section.
S.P. G
Reflected Ray
Layer 1, V1
Layer 2, V2
Reflector
٥٠
* With reflection method one can locate and map
such features as anticlines, faults, salt domes and
reefs. Many of these are associated with the
accumulation of oil and gas.
Seismic refraction method:
* In refraction method, the detecting instruments
recorded the arrival times of the seismic waves when
refracted from the surface of discontinuity.
* These times give information on the velocities and
depths of the subsurface formations along which
they propagate.
Refracted Ray
Refractor Layer 1, V1
Layer 2, V2
S.P. G
٥١
* Refraction method makes it possible to cover a
given area in a shorter time and more economically
than with the reflection method.
Radioactive Method :
* This method is used to detect radioactive minerals
such as uranium and thorium.
Well logging method:
* This involves probing the earth with instruments
which give continues readings recorded at the
surface as they are lowered into boreholes.
* The rock properties which are covered by well
logging techniques are electrical resistivity, self
potential, gamma ray generation density, magnetic
susceptibility and acoustic velocity.
* Well logging is one of the most widely used of all
geophysical techniques
٥٢
GEOPHYSICAL ANOMALIES
* It is the local variation in a measured parameter,
relative to some normal background variation is
attributed to a localized subsurface zone of
distinctive physical property and possible
geological importance.
* A local variation of this type is known as a
geophysical anomaly.
Example:
* The Earth's gravitational field after the application
of certain corrections would everywhere be
constant if the subsurface were of uniform density.
* Any lateral density variation associated with a
change of subsurface geology results in a local
deviation in the gravitational field
٥٣
* This local deviation from the otherwise constant
gravitational field is referred to as a gravity
anomaly .
* It may be positive (high anomaly) or negative (low
anomaly).
10 20
30 10
30
20
Positive (high anomaly) Negative (low anomaly)
٥٤
AMBIGUITY IN THE INTERPRETATION OF
GEOPHYSICAL ANOMALIES
* In studying the Earth's hidden features, most
problems are of an Inverse type (i.e. deducing the
source from the observed anomaly).
* The measured physical effect ( e.g. surface
variations in gravity, magnetic or electrical fields)
can not be interpreted in terms of a unique source
occurring at a particular depth inside the earth (
i.e. the same anomaly gives more than one
interpretation).
* This is because a variety of sources with varying
parameters at different depths can theoretically
produce the same affect.
* A combination of several geophysical methods and
the different geological information often yields
more information that can help reduce the
ambiguity by narrowing down the range of possible
solutions.
٥٥
NOISE IN THE INTERPRETATION OF
THE GEOPHYSICAL DATA
* Noises are undesired readings recorded during
geophysical measurements and make the
interpretation more difficult.
* The reliability of geophysical mapping is strongly
dependent upon the quality of the field records.
* We use the term signal to denote any event on the
geophysical record from which we wish to obtain
valuable information. Everything else is called
noise.
* The signal / noise ratio, is the ratio of the signal
energy in a specified position of the geophysical
record to the total noise energy in the same
portion.
* Poor geophysical records result whenever the
signal/ noise ratio is small.
٥٦
* When signal / noise ratio is less then unity, the
record quality is usually marginal and deteriorates
rapidly as the ratio decrease further.
* Some noise can be anticipated on the basis of
existing information possible sources of terrain
noise (swamps, conductive overburden) may be
identified.
* Sources of cultural noise- mines, pip lines, and
abandoned town sites may be known.
* Noise can be attenuated by applying some
processing and treatment techniques to the
geophysical field data to increase the signal/ noise
ratio.
٥٧
FIELD GEOPHYSICAL SURVEYING
* The field surveying in geophysics can be carried out
in the form of profiles or traverses.
* These profiles must be, as possible as it can
perpendicular to the strike of the causative body.
* The distance (interval) between the measuring
points ( e.g. stations) depends up the purpose of
the surveying ( e.g. regional or detailed studies)
Example:
1- In oil exploration, we look for oil traps ( geologic
structures) which may be extended for several
hundreds of meters or even several kilometers . In
this case, the station interval may be of as large as 1
to 2 kilometers.
2- In mineral exploration, we look for mineralized
zone of few tens of meters.
٥٨
* In this case, the station interval should be as small
as possible to cover the target body with enough
number of measuring points
* The field geophysical measurements can be carried
out in more than one profile, parallel to each other
to form what is called Grid pattern system.
GLOSSERY
Anomaly : An irregularity in observed or theoretically
calculated geophysical effect caused by a
significant change in some physical
property ( e.g. density, magnetization ,
seismic velocity) of rocks.
Aquifer: A permeable rock formation that stores and
transmits groundwater to wells.
Disseminated ore: An ore body in which metal is
distributed in small amounts throughout the
rock.
٥٩
Geomagnetic reversal : A reversal of the polarity of
the earth's magnetic field.
Hydrothermal activity: Any process involving high
temperature groundwater.
Isostasy : the concept that areas of the crust are in
gravitational balance by a mechanism
which compensates for the broad
topographic variations .
Magnetic epoch : A period of the order of one million
years during which the earth's magnetic
field was predominantly of one polarity.
Magnetic event: A short period within a magnetic
epoch during which the earth's field had a
polarity opposite to that of the epoch.
Prospecting: Exploration of an area with the aim of
locating minerals, oil gas, water, …….etc
٦٠
REPRESENTATION OF GEOPHYSICAL
MEASUREMENTS
* The geophysical data can be represented in TWO
forms:
Profiles: when the measurements are taken along a
single traverse, the measured parameter is
plotted on the "Y" axis and the measuring
points on the "X" axis.
* The measurements can also be potted on parallel
profiles, called stacked profiles.
Value
Stations
٦١
b- contour maps: when the measurements are
recorded on a grid pattern system they can be
contoured in the form of maps.
PROCESSING OF GEOPHYSICAL DATA * The field geophysical data are affected by
interference from undesired sources (e.g. noises).
* This data must be subjected to different correction
and processing techniques before being interpreted.
* Rapid advances in digital computer technology
made extensive calculations for this purpose are
available.
٦٢
Examples:
* Gravity field measurements are affected by
latitudes, terrains, drifts and should be corrected for
these effects before interpretation .
* Magnetic measurements are usually affected by
daily variations in the earth's magnetic field and must
be corrected.
* Some electromagnetic methods are affected by
variations in topography and must be corrected
before interpretation.
INTERPRETATION OF GEOPHYSICAL DATA
* Interpretation of geophysical field measurements
means that the transformation of digital data into
understandable geological forms (e.g. structures,
groundwater occurrences, mineral deposits, …..)
* It can be divided into Qualitative and Quantitative.
٦٣
Qualitative interpretation * A first step towards interpretation is the
preparation of a contour map on which the intensity
values at different stations are plotted and on which
the contours of equal values are drawn at suitable
intervals.
* Contouring of geophysical maps is nowadays often
done on automatic plotters using computer programs
for interpretation.
* Qualitative interpretation means general inspection
of the contour map or profile without making any
calculations.
* Most geophysical anomaly maps are colored using
suitable color schemes and color gradations for the
areas enclosed between successive contours.
* Coloring is a very valuable aid in the qualitative
interpretation of a geophysical map in general .
٦٤
* Many features of geological interest is first
discernible when a map is suitably colored.
* An important point in considering the anomalies
in an area is the zero level, that is the reading of the
instrument at points where the field is the normal
undisturbed field.
* The qualitative interpretation of geophysical map
begins with a visual inspection of the shape and
trend of the major anomalies.
* Each contour pattern should have its geological
counterpart.
* After delineation of the structural trends, a closer
examination of the characteristic features of each
individual anomaly is made. These features are:
a- The relative locations and amplitudes of the
positive and negative parts of the anomaly.
٦٥
b- The elongation and areal extent of the contours
which suggests the strike of the corresponding
geological feature.
c- The sharpness of the anomaly as seen by the
spacing of contours (e.g. high horizontal
anomaly gradients are often associated with
contacts between rocks and with bodies at
shallow at depths).
d- Circular patterns of contours are associated with
circular bodies such as ore body.
e- Long narrow patterns are due to long narrow
bodies such as dike, tectonic shear zones,
isoclinally folded strata.
f- Dislocations, when one part of an anomaly
pattern is displaced with respect to the other
part, are indicative of geological faults.
٦٦
Quantitative interpretation
* After completing qualitative study it is important to
extract some quantitative information (e.g. the
important parameter to be estimated is the depth to
the anomalous structures).
* From the relative spreads of the maxima and
minima of the anomaly, the approximate location
and horizontal extent of the causative body may be
determined.
* From the from of the anomaly, the other parameter
of the body, its shape and depth may be
determined.
* The usual procedure in quantitative interpretation
is to guess a body of suitable from, calculate its field
at the points of observation and compare it with the
measured values.
* It is then possible to adjust the depth and
dimensional parameters of the body by trial and
٦٧
error or by automatic optimizing methods until a
satisfactory agreement is achieved between the
calculated and observed values.
* The geometrical parameters must then be
translated into structural terms. In the light of
know geology.
THE PLACE OF GEOPHYSICS IN SOLVING
GEOLOGICAL AND ENVIRONMENTAL
PROBLEMS
1- In hydrocarbon ( petroleum) exploration:
* Petroleum, when in an accumulation, forms only a
small proportion of the total fluids present in a
rock section, and none of its properties differs
sufficiently from those of the salt water.
* Since rocks can vary considerably in their physical
properties such as densities, magnetic properties,
electrical conductivities, and the seismic velocities.
It has proved possible to use these variations in
٦٨
rock properties to assist in the location of
subsurface structures which are favorable for the
accumulation of petroleum.
* All the geophysical methods concentrate on the
discovery of anomalies in the rock which overlie or
surround possible petroleum accumulations.
* Nowadays geophysical surveys are generally
considered to be standard pre- requisites before an
exploration drilling program.
* Geophysics was first applied to petroleum
exploration in the U.S.A in the early 1920's.
* Hydrocarbons (oil and gas) are normally found in
association with thick sedimentary sequences in
major sedimentary basins.
* The hydrocarbons are accumulated in commercial
quantitative in suitable geological environments
called traps.
٦٩
* There are many types of traps including tectonic
structures such as anticlines, tilted fault blocks, salt
domes and stratigraphic traps such as local sand
bodies surrounded by clay envelops or local reef
developments in limestone sequences.
* Geophysical exploration for hydrocarbons normally
employs an indirect approach, searching for the
traps, depending on the great variations in the
physical properties of the earth's rock such as
density, magnetic properties , electrical
conductivities and seismic wave velocities.
* The only techniques which are believed to be
directly related to the properties of petroleum itself
are the geochemical and radioactive surveys.
* All the other geophysical techniques concentrate on
the discovery of anomalies in the rocks which
overlie or surround possible petroleum
accumulations.
٧٠
* Exploration is usually carried out in several phases:
A-In cases where the subsurface geology is unknown
(unexplored areas), the initial reconnaissance may
involve gravity and/ or aeromagnetic surveying.
* Gravity surveying is capable of identifying areas of
thick sediments by their relatively low densities and
the large scale negative Bouguer anomalies.
* Gravity is also used to determine the subsurface
structures by the lateral changes in density. It is
employed as a preliminary to the seismic survey
enabling areas of maximum interest to be
delineated.
٧١
* Gravity method is an ideal technique for detecting
the salt domes often associated with oil
accumulations, because the density of the salt is low
compared with the surrounding sediments.
* Gravity "highs" are usually due to buried anticlines.
Gravity
Anticline
Salt dome
٧٢
* Aeromagnetic surveying can be used to estimate
variations of depth to an igneous or metamorphic
basement underlying a sedimentary sequence ( i.e.
thickness of sediments) and hence to determine
indirectly the areas of main sediment accumulation.
* Aeromagnetic measurements depends mainly on the
great difference in magnetic susceptibility between
the sedimentary rocks and the underlying
basement rocks.
* The aeromagnetic survey is usually used in
petroleum exploration more than ground survey
for the following reasons:
- The speed of the survey.
- The possibility of reaching inaccessible area.
- Local influences which would affect the accuracy
of the ground instrument are avoided.
- The aeromagnetic survey provides a rapid and
effective method of estimating the depth and
shape of the crystalline basement and hence
٧٣
approximate thickness of the overlying
sedimentary material.
*The presence of oilfields may sometimes be directly
indicated by the results of aeromagnetic surveys
which detect the presence of concentrations of
diagenetic magnetite. These concentrations are
produced by the reduction of hydrated iron oxides
and/or hematite as a direct result of micro –
seepage from buried oil accumulations.
* Once a prospective sedimentary basin environment
has been identified, further geophysical surveying
normally carried out using seismic methods,
especially reflection profiling .
* Reconnaissance seismic exploration surveying
involves measurements along widely spaced profile
lines covering large areas in order to detect
regional structural elements.
٧٤
* Detailed refection seismic surveying involves closely
spaced, intersected profile lines in more restricted
areas containing the main prospective targets, to
delineate the most promising structures.
* The main job of seismic interpretation is to involve
structural mapping in the search for the structural
closures that may contain oil or gas. Geochemical
investigations may help to differentiate between
those which are hydrocarbon – bearing and those
which are barren.
* The petroleum geologist must be able to relate the
resultant sections and maps to the surface
geological evidence and the subsurface data
furnished by well samples and cores.
* Exploration boreholes are normally sited on
seismic profile lines so that the borehole logs can
correlated directly with the local seismic section.
٧٥
* Exploration boreholes are normally sited on seismic
profile lines so that the borehole logs can be
correlated directly with the local seismic section.
*Seismic stratigraphic provides additional criteria
on which to select areas for detailed study, for
example, the definition of local deltaic of reef faces
with an associated high reservoir potential.
*Geochemical investigations may help to differentiate
between those which are hydrocarbon bearing and
those which are barren.
*Radioactive survey is a method of surface
exploration for oil. It is based on the hypothesis
that most crude oils contain radioactive material,
some of which notably dissolved radium salts or
radon gas, may be carried to the surface by
percolation and thus are areas under which oil may
lie.
٧٦
* Tests for radioactivity may be made on gas samples
drawn from shallow surface holes or on soil
samples.
* These samples are collected along closely spaced
profiles or grids covering the area under test, and
the relative radioactivity of each sample is then
measured and plotted against its map position.
* By such measurements radioactivity haloes could be
defined over oil fields.
2- The place of geophysics in mineral exploration:
* Geophysical methods are extensively used in the
search for economically valuable mineral deposits,
including non- metallic deposits such as sand,
gravel and limestone and metallic deposits such as
massive and disseminated sulphides and iron ores.
* These deposits differ significantly from their host
rocks in their physical properties and consequently
give rise to geophysical anomalies of various types.
٧٧
* The initial aim of a geophysical survey for ore
deposits is to locate mineralized areas of potential
interest.
* Airborne magnetic and electromagnetic techniques
are suitable since large areas can be surveyed
rapidly at relatively low cost .
* Once possible target area are determined, further
information on causative bodies within the
anomalous zones is obtained by ground surveys
which enable the prospector to determine whether
the anomalous bodies are of economic importance.
* If ore bodies are present, the ground geophysical
data will provide information on their depths,
extent and attitude and consequently control the
location of exploratory boreholes or trenches.
* The return - ratio is very important in geophysical
surveys. It is the ratio of the estimated value of the
٧٨
ore to the cost of the geophysical work. This ratio
must be several hundred to one.
A – Massive sulphide ores:
* They are considered to be a single mass with a cross
sectional area of at least 100m2 comprising 50% or
more of metallic sulphides.
* Such ore may contain magnetic minerals pyrrhotite
and magnetite. If these minerals are present in
reasonable quantities, the ore will produce large
magnetic anomalies.
* The electrical conductivity of massive sulphides is
very high, in the range 102 – 104 S.m-1.
* The geophysical methods applicable to the search
for such ores those responding to very dense
material (gravity), high magnetic (magnetic) and
conductive materials (electrical and
electromagnetic).
٧٩
* Airborne prospecting techniques for massive
sulphides usually exploit the property of high
conductivity ( i.e. electromagnetic).
* Airborne prospecting techniques for massive
sulphides usually exploit the property of high
conductivity ( i.e. electromagnetic methods are
used).
* The survey aircraft usually also carries a
magnetometer to provide additional information
at little extra cost.
* Ground geophysical surveys employ electrical and
electromagnetic methods. Self potential methods
are cheap and effective if the correct subsurface
conditions exist and the ore body lies at a depth of
less than a bout 30m.
* Gravity surveying is a secondary ground
exploration tool because of the high cost and
ambiguities in interpretation.
٨٠
* It provides accurate estimates of ore tonnage on the
basis of the total mass anomaly.
* Although electrical and electromagnetic methods
are the major exploration techniques, they suffer
from drawback that anomalies may result from
non- economic sources such as graphite or water
filled shear zones.
* It is possible to eliminate such non – economic
sources by using a combinations of electrical,
magnetic and gravity methods.
B- Disseminated sulphides ores :
* Disseminated sulphide deposits are those bodies in
which sulphides are scattered as specks and
veinlets throughout the rock and constitute not
more than 20% of the total volume.
٨١
* Magnetic method is not an effective tool in the
exploration for disseminated sulphides because
their magnetic susceptibility is low.
* The electrical and electromagnetic methods appear
to be the most suitable survey techniques.
* The conductivity of a disseminated sulphide ore
body is highly variable because of the irregular
dispersion of the sulphides throughout the host.
Consequently, Resistivity and electromagnetic
anomalies are encountered.
* Since electrical conduction through the metallic
sulphides is not electronic, but electrolytic through
the host rock, disseminated sulphides produce
strong induced polarization anomalies. So that, the
induced polarization method is the most
appropriate to detect such bodies.
* The physical properties of economically important
sulphides such as chalcopyrite ores are not great
٨٢
different from zones of disseminated uneconomic
minerals such as pyrite. Hence, the economic
importance of a deposit cannot be judged solely
from its IP response and further geological and
geochemical surveying need to be executed prior to
any costly drilling program.
Iron ores:
* The most widely exploited physical property of iron
ores in geophysical exploration is their magnetic
susceptibility.
* The ratio of magnetite to haematite must be high
for the ore to produce significant magnetic
anomalies, as haematite is non – magnetic.
C- Geophysical in Hydrogeology:
* Many geophysical methods find application in
locating and defining subsurface water resources.
* The magnetic method is rarely used, but it can be
used to locate faults and shear zones which could
٨٣
affect the pattern of ground water flow and
determine the basement configuration underlying
the alluvial deposits.
* The gravity method is widely used in regional
reconnaissance surveys to delineate the from and
extent of porous sedimentary deposits such as
buried valley–fill, determining the configuration of
the bedrock surface over an area of recent surface
cover.
* The gravity method has also been used to determine
groundwater volumes from anomalous mass
calculations.
Observed Gravity
Anomaly
٨٤
* Seismic refraction method is widely used in hydro
geological investigations. They provide direct
information on the level of the water table since an
increase in water content causes a significant
increase of seismic velocity.
* The technique of fan – shooting may be adapted to
the location of buried channels and gravel filled
valleys which are important sources of
groundwater in regions of largely impermeable
bedrock.
* The most widely used geophysical methods in
hydrogeology are the electrical techniques.
Buried Valley Shot
point
Impermeable rock
Impermeable rock
Fan shooting
٨٥
* Resistivity surveys are routinely employed in
ground water exploration to locate zones of high
conductivity corresponding to saturated strata at
depths down to 400 ms.
* Resistivity surveys may also provide indications of
ground water quality.
* The Resistivity of the rock is controlled by the
volume of water present and will decrease as the
salinity of the water increases.
* In a homogeneous aquifer, it is possible to
distinguish fresh from saline ground water and
even to trace the subsurface flow of contaminated
ground water resulting from polluted water has a
distinctive resistivity.
D- Geophysics in engineering geology:
* Geophysical methods are frequently used in an
initial site investigation to determine subsurface
٨٦
ground conditions prior to excavation and
construction work.
* Both seismic refraction and vertical electrical
soundings are routinely employed in the
determination of overburden thickness for
foundation purposes.
* Magnetic surveys are occasionally used to delineate
zones of faulting in bedrock, and may be employed
in the location of buried , metallic, man made
structures such as pipelines or old mine working.
* Micro-gravimetric method may be used to detect
subsurface cavities, buried valleys, faults within
bedrock, underground workings and various
archaeological features.
* Resistivity method is used to detect the presence of
the subsurface voids which constitutes highly
resistive zones.
٨٧
* A recent ground based radar transmitter can be
used successfully to detect the subsurface voides. It
provides a shallow penetration continuous profile
of the subsurface similar to a seismic section.
* It may be noted that all the above survey
techniques find application in archaeological
investigations, where they may be used in the
delineation of buried buildings, walls, tombs and
other artifacts .
* Geophysical techniques have a major role in
offshore engineering activities such as: the
construction of harbors, tidal barrages and
offshore platforms, the laying of submarine
pipelines, and dredging.
* Such offshore constructions usually require detailed
information on the nature of the sea bed and the
thickness of any unconsolidated sediment layers.
٨٨
* Dredging , which may be carried out either to
establish and maintain a navigation channel in the
approaches to a harbour or to extract sand or
gravel from offshore banks, requires information
on the thickness and distribution of sediment
layers.
E- Geophysics in the investigation of the
Earth's crust.
* The crust is defined as that part of the Earth lying
above the Mohorovici discontinuity ( i.e. Moho,
after a Yugoslavian seismologist Andrijia
Solid Crust
5-65 Km Mantle
2900 Km
Solid core
2400
Km
Fluid Core
1100 Km
٨٩
Mohorovicic 1909) which is the boundary between
the crust and the mantle, below which the velocity
of compressional seismic body waves increases
abruptly to about 8.0 km/s.
* It is composed of a series of lithospheric plates in
relative of rocks.
* Large scale seismic refraction surveys, using
explosions as seismic sources, have been carried
out to study crustal structure in most continental
areas.
* Such surveys show that continental crust is typically
30-40 Km. Thick and it is internally layered. These
layers are:
Continental Crust
Oceanic Crust
Sedimentary layer
Moho
Upper mantle
٩٠
* Upper crust : which has seismic velocities in the
range 5.8to 6.3 km/s and may represent mainly
granitic or granodioritic rocks.
* Lower crust : which has seismic velocities in the
range 6.5 to 7 km/s and may represent igneous and
metamorphic rocks, including gabbro and basic
granulite.
* Marine seismic refraction surveys show that the
thickness of the ocean crust is 6 to 8 km, composed
of three layers with different seismic velocities
which are:
Layer Thickness
(km)
Velocity
(km/s)
Rock type
1 0 : 1.0 1.6 : 2.5 Sediments
2 1: 2 4 : 6 Pillow lava
3 4.5 : 5.5 6.5 : 7.0 Dolerite dykes and
gabbro
* Gravity surveying estimated regional variations of
crust thickness on the basis of variations in the level
of the Bouguer anomaly field.
٩١
F- Geophysics in the investigation of the
Earth's interior
* Most of the our methods for studying the interior of
the Earth are geophysical in nature.
* Our knowledge about the earth's interior is gained
from large earthquakes whose waves pass through
the entire earth.
* These knowledge comes from the behavior of these
waves as they travel through the earth ( i.e.
Moho
Surface
Lithosphere
Astenosphere Astenosphere
Zone of low
S-Wave velocity
Zone of slowly increased
S-Wave velocity
Zone of rapidly increased
S-Wave velocity
Mesosphere
70 Km
400 Km
٩٢
includes the changes in the velocities and paths of
the waves as they travel through different kind of
rocks and from solids to fluids).
* The key to our understanding of the earth's interior
is the knowledge of seismic wave velocities, because
from this we learn what kind of materials lie at
depth and how these materials are distributed.
* Earthquakes produce compression waves ( P- waves)
and shear waves or secondary ( S- waves) , together
called body waves because they travel through the
earth.
* P- waves vibrate in the direction of wave
propagation, and S- waves vibrate at right angles to
the direction of propagation.
* P- waves travel faster than S- waves and therefore
at recording station first.
٩٣
* From the focus of an earthquake, P and S waves
spread outward in all directions.
* The velocity of waves depends on both the elasticity
and the density of rocks through which the waves
travel.
* Elasticity is a measure of the degree to which a rock
deforms when subjected to stress. It generally
increases with depth.
* Density also increases with depth.
* Greater elasticity allows seismic waves to travel
faster, greater density slow them down.
٩٤
Layers of the Earth
1- Crust:
* In crust, there is a general increase
٩٥
* The Moho discontinuity is a boundary between
different types of rocks and is marked by sharp
increase in the velocities of both P and S waves.
2-Mantle:
* It has the greatest share of the earth's volume,
extending from a depth of about 20 Km. to 2900
Km.
* The mantle can be subdivided, based on seismic
wave behavior, into a number of layers:
a- lithosphere:
* It is the most important in the theory of plate
tectonics.
* It comprises the top part of the mantle and all the
crust, about 70 to 100 Km. thick composed of
strong, brittle rocks.
* The lithosphere is broken up into about two dozen
sections called plates, which are shifting position
٩٦
with respect to one another over the earth's
surface.
b- Astenosphere:
* It lies below the lithosphere and extends from about
70 Km. under the oceans and 100 Km. under the
continents to a depth of about 700 km.
* It is a weak material in contrast to the stronger
lithosphere.
* It is characterized by low seismic velocities,
particularly in the top part from 70 km (100 Km.
under continents) to about 400 km.
* In this section, S-wave velocities decrease from 4.7
km/s to 4.2 km/s. P-wave velocities also decrease.
This zone is called the low velocity zone.
* The decease in velocities is interpreted to mean that
partial melting occurs in the low velocity zone such
٩٧
decrease is called attenuation. A weak material has
greater attenuation than a strong material.
_____________________________
Example:
* The vibration of a bell: A good bronze bell low
attenuation and will vibrate when struck because
bronze is a strong material. But, a ball made of
lead is weak and has a high attenuation when
struck.
* The low velocities and high attenuation of seismic
waves in the astenosphere indicate that the
astenosphere is not nearly as rigid as the overlying
lithosphere.
* The significance of the partially molten
astenosphere is that the lithosphere can slide over
it. This movement is a vital part of the theory of
plate tectonics.
Amplitude Attenuation
٩٨
C- Mesosphere:
* It is be the lower part of the mantle (400 to 2900
km). In it, rocks are dense and highly elastic and
seismic wave velocities increase.
* At its base lies a thin transition zone in which S
waves die out quite rapidly.
3- core:
* It lies below the mesosphere and the transition zone,
little is known about the core.
* It plays no role in the movement of lithospheric
plates, but it is the source of the earth's magnetic
field.
* The P- seismic waves show a sharp drop in velocity
when they reach the core and that their velocities
increases as they travel through it, out with slower
velocities than in the mantle. Scientists conclude
٩٩
that the core is of much greater density than the
mantle.
* There is a discontinuity at about 5100 km. velocities
increase there, the wave behavior has led
seismologists to postulate a solid inner core.
* S- waves cannot travel through fluids because they
act to change the shape of a body. Water and air
can have their volumes changed by contraction or
expansion, but they cannot have their shapes
changed.
* S- waves are not refracted down into the core but
die out at the core-mantle boundary. This
convincing evidence for a molten outer core.
Continental crust versus oceanic crust
* The structure and composition of oceanic crust is
relatively simple compared with continental crust.
* The igneous part of the oceanic crust consists of
basalt, rich in iron and magnesium.
١٠٠
* The oceanic crust is uniform in thickness, being
about 10 km thick.
* Continental crust may be thin as 20 km and as thick
as 70 km under mountain ranges. It averages about
35 km thick.
* The rock in the upper 10 to 20 km. Have the
average composition of the igneous rock
granodiorite. Downward, the continent is composed
of the common metamorphic rock gneiss( i.e.
metamorphic equivalent of granodiorite).
* Continental and oceanic crust also differs in density.
Basalt is denser than granodiorite. For that, the
average density for continental crust is 2.7 gm/cc
and for oceanic crust is 3.0 gm/cc.
* About 65% of the earth's surface is underlain by
oceanic crust and about 35% by continental crust.
Although the oceans cover approximately 71% of
١٠١
the earth's surface, part of the ocean waters lie over
the edges of the continents and thus over
continental crust.
• Oceanic crust is relatively young, not greater
than 200 million years. Continental crust range
to as old as 3.8 billion years.
PLANNING AND COORDINATING
GEOPHSICAL WORK
* There is a special need to coordinate geophysical
work with geological investigations because they
are so interdependent.
* A geophysicist chooses field methods and traverses
on the basis of interpreted geology.
* A geologist uses geophysical information in making
an interpretation.
١٠٢
Preparing for geophysical surveys
A – preliminary considerations
1- Geophysical exploration models:
* These models depend on the information gained
from:
a- Geology of the area.
b- Contrasts between physical properties.
c- Probable range in depth of occurrence.
2- Objectives:
* It is important that the objectives of a geophysical
survey should be clear at the beginning.
* The geophysical survey produced poor results for
the following reasons:
1- Inadequate and / or bad planning of the
survey.
2- Incorrect choice or specification of technique.
3- Insufficient experienced personnel conducting
the investigation.
١٠٣
* For cost effective, experienced geophysical
consultants are employed for survey design, site
supervision and final reporting.
١٠٤
* The objective will be to do the work within the best
of some sequences such as:
a- Limits in cost.
b- Time.
c- Scheduling.
Survey constraints (limitations):
1- Finance: how much is the survey going to cost
and how money is available?
* The cost of the survey will depend on:
• Where the survey is to take place?
• How accessible the proposed field site is?
• What scale the survey is to operate?
• The more complex the survey in terms of
equipments and logistics, the greater the cost is
likely to be.
• It is important to remember that the geophysics
component of a survey is a part if an exploration
program and thus the costs of the geophysics
١٠٥
should be viewed in relation to those of the whole
project.
• The factors that influence the various components
of a budget also vary from country to country
and form job to job.
• Some of the basic elements of a survey budget are
given in the following table:
Staffing Management, technical, support,
administration, etc.
Operating costs Including logistics
Cash flow Assets versus usable cash
Equipment For data acquisition and data reduction
analysis – computers and software
whether or not to hire or buy.
Insurance To include liability insurance as
appropriate
Overheads Administration, consumables, etc.
Development
costs
Skills, software, etc.
Contingencies Something is bound to go wrong at some
time, usually when it is most convenient.
١٠٦
• The main people to be involved in a survey are:
Geologists, Geophysicists, Surveyors.
• Vehicles and equipments will need maintaining ,
so skilled technicians and mechanics may be
required.
• Everybody has to eat and it is surprising how
much better people work when they are provided
with well prepared food: a good cook at base
camp can be a real asset.
• Due considerations should be paid to health and
safety and any survey team should have staff
trained in First Aid.
• Local labor (workers) may be needed as porters
(carriers), guides, translators, guards.
• In some countries, access to a survey site in dry
season may be possible whereas during the rains
١٠٧
of the wet season, roads may be totally
impossible.
• Also access to land for survey work can be
severely hampered during the growing season
with some crops reaching 2-3 meters high.
• some survey such as seismic refraction and
reflection may cause a limited amount of
damage for which financial compensation may
be sought.
• Consideration has to be given to the transport
of the geophysical and other equipments.
• It may even be necessary to make provision (
arrangement ) for a bulldozer to excavate a
rough road to provide access for vehicles.
• Other constraints (limitations) are those
associated with politics, society, and
religion:
١٠٨
Political limitations:
• This means gaining permission form land
owners tenants ( ]^_`abcdefا) for access to land
and communications with clients which
often requires great diplomacy.
• It is important to have a permission from
the appropriate authority to carry out
geophysical field work.
Examples: permissions from a local council if
survey work along a major road is being
considered. Permissions from the local
harbour master in case of marine surveys to
safe other shipping.
Social limitations:
• In designing the geophysical survey, the
questions must be asked" Is the survey
technique socially and environmentally
acceptable?
١٠٩
• It is always best to keep on good terms with
the local people. Treating people with
respect will always bring dividends (hiاjk).
• Each survey should be socially and
environmentally acceptable and not cause a
nuisance (problems). An example is in not
choosing to use explosives as a seismic
source for reflection profiling through
urban areas or night . Instead, the seismic
vibrator technique should be used.
• Another example: an explosive source for
marine reflection profiling would be
inappropriate in area associated with a
fishing industry because of possibly
unacceptable high fish kill.
Religious limitations:
• Religious traditions must be respected to
avoid difficulties. The survey should take
into account local social customs such as:
١١٠
• Muslims like to go to their mosques on
Friday afternoon and are thus unavailable
for work then.
• Similarly , Christian workers do not like to
work on Sundays or Jews on Saturdays.
Geophysical survey design:
A- Target identification:
• Geophysical methods locate boundaries
across which there is a marked contrast in
physical properties. Such a contrast gives
rise to geophysical anomaly which indicates
variations in physical properties relative to
some background value.
١١١
١١٢
o The physical source of each anomaly is termed
the geophysical target. Some examples of targets
are trap structures for oil and gas, mineshafts,
pipelines, ore bodies, cavities , groundwater,
buried rock valleys.
o In designing a geophysical survey, the type of
target is of great importance. Each type will
dictate (tell) to a large extent the appropriate
geophysical method (s) to be used.
١١٣
Examples:
- Consider the situation where saline water intrudes
into a near- surface aquifer, saline water has a high
conductivity ( low resistivity ) in comparison with
fresh water and so is best detected using electrical
resistivity or electromagnetic conductivity methods.
- Gravity method would be in–appropriate in this
case because there would be no density contrast
between the saline and fresh water.
- Similarly, seismic methods would not work as there
is no significant difference in seismic wave
velocities between the two saturated zones.
- Also, the shape and size of the target is important to
know. In the case of a metallic ore body, a mining
company might need to known its lateral and
vertical extent. This comes from the amplitude of
the anomaly (I .e. its maximum peak-to–peak
value).
١١٤
B- optimum line configuration:
• There is an important question in this case:"
How are the data to be collected in order to
define the geophysical anomaly ? Two concepts
need to be introduced, namely: profiling and
mapping.
a- profiling :
* It is a mean of measuring the variation in a
physical parameter along the surface of a two
dimensional cross section.
* The best orientation of a profile is normally at right
angles to the strike of the target. Indication of
١١٥
geological strike may be obtained from existing
geological maps, mining records, etc..
• The length of the profile should be greater than
the width of the expected geophysical anomaly to
define a background value.
• Data values from a series of parallel lines or from
a grid can be contoured to produce a map on
which all points of equal values are joined by
isoclines
• A great care has to be taken over the methods of
contouring or else the resultant map can be
misleading.
١١٦
C- Selection of station intervals:
* The point at which a geophysical measurement is
made is called a station and the distances between
successive measurements are station intervals.
* The success of a geophysical survey depends on the
correct choice of station intervals. It is a waste of
time and money to record too many data and
equally wasteful if too few are collected.
* How is a reasonable choice of station intervals to be
made?. This requires some idea of the nature and
size of the geological target.
* Any geophysical anomaly found will always be
larger than the feature causing it. For example: to
find a mineshaft with a diameter of two meter, an
anomaly with a width of at least twice this might be
expected. Therefore, it is necessary to choose a
station interval that is sufficiently small to be able
to resolve the anomaly.
١١٧
* Reconnaissance survey tend to have coarser station
intervals in order to cover a larger area quickly
and to indicate zones over which a more detailed
survey should be conducted with a reduced station
interval and a more closely spaced set of profiles.
١١٨
* The figure (A) shows a typical electromagnetic
anomaly for a buried gas pipe. The whole anomaly
is 8m wide. If a 10m sampling interval is chosen,
then it is possible either to clip the anomaly as
shown in figure (B) or to miss it entirely (fig. C).
* The resultant profile with 2 m and 1m sampling
intervals are shown in figures (D) and (E)
respectively.
* The smaller the sampling interval, the better the
approximation is to the actual anomaly.
* The loss of high- frequency information, as in
figures (B) and(C), is a phenomenon known as
know as spatial aliasing .
* Another from of spatial aliasing may occur when
gridded data are contoured, particularly by
computer software. For example figure 1.8 A
shows a hypothetical aeromagnetic survey. This
١١٩
map was complied from contouring the original
data at line spacing of 150m.
* Figures (B) and (C) were contoured with line
spacing of 300m and 600m respectively.
* The difference between the three maps is very
marked, with a significant loss of information
between figures (A) and (C).
* The higher frequency anomalies have been aliased
out, leaving only the longer wavelength (lower
frequency) features.
١٢٠
* In addition, the orientation of the major anomalies
has been distorted by the crude contouring in
figure (C).
* The spatial aliasing can be removed or reduced
using mathematical functions, which provide
١٢١
means of developing a better gridding scheme for
profile line- based survey.
* Similar aliasing problems associated with
contouring can arise from radial survey lines
and/or too few data points as shown in figure 1.9.
١٢٢
* Figures (A) and (B) both have 64 data points over
the same area. In figure (A) the orientation of the
contours follows that of the line of data points to
the top left – hand corner, whereas the orientation
is more north- south in figure(B).
* Figure (C) shows the inadequacy of the number of
data points, which is based on only 13 data values,
forming concentric rounded contours. On the
other hand, figure (D) has been compiled on the
bases of 255 data points and exposes the observed
anomalies much more realistically.
D- Noises
* When a field survey is being designed it is
important to consider what extraneous data
(noise) may be recorded.
* There are various sources of noises:
a- Man made sources (cultural noise): such as
electric cables, vehicles, pipes drains.
١٢٣
b- Natural sources: such as wind and rain, waves,
and electric and magnetic storms.
• Electrical resistivity survey should not be
conducted close to or parallel to metal pipes, nor
parallel to cables as power lines will induce
unwanted voltages in the survey wires.
• Before a survey start, it is always advisable to
consult with public utility companies to provide
maps of their underground and overhead
facilities.
* It is important to check on the location of water
mains, sewers ( underground pipes for carrying
off sewage or rainwater), gas pipes, electricity
cables, telephone cables and cable-television
wires.
* Such utilities may mask any anomalies caused by
deeper- seated natural bodies.
١٢٤
* It is also worth checking on the type of fencing
around the survey area. Wire mesh and metal
sheds can affect on the electromagnetic and
magnetic surveys.
* Cultural and unnecessary natural noise can often be
avoided or reduced significantly by careful survey
design.
* Modern technology can help to increase the signal –
to – noise ratio, so that even when there is a degree
of noise present, the important geophysical signals
can be enhanced above the background noise levels.
١٢٥
E – data analysis
• As automatic data logging and computer analysis
are becoming more common, it is increasingly
important to standerdise the format in which the
data are recorded to ease the portability of
information transfer between computer systems.
• This also makes it easier to download the survey
results into data processing software packages.
• To make computer analysis much simpler it
helps to plan the survey well before going into
the field to ensure that the collection of data and
the survey design are appropriate for the type of
analyses anticipated .
١٢٦
• To get a reliable analysis, the following questions
must be considered:
1- How reliable is the software?
2- Has it been calibrated against proven manual
methods, if appropriate?
3- What are the assumptions on which the software
is based and under what conditions are these no
longer valid, and when will the software fail to
cope ( succeed) and then to produce erroneous
results?
* Unfortunately, there are no guidelines are accepted
standards for much geophysical software apart
from those for the major seismic data processing
systems.
* However, the judicious (having sound judgment)
use of computers and of automatic data – logging
methods can produce excellent results.
١٢٧
F- Procedure:
* One or more organizations will be capable of doing
the job. In order for then to set up a tentative
procedure and offer their services, the following
conditions must be taken into account:
a- Size of the area.
b- Degree of detail needed.
c- Orientation of survey lines and spacing of survey
stations.
d- Type of coverage needed (complete or partial).
e- Sensitivity required in each proposed method.
f- The format of the data to be delivered (raw data,
contoured data, interpreted data).
g- Scheduling of the job.
h- Kind of terrain involved, seasonal
characteristics, and field base facilities.
١٢٨
B- Preparations for geophysical work.
1- Before the job gets underway, the geologist and
geophysicist will design a specific program in
which the following points should be covered:
a- Geological conditions :
• Using existing geologic maps and prior
geophysical surveys to indicate discontinuities
and lithologic contrasts, the geologic pattern
will be related in detail to physical properties
such as density, conductivity and magnetic
susceptibility.
b- Sources of noise:
• Possible sources of terrain noise are swamps
and conductive overburden.
• Sources of cultural noise are mines, pipelines
and town sites.
c- Access:
• The geologist should have some information on
physical conditions of access such as roads,
terrain and weather.
١٢٩
• There will be some legal conditions of access. For
example, the geophysicist may need formal
permission to enter the land, permits to bring
geophysical equipment into the country and
work permits for personnel.
d- facilities:
* If the geologist or geophysicist is going to work in
an area, something must be known about the
facilities such as: supplies, campsites maintenance
and repair
2-Scheduling:
• The season, the time allowable for completion of
the job and possible delay and extensions are
taken into account.
a- Season:
• Weather conditions may decide the best
flying season and the best season for ground
access.
١٣٠
• In tropical areas man soon rains may make
geophysical work impossible during certain
months.
• In arctic areas: winter weather and
darkness will restrict certain types of
geophysical work.
• The best season for geophysics in a
particular region will also be the busiest for
contractors and there may be a shortage of
available crews.
b- Delays:
• It is almost impossible to avoid some delay due
to weather, equipment malfunction, magnetic
storms and unexpected problem with land,
govemment and people.
• The scheduling should therefore be flexible
enough to permit alternative geophysical
methods and traverses.
١٣١
3- Extension:
• Extra geophysical traverses may be needed
while the work is in progress. Survey lines
may have to be extended into nearby areas
(sometimes into areas not yet controlled by
permits or claims).
4-Sampling and orientation:
• Samples for the laboratory determination of
geophysical parameters can be furnished by
the geologist.
• The geologist and geophysicist may take an
orientation tour of the most significant
outcrops, across a known ore body to identify
a representative ore body in the area or in
some analogous area.
5-Survey control:
* Existing maps and aerial photographs will need
to be studied.
١٣٢
6- Subsurface information :
* Key information from stratigraphic sequences,
samples from depth and dimensions from
profiles are important to geophysical work.
* Drill holes may be planned to obtain
information in the most critical locations.
* In some instances, a few extra meters of
drilling to intersect a significant boundary in
physical characteristics or an inexpensive
noncore hole to the base of overburden may be
worthwhile with respect to geophysics.
* Down hole geophysical information is directly
applicable to surface geophysics.
* Certain drill holes may therefore be filled with
heavy mud or lined with plastic casing and
kept open for geophysical logging.
١٣٣
C- Coordination work during a geophysical
survey.
1- Sorting of apparent anomalies:
Some specific work may be needed to strengthen or
verify preliminary interpretations.
2- Key drilling and trenching:
Subsurface information may be needed for depth
control points.
3- Providing for extended coverage:
Earlier ideas on the limits of an exploration target
may be changed by the geophysical data.
Additional work may be needed.
D- Follow–up work:
* After the job has been completed, the geophysicist
will interpret the data and additional work may
be needed to confirm geophysical interpretations,
appropriate targets will be drilled.
١٣٤
* Geophysical surveys are keys to the depth
dimension but they are delicate keys.
* A geophysical survey is a job for specialists and the
interpretation of geophysical data is a job for
experts.
* Both specialists and experts know that their work
would be of limited value if it had no geologic
guidelines.
* Geologic specialists and experts know that their
work would be severely limited without geophysical
information.
References
1-Reynolds J.M. (1997): An introduction to applied
and environmental geophysics. John Wiley and
sons Ltd, England.
2-Telford W.M., Geld art L.P., sheriff R.E. and Keys
D.A (1990): Applied geophysics, 2nd ed. Cambridge.
١٣٥