1. The document summarizes various geophysical methods used in hydrocarbon exploration, including gravity, magnetics, electromagnetics, seismic reflection, and others. 2. It provides details on the basic theories, principles, and applications of each method. Gravity relies on density variations, magnetics on magnetic susceptibility. 3. Seismic reflection involves analyzing travel times of reflected seismic waves to image subsurface geology based on elastic properties and density.

1. Soran University
Faculty of Engineering
Department of Petroleum Engineering
Petroleum Geophysics Report
Supervised by: Sarkar Muheddin
Geophysical Methods in Hydrocarbon Exploration
by:
Raboon Redar
Usama Yusif
Summer 2020

2. Contents
Introduction: ...............................................................................................................................................................3
Geophysical Methods:.................................................................................................................................................3
Table 1 Geophysical methods..............................................................................................................................4
1. Gravity....................................................................................................................................................................4
1.1 Basic Theory.....................................................................................................................................................4
1.2 Units of gravity..................................................................................................................................................5
1.3 Measurement of gravity.....................................................................................................................................5
Figure 1.1 Principle of stable gravimeter operation. .............................................................................................5
Figure 1.2 Principle of the LaCoste and Romberg gravimeter. .............................................................................6
Figure 1.3 Disturbances in acceleration ...............................................................................................................6
1.4 Rock densities ...................................................................................................................................................6
Table 1.2 Approximate density (Mg m-3
) ranges of some common rock types and ores. .......................................7
2. Magnetic Surveying ................................................................................................................................................7
2.1 Basic concepts...................................................................................................................................................8
Figure 2.1 The magnetic Flux surrounding a magnet............................................................................................8
Figure 2.2 Vector diagram illustrating the relationship between (Ji), (Jr) and (J). .................................................9
Figure 2.3 Histogram showing mean values and ranges in susceptibility of common rock types. ..........................9
2.2 Rock Magnetism ...............................................................................................................................................9
Figure 3.1 Magnetic Field variations..................................................................................................................10
3. Electromagnetic surveying ....................................................................................................................................10
Figure 3.2 General Principal of Electromagnetic Surveying. ..............................................................................10
3.1 Electromagnetics in Sea Bed Logging..............................................................................................................11
Figure 3.3 SBL mapping of resistivity. ..............................................................................................................11
4. Seismic Reflection Surveying................................................................................................................................11
4.1 Geometry of reflected ray paths.......................................................................................................................11
Figure 4.1 Vertical reflected ray paths in a horizontally-layered ground. ............................................................11
4.1.1 Single horizontal reflector.........................................................................................................................12
Figure 4.2......................................................................................................................................................12
4.2 The reflection seismogram...............................................................................................................................12
4.2.1 The seismic trace......................................................................................................................................12
Figure 4.3......................................................................................................................................................13
4.2.2 The shot gather .........................................................................................................................................13
Figure 4.4......................................................................................................................................................13
4.2.3 The CMP gather .......................................................................................................................................14
Figure 4.5......................................................................................................................................................14

3. Introduction:
The first pages (introduction and geophysical methods) are for readers not previously acquainted with
geophysical approaches and are lined up at the basic level. Readers who are already acquainted with the
basic concepts and limitations of geophysical analysis and would like to gain more information about
seismic reflection surveying should pass them over. The geophysics research applies to the earth's analysis
the principles of physics. Geophysical studies of the earth's interior include measurements on or near to the
earth's surface that are dependent upon the physical properties of the internal distribution. Examination of
these measurements can show how vertically and laterally the physical properties of the Earth's interior
differ. Through operating on different scales, geophysical techniques can be used to investigate the localized
upper crust area for engineering or other purposes (e.g. Vogelsang 1995, McCann et al. 1997) through a
range of investigations ranging from studies of the entire world (global geophysics; e.g. Kearey & Vine
1996). Of course, an alternative method of studying subsurface geology is to boil boreholes, but they are
costly and provide information at specific locations only. It is such that the fundamental principles and the
scope of the methods and their key application fields should be understood by any practicing Earth scientist
that geological examinations are necessary to extract geological knowledge from the subsurface. The main
purpose of this study is to provide a summary of the fundamental geophysical methods for the discovery of
hydrocarbons.
Geophysical Methods:
In a natural field (passive) system, seismic waves produced with the velocity of propagation and the
transmission paths through the surface are monitored to provide information regarding the distribution of
geology boundary information at depth, using the gravitational, magnetic, electrical, and electromagnetic
fields of the Earth. Natural field methods can typically provide earth property knowledge to far greater
depths and are logistically simpler to implement than artificial source methods. Nevertheless, the energy
provided by artificial means is able to provide the sub-surface geology with a comprehensive and better
resolved image. Various methods of geophysical analysis can be used at sea or in the soil. Increased
operational speed and gain from being able to track areas where access to land is difficult or impossible
compensate for the greater capital and operating costs associated with marine or aerial work. There exists a
wide range of geophysical methods, each of which is subject to a "operational" physical property. The
methods are listed in Table 1.

4. Method Measured Parameter Operative physical Property
Seismic Travel times of reflected/refracted seismic
waves
Density and elastic moduli, which
determine the propagation
velocity of seismic waves
Gravity Spatial variations in the strength of
the gravitational field of the Earth
Density
Magnetic Spatial variations in the strength of the
geomagnetic field
Magnetic susceptibility and
remanence
Electrical
- Resistivity Earth Resistance Electrical conductivity
- Induced
polarization
Polarization voltages or frequency-dependent
ground resistance
Electrical capacitance
- Self-potential Electrical potentials Electrical conductivity
- Electromagnetic Response to electromagnetic radiation Electrical conductivity and
inductance
- Radar Travel times of reflected radar pulses Dielectric constant
Table 1 Geophysical methods.
1. Gravity
Gravity studies analyze the surface geology based on variations in density between subface-surface rocks in
the Earth's field in gravity. The notion of a causative body is a fundamental definition, a rock unit of
different density than its surroundings. The origin body is an anomalous mass subsurface region which
creates a localized disruption in the gravity field known as an anomaly. The ability to perform gravity tests
in marine or, to a lesser degree, airborne regions expands the method's reach so that the procedure can be
used in most regions around the world.
1.1 Basic Theory
Newton's gravity law, which states that the attraction force F between two masses (m1 and m2) whose
dimensions are small relative to the distance r between them, is the basis of the gravity survey system
………………………………………………………………………………………….. (1.1)
G: Gravitational constant (6.67 × 10-11
m3
kg-1
s-2
)

5. Consider the spherical, non-rotating, homogeous earth of mass (M) and radius (R) gravitable attraction on its
surface with a small mass (m). By substitution in equation 1.1
…………………………….……………………………………………. (1.2)
r: the Earth’s radius
M: the mass of the Earth
m: the mass of a small object
Force is related to mass by acceleration and the term g = GM/r2
is known as the gravitational acceleration or,
simply, gravity. The weight of the mass is given by mg.
In terms of gravitational potential U, the gravitational field is best defined:
…………………………………………………….……………………………………………. (1.3)
Whereas the vector quantity of the gravitational acceleration g is both of size and direction (vertically
downward), the gravitational potential (U) is a scalar, with only distance. The first (U) derivative in any
direction gives the gravitational component in this direction.
1.2 Units of gravity
The average gravity value at the Earth's surface corresponds to approximately 9.8 ms-2.Differences in
gravity caused by differences of density on the surface are in the range of 100 mms-2. A gravity survey on
the land quickly achieves an accuracy of ±0.1 gu, which equates to around 100 millionths of the natural
gravitational field. In terms of precision, roughly ±10 g can be obtained. The milligal (1 mgal=10-3 gal=10-
3cms-2 or 10-5ms-2) gram of the second centimeter is the equivalent of 10 gu.
1.3 Measurement of gravity
Since gravity is acceleration, it can be measured merely with length and time measurements. Nevertheless,
the precision and accuracy of the gravity survey needed for these seemingly
simple measurements are not easily achieved. Measuring an absolute gravity
value is challenging, requiring complex instruments and a lengthy observation
time. Gravitational measures are referred to as gravitational meters or
gravitational meters. Gravimeters are primarily balances with a constant
weight. Changes in body weight due to changes in gravity contribute to
differences in spring length and quantify changes in gravity. In Fig. 1.1 a
spring of initial length s has been stretched by an amount (ds) as a result of an
increase in gravity (dg) increasing the weight of
the suspended mass (m). The extension of the spring is proportional to the
extending force (Hooke’s Law) thus,
Figure 1.1 Principle of
stable gravimeter operation.

6. …………………………………………………………………………......... (1.4)
k: the elastic spring constant.
(ds) must be measured to a precision of 1 : 108 in instruments suitable
for gravity surveying on land.
The LaCoste and Romberg gravimeter are an example of an unstable
instrument. The meter consists of a hinged beam, carrying a mass,
supported by a spring attached immediately above the hinge Fig. 1.2.
The magnitude of the moment exerted by the spring on the beam is
dependent upon the extension of the spring and the sine of the angle q.
Rising gravity would compress the beam and further increase the
spring. Although the restoring force of the spring is increased, the angle
q is decreased to q¢.
Gravimeters for general surveying use are capable of registering
changes in gravity with an accuracy of 0.1 gu. Gravity can be measured at discrete locations at sea using a
remote-controlled land gravimeter, housed in a waterproof container, which is lowered over the side of the
ship and, by remote operation, leveled and read on the sea bed.
Gravity anomalies (in milligal, 10-5
ms-2
), i.e. deviations from a predefined reference level, geoid (a surface
over which the gravitational field has equal value)
1.4 Rock densities
Gravity anomalies are the result of the difference between a rock body and its surroundings in density or in
density contrast. For a body of density ρ1 embedded in material of density ρ2, the density contrast Δρ is
…………………………………………………………………………………………….. (1.5)
Figure 1.2 Principle of the
LaCoste and Romberg gravimeter.
Figure 1.3 Disturbances in acceleration

7. Most common rock types have densities in the range
between 1.60 and 3.20Mgm-3
. The rock's density
depends on its structure and porosity. The principal cause
of variation in density in sedimentary rocks is porosity
variation. Thus, due to the compaction and aging, density
in sedimentary rock sequences tends to increase with
depth. The majority of ignorant and metamorphic rocks
have negligible porosity, and the main cause of density
variability is their composition. The rock's density
depends on its structure and porosity. The principal cause
of variation in density in sedimentary rocks is porosity
variation. Thus, due to the compaction and aging, density
in sedimentary rock sequences tends to increase with
depth. The majority of ignorant and metamorphic rocks
have negligible porosity, and the main cause of density
variability is their composition.As acidity decreases, the
density usually increases; hence the density rises from
acid to critical to ultrabasic igneous rock. Table 1.1
includes levels of density for ordinary forms of rock and ores. Direct measurements on rock samples usually
assess density In air and water a sample is measured. The weight difference provides the sample volume and
so it is possible to achieve the dry density. The saturated density can be measured after saturating the rock
with the water if the rock is porous. It should be stressed that the density of any particular rock type can be
quite variable. Consequently, it is usually necessary to measure several tens of samples of each particular
rock type in order to obtain a reliable mean density and variance.
2. Magnetic Surveying
The purpose of a magnetic survey is to investigate the subface geology based on magnetic field variations in
the Earth arising from the magnetic properties of the rock underneath. While most minerals that shape rock
are essentially non-magnetic, certain forms of rock contain magnetic minerals necessary to cause significant
magnetic anomalies. Magnetic irregularities are often caused by man-made iron artifacts. Magnetic surveys
therefore have a broad range of uses, from small-scale scientific or archeological surveys to the
identification of buried metal artifacts, to large-scale surveys to investigate regional geological structures.
On land, at sea and in the air, you can carry out magnetic surveys. The technique is therefore widely used,
and the speed at which airborne surveys work is carried out makes the process very attractive in the search
for ore deposition forms containing magnetic minerals.
Table 1.2 Approximate density (Mg m-3
) ranges of
some common rock types and ores.

8. 2.1 Basic concepts
A magnet flux flowing from one end of the magnet to the other may be produced in the vicinity of a bar
magnet Fig. 2.1. This flow can be traced from the directions of an inside compass needle. The magnet points
where the stream converges are known as the magnet's poles. The flux of the Earth's magnet field is
similarly connected by a freely suspended bar magnet. The pole of the magnet which tends to point in the
direction of the Earth’s North Pole is called the north-seeking or positive pole, and this is balanced by A
pole of the same strength south or negative at the opposite end of the magnet. The force F among two m1
and m2 magnetic poles, divided by distance r:
……………………………………………………………………………………………… (2.1)
Where μ0 and μR are constants corresponding to the magnetic permeability of vacuum and the relative
magnetic permeability of the medium separating the poles.
The magnetic field (B) due to a pole of strength mat a distance r from the pole is defined as the force
exerted on a unit positive pole at that point.
…………………………………………………………………………………………........ (2.2)
In the same way as in gravitational fields, magnetic fields can be described as magnet potential. For a single
pole of strength (m), the magnetic potential V at a distance (r) from the pole is given by:
………………………………………...………………………………………………………. (2.3)
A partial derivative of the potential against that direction is given to the magnetic field portion in either
direction. The magnetic parameters of the flow of electrical current are
described in the SI system of units. A partial derivative of the potential
against that direction is given to the magnetic field portion in either direction.
The magnetic parameters of the flow of electrical current are described in the
SI system of units. The magnitude of H is proportional to the sum of turns in
the coil and current power, which is inversely proportional to the length of
wire so that H is expressed in Am-1. The density of the magnets is, as it is,
measured in Am. The magnitude of a currents is measured in Am-1. B is
proportional to Hand defined as magnetic permeability as the constant of
proportionality μ.
As in Fig. 2.2 any rock containing magnetic minerals may possess both
induced and remanent magnetizations Ji and Jr. The relative intensities of
induced and remanent magnetizations are commonly expressed in terms of
the Königsberger ratio, Jr : Ji.These may be in different directions and may differ significantly in magnitude.
Figure 2.1 The magnetic
Flux surrounding a magnet.

9. The magnitude of J controls the amplitude of the magnetic anomaly and the orientation of J influences its
shape. The intensity of induced magnetization Ji of a material is defined as the dipole moment per unit
volume of material:
………………………………………………………………… (2.4)
M: is the magnetic moment of a sample
L: Length
A: Cross-sectional area
Ji is consequently expressed in Am-1
. In the c.g.s. system intensity of
magnetization is expressed in emucm-3
(emu = electromagnetic unit), where
1 emucm-3
= 1000Am-1
. The induced intensity of magnetization is
proportional to the strength of the magnetizing force H of the inducing
field:
…………………………………………………………………………………………………… (2.5)
K: the magnetic susceptibility of the material.
2.2 Rock Magnetism
The most common rock minerals are very
magnetically sensitive and rocks owe their
magnetic properties to a limited proportion
of magnetic minerals. These minerals are
provided by only two geochemical groups.
Magnetite with a curie temperature of 578 °
C is by far the most common magnetic
mineral. Specific igneous rocks are
typically highly magnetic despite their
relatively high amounts of magnetite. The
amount of magnetite in the igneous rocks
continues to decrease as acidity increases,
meaning that acidic ignorant rocks are less
magnetic than simple rocks while they have
varying magnetic behavior. Through their
magnetic character, metamorphic rocks are also variable. If partial oxygen pressure is relatively low, then
magnetite is resorbed and, as the degree of metamorphism increases, iron and oxygen are applied to other
mineral stages. Magnetic sensitivity (K): A dimensional less property that basically quantify the
Figure 2.2 Vector diagram
illustrating the relationship
between (Ji), (Jr) and (J).
Figure 2.3 Histogram showing mean values and ranges in
susceptibility of common rock types.

10. susceptibility of a substance to magnetization and shows in an histogram the susceptibilities of different rock
types Fig. 2.3. Lateral variation in magnetic resistance
and remanence contributes to magnetic field spatial
variations. It has so-called magnetic anomalies, that is to
say deviations from the Earth’s magnetic field. The unit
of measurement is the tesla (T) which is volts·s·m-2
in
magnetic surveying the nanotesla is used (10-9
T)
Examples of natural magnetic elements are iron, cobalt,
nickel, and gadolinium. Examples for Ferromagnetic
minerals are magnetite, ilmenite, hematite and
pyrrhotite.
3. Electromagnetic surveying
Electromagnetic methods use the soil responses in a plane perpendicular to the direction of travel to
disperse incident, alternating electro-magnetic water waves, compounded by two orthogonal vector
elements, the electrical strength (E) and the magnetizing force (H). Primordial electromagnetic fields can be
produced through a small belt consisting of many wires or a large wire loop, by moving alternating current.
The soil's reaction is secondary electromagnetic fields, and the resulting fields can be sensed via the
alternate currents that enable the mechanism of electromagnetic induction to flow through a receiver coil.
The main electromagnetic field travels through paths above and below the surface from the transmitter spool
to the receiver spiral. There is no distinction between the areas distributed above the surface where the
subsurface is homogeneous. Nevertheless, if a
conductive body is present, it causes alternating currents
or eddy currents to flow in the conductor through a
magnetic constituent of the electromagnetic field that
penetrates the ground Fig 3.2. Eddy currents generate its
own electromagnetic secondary field that travels to the
receiver. The receiver then replies to the resulting
primary and secondary fields such that the response
varies from the answer to the primary field alone in phase
and amplitude. These variations between the
electromagnetic fields transmitted and obtained show the
existence of the driver and provide details on its geometry and electrical properties. The current flow
induction results from the electromagnetic field magnetic portion. Therefore, either the transmitter or the
receiver is not physically in contact with the earth. Surface EM surveys can therefore be performed much
Figure 3.1 Magnetic Field variations.
Figure 3.2 General Principal of
Electromagnetic Surveying.

11. faster than electrical surveys, where contact with the ground is important. Most specifically, the transmitter
and receiver may be mounted in or hidden behind aircraft.
3.1 Electromagnetics in Sea Bed Logging
Fig 3.3 SBL is a marine electromagnetic system that can
map the resistivity of the surface remotely. SBL is based
on the use of a HED source which transmits a low
frequency electromagnetic signal and a number of
Seafloor electric field receptors. In a hydrocarbon filled
reservoir, shale and water filled reservoirs typically have
high resistant characteristics. SBL thus has the unusual
ability to differentiate between a filled hydrocarbon and
a reservoir filled with water. If there is salty water in a
porous structure, the overall strength is small. The restiveness of the shape is high whether it contains
hydrocarbons or has very low porosity. When oil-based mud is used and water flows, deeper logs are more
conductive than the zone under invasion.
4. Seismic Reflection Surveying
The most famous and renowned geophysical technique is seismic reflection surveying and the most
powerful method of 2D/3D subsurface mapping. This is commonly used in hydrocarbon fields by the oil and
gas industry. Seismology for reflection can be considered as echo and
depth sounding and is done at sea more effectively than on land.
Sections can now be produced to expose geological structural features
on scales from the top ten meters to the entire lithosphere.
4.1 Geometry of reflected ray paths
Seismic reflecting studies reflect seismic energy pulses from surface
interfaces and record them on the surface at near-normal conditions.
The time of traveling is estimated and the depths to interfaces can be
determined. Due to the different physical properties of each layer
velocity varies depending on depth. Owing to the lateral lithological
shifts within the various layers, velocity can also vary horizontally but
ignored for the first approximations. Fig 4.1 shows a simple physical
model of horizontally-layered ground with vertical reflected ray paths
from the various layer boundaries. If zi is the thickness of such an
Figure 3.3 SBL mapping of resistivity.
Figure 4.1 Vertical reflected ray
paths in a horizontally-layered
ground.

12. interval and 𝜏i is the one-way travel time of a ray through it, the interval velocity is given by:
For several depths, Vaverage is
∑
∑
4.1.1 Single horizontal reflector
Fig 4.2(a) shows the basic geometry of the reflected ray
path for the simple case of one horizontal reflector under a
homogeneous top-floor speed layer V lying at depth z.
√
………………………………………. (4.1)
Substituting x = 0 in equation (4.1), the travel time t0 of a
vertically reflected ray is obtained:
This form of the travel-time equation (4.2) suggests the
simplest way of determining the velocity V.
………………………………………. (4.2)
4.2 The reflection seismogram
The picture of an individual detector 's output in a reflection period is a visual representation, for a short
time after a near seismic source is triggered, of the local Vertical ground movement pattern (on ground) or
variation of pressure (on sea). This seismic trace is the combined layered soil and seismic pulse sensitivity
of the recording system. Any show of one or more seismic traces is referred to as a seismogram. A selection
of such traces that reflect a number of sensors' answers to the energy of a single shot is known as a shot
gather. A collection of traces of the seismic response at one mid-point surface is called a common mid-
point gather (CMP gather). The main purpose of seismic reflection processing is to collect and transform
seismic traces for the individual CMP into a component of the image presented as a seismic section.
4.2.1 The seismic trace
A proportion of the incident energy is returned to the detector at each layer boundary. The proportions are
determined by the contrast in both layers' acoustic impedances, and the reflective coefficient can be easily
Figure 4.2 (a) Section through a single
horizontal layer showing the geometry of
reflected ray paths and (b) time–distance curve
for reflected rays from a horizontal reflector.
DT = normal moveout (NMO).

13. measured for a vertically moving ray. The relationship of geologic layering, changes in acoustic impedance
and reflection coefficients as a function of depth are shown in Fig 4.3 the detector is supplied with a number
of reflected pulse sizes according to the distance and reflective coefficients from the different limits of the
layer. The pulse happens periodically by the depths of the limits and the rates of propagation.
In the seismic reflection survey,
seismic traces are registered and an
attempt can be made to reconstruct the
various columns of Fig. 4.3 Seismic
processing, moving from the right to
the left. This will involve:
• Noise removal
• Determination and removal of the input pulses to provide reflectivity
• Determine velocity for conversion from axis to axis of depth
• Acoustic impedances of the formation (or the associated characteristics).
4.2.2 The shot gather
Ordinarily the original display of seismic profile data is recorded from a common firefight known as a
common firefighting point or, simply, firefighting groups. The seismic detectors can be distributed on either
side, or just on one side, as shown in the Fig 4.4.
Figure 4.3 The convolutional model
of the reflection seismic trace,
showing the trace as the convolved
output of a reflectivity function with
an input pulse, and th relationship of
the reflectivity function to the
physical properties of the geological
layers.
Figure 4.4 Shot–detector configurations used in multichannel seismic reflection profiling. (a) Split
spread, or straddle spread. (b) Singleended or on-end spread.

14. 4.2.3 The CMP gather
Each seismic trace is characterized by three principal geometric
factors. The position of the shot and the recipient are two of
these. The location of the reflective surface is the third and
perhaps most important. This position is uncertain before the
seismic processing, but a good estimate is possible if this point
of reflection is assumed to lie vertically below the place on the
surface between the snap and the receivers for the trace.
For two main reasons, the CMP is the heart of seismic
process:
1. Section 4.2 provides simple equations with horizontally
uniform layers. They can be applied to a set of traces passing
through the same geological structure with less error. The CMP
collection is the simplest approach to such a set of traces. For
horizontal layers, events for reflection at a common depth point
in every CMP collection is reflected (CDP-see Fig. 4.5(a)). The
shift in travel time with offset, movement, depends solely on the
speed of the surface layers and therefore on the sub-surface
level.
2. The seismic energy reflected is generally very low. The signal-to - noise ratio of most data needs to be
increased. Once the velocity is known, NMO can fix traces in a CMP to fix each trace to the zero-offset
trace equivalent. They all have identical reflected pulse, but different random and consistent noise at the
same time. The average noise output will increase the signal-to - noise ratio (SNR) when all traces in a CMP
are joined together. This is called stacking.
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Figure 4.5 Common mid-point (CMP)
reflection profiling. (a) A set of rays from
different shots to detectors reflected off
common depth point (CDP) on a horizontal
reflector. (b) The common depth point is
not achieved in the case of a dipping
reflector.