Seismic Wave
Seismic waves are oscillating displacements or vibrations from an undisturbed position that travel through
the ground from a source, such as an explosion or mechanical vibrator. As seismic waves, the energy
generated by the disturbance travels away from its origin.
In another way, Seismic waves are parcels of elastic strain energy that propagate outwards from a seismic
source such as an earthquake or an explosion. Sources suitable for seismic surveying usually generate shortlived wave trains, known as pulses, that typically contain a wide range of frequencies.
There are two groups of seismic waves1. Body waves.
2. Surface waves.
Body waves: A seismic wave that travels through the layers of the Earth rather than across its surface.
Body waves usually have smaller amplitudes and shorter wavelengths than surface waves and travel at
higher speeds.
a. P waves (Primary waves): P waves are also called compressional waves or longitudinal waves.
The longitudinal wave is the fastest of all seismic waves. When an earthquake occurs, this wave is
the first to arrive at a recording station. As a result, it is called the primary wave, or P-wave. Pwaves can travel through solids, liquids, and gases, all of which are compressible. Liquids and
gases do not allow shear. The longitudinal (or compressional) body wave passes through a medium
as a series of dilatations and compressions. P-waves travel fastest, at speeds between 4-8 km/sec
(14,000-28,000 km/h) in the Earth's crust. P-waves shake the ground in the direction they are
propagating.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
b. S waves (Secondary waves): Are also called transverse waves or shear waves. Shear waves from
an earthquake travel more slowly than P-waves and are recorded at an observation station as later
arrivals. S wave is perpendicular to the direction of wave propagation, and the main restoring force
comes from shear stress. They are much slower than P waves and can travel only through solids.
S-waves travel more slowly, usually at 2.5-4 km/sec (9,000-14,000 km/h). S-waves shake
perpendicularly or transverse to the direction of propagation (i.e. they displace material at right
angles to their path).
Surface Waves: Seismic surface waves, also known as surface waves or ground waves, are a type of
seismic wave that travels along the Earth's surface, primarily in the crust. These waves are generated by
seismic events such as earthquakes and can cause significant ground shaking and damage. Seismic surface
waves are distinct from body waves, which travel through the Earth's interior.
a. Love waves: Love waves are a type of horizontally polarized surface wave that move with a sideto-side motion in a horizontal plane. They are the fastest surface waves and can travel at speeds
slightly slower than the shear waves (S-waves) that propagate through the Earth's interior. Love
waves are responsible for the horizontal shaking of the ground during an earthquake.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
b. Rayleigh waves: Rayleigh waves, also known as ground roll or rolling waves, are a combination
of vertical and horizontal motion that results in an elliptical, rolling motion. They are named after
the British scientist Lord Rayleigh, who studied them extensively. Rayleigh waves are slower than
Love waves and are responsible for the up-and-down, rolling motion of the ground during an
earthquake.
Propagation of seismic waves
Compressional waves (the longitudinal, primary, or P-waves of earthquake seismology) propagate by
compressional and dilational uniaxial strains in the direction of wave travel. Particle motion associated with
the passage of a compressional wave involves oscillation, about a fixed point, in the direction of wave
propagation. Shear waves (the transverse, secondary, or S-waves of earthquake seismology) propagate by
a pure shear strain in a direction perpendicular to the direction of wave travel. Individual particle motions
involve oscillation, about a fixed point, in a plane at right angles to the direction of wave propagation. If all
the particle oscillations are confined to a plane, the shear wave is said to be plane-polarized.
If the surface is layered and the surface layer shear wave velocity is lower than that of the underlying layer,
a second set of surface waves is generated. Love waves are polarized shear waves with a particle motion
parallel to the free surface and perpendicular to the direction of wave propagation. The velocity of Love
waves is intermediate between the shear wave velocity of the surface layer and that of deeper layers, and
Love waves are inherently dispersive. Rayleigh waves propagate along a free surface, or along the boundary
between two dissimilar solid media, the associated particle motions being elliptical in a plane perpendicular
to the surface and containing the direction of propagation. The amplitude of Rayleigh waves decreases
exponentially with distance below the surface. They have a propagation velocity lower than that of shear
body waves and in a homogeneous half-space, they would be non-dispersive. In practice, Rayleigh waves
traveling around the surface of the Earth are observed to be dispersive, their waveform undergoing
progressive change during propagation as a result of the different frequency components traveling at
different velocities. This dispersion is directly attributable to velocity variation with depth in the Earth’s
interior.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Attenuation of seismic waves
Attenuation is a natural phenomenon that occurs due to various factors, including the intrinsic properties of
the Earth's materials and the spreading of energy over distance. Seismic wave attenuation refers to the
decrease in the amplitude or energy of seismic waves as they travel through Earth's subsurface or other
geological materials.
A seismic wave propagating in the earth encounters several discontinuities (boundaries) between rock types
of different physical properties and produces wave phenomena such as reflections, diffractions, absorptions,
scatterings and transmissions (refractions). At each boundary or interface between two different rocks, a
part of the incident energy is reflected back to the surface and the rest of energy is transmitted to the
underlying rocks. As the wave of energy (seismic pulse) travels downwards in solid media, it undergoes
gradual loss of energy (attenuation) depending on the rock-fluid properties.
1. Absorption: The seismic source wave, generated
at the surface, as stated earlier, propagates through
a rock by transferring energy from one particle to
another. In the process, a part of the energy is
attenuated due to conversion of mechanical energy
to heat energy through frictions at grain contacts,
cracks and fractures and fluids present in pores of a
rock. The frictional loss primarily due to motion
between rock particles at the point of grain contacts
is known as absorption. Absorption effects are
severe within shallow weathering zones and
decrease with depth. Magnitude of absorption
(friction) loss in a hard rock is liable to be much
higher than that in a fluid saturated rock as friction
in fluid, considered as a slushy medium, is likely to
be small.
2. Scattering: Scattering losses are irregular dispersions of energy due to heterogeneity in rock
sections, usually considered as apparent noise in seismic records. Geological objects of very small
dimensions tend to scatter wave energy and produce diffractions rather than continuous reflections.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Highly tectonised shear zones with faults and fractures, very narrow channels, pinnacle mounds
etc., are some of the geologic features, most prone to scattering effect.
3. Transmission: Transmission loss is the loss of energy the wave undergoes at every lithologic
boundary, as a part of the energy is reflected back to the surface allowing less to go deeper. The
loss thus depends on the type and number of reflecting interfaces. It is often believed that a strong
reflector like a limestone or an intrusive body reflects most of the energy upwards and transmits
less in the process, causing poor reflections or shadows below. Transmission losses reduce
amplitudes at all frequencies.
4. Spherical (Geometrical) divergence: A
seismic wave (usually considered travelling
in the form of spherical wave fronts),
suffers from reduction of energy as it
continually moves away from source and
spreads through the subsurface rocks with
time (distance). This is also known as
geometrical loss as it is linked to the wavepath geometry. The decay is dependent on
distance from the source and increases with
higher velocities due to greater distance
travelled.
Geological significance of energy attenuation
Large attenuation losses in rocks, besides the amplitude, lower the frequencies of seismic wave which lead
to lower velocities due to dispersion effect. Measurement of both attenuation and velocity can thus provide
complimentary information about the rock and fluid properties. Further, attenuation affecting the frequency
and the amplitude content of the wavelet also results in changing the seismic wave shape. Analysis of
propagation loss in rocks from the resulting changes in wave shapes can then lead to important geological
information about rock and fluid properties. Some significant geological conclusions from analysis of
attenuation effect can be as below.
1. An indication of high energy loss considered owing to absorption, may give a clue to the type and
texture of the reservoir rock. Unconsolidated, fractured, and poorly-sorted rocks having angular
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
grain contacts are likely to have considerable friction (Anstey 1977). A rock, well-sorted and with
well-cemented pore spaces, on the other hand, will show negligible loss due to absorption.
2. Seismic evidence of high transmission loss can be suggestive of a formation consisting of cyclically
alternating impedance contrasts typified by multiple thin sands with intervening shale, the
cyclothems, in a deltaic environment. Cyclothems are potentially important geological plays that
are commonly sought after by the explorationists.
3. Scattering losses due to heterogeneity in strata may provide a clue to the order of irregularities in
reservoirs suggesting rapid facies change in a continental depositional environment. Similarly,
scattering losses may result in poor to no seismic reflections indicating the presence of mélanges
in highly tectonised zones of subduction which can lead to planning suitable acquisition and
processing techniques to achieve better seismic images.
Seismic Properties
The propagating seismic wave has two very important intrinsic elements, which are indispensable to the
framework of exploration seismic technology. These are:
(1) Amplitude of the seismic wave–particle velocity measured by geophones on land or the acoustic
pressure measured by hydrophones in marine streamer surveys and
(2) Velocity of the wave with which it passes through the rocks.
Amplitudes and velocities are the seminal seismic properties and differ over a wide range, depending on
rock-fluid properties. Seismic wave propagation with its associated effects brings out vital geological
information about different types of subsurface rocks and their fluid contents. The rock and fluid properties
therefore can be determined from seismic properties.
Rock properties
a. Elasticity: The elasticity of a rock may be defined as the resistance that it offers to stress. In a
geological sense, elasticity may be likened to a measure of the hardness of a rock, which depends
on lithology and commonly increases with depth. A hard rock is difficult to compress because of a
high bulk modulus (incompressibility), which ensures high seismic velocity. Likewise, a soft rock
with a large compliance has lower elastic modulus and consequently exhibits lower velocity.
b. Bulk density: The bulk density of a sedimentary rock includes the density of the rock matrix and
the density of the fluid in the pore spaces. Density of a rock is defined as its mass per unit volume
and commonly increases with depth. This is a result of compaction as the rock undergoes burial.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Compaction is a diagenesis process that squeezes out water from the pore space of sediments with
time (depth) by overburden pressure as they get buried beneath successive layers of sediments.
Compact rocks show higher densities whereas under-compacted formations demonstrate lower
density values.
c. Pore geometry (Porosity, pore size and shape): Porosity is a measurement of the void space in a
given volume of rock. In general, an increase in porosity lowers the density and more so the
elasticity of a rock resulting in decreased seismic velocity.
Porosity and pore shape varies in different kinds of rock. Unconsolidated sandstones and vuggy
carbonates (cavernous porosity) have open, irregular pores whereas partially cemented sedimentary
rocks show porosity that occurs between the grains known as intergranular porosity. Cemented
sands and tight carbonates, on the other hand, may have dominantly crack-like pore space (fracture
porosity).
Behavior of velocity, however, is known to be affected more by the shapes of pores than the
porosity, as the shape plays a major role in determining the compliance of a rock to stress. For
example, flat-pore shapes in a relatively low-porosity reservoir may indicate lower seismic velocity
(more compliance) than that in a high-porosity reservoir with spherical pore shapes.
d. Texture: Grain sizes, roundness, sorting and cementation describe the texture of a rock. Elasticity
and density of a rock depend on contacts of grains, their size and angularity. Large grain sizes and
compact sands have generally higher seismic properties whereas, unconsolidated sands with
angular grains are likely to show lower seismic properties.
e. Fracture and cracks, Geometry: Seismic properties are greatly affected by presence of open
fractures in a rock. Fractures and cracks facilitate compressibility (compliance) and significantly
lower the velocity and impedance. Fractures also create anisotropy in rock sections. Tectonic stress
causing fractures, cracks, and vugs may change the texture of a rock to induce seismic anisotropy.
Fluid Properties
a. Pore fluid and saturation: Most rocks have fluid in pore space. Fluids
typically are known to have negligible shear modulus but affect
compressional seismic properties depending on its compressibility and
density. In a fully water or brine-saturated reservoir rock, water or brine
offers resistance to stress and tends to increase velocity though not to the
same extent as in a tight rock having little water. Oil in rock pores lowers
velocity marginally compared to that with water. Gas, on the other hand,
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
has the least bulk modulus (highly compressible) and density, and the velocity and impedance of a
rock with gas in the pore space thus tend to show significantly lower values than that of rocks
saturated with water and/or oil.
b. Viscosity: Rocks tend to exhibit increasing elasticity and density with an increase in the viscosity
of oil. Heavy oil has a large bulk modulus and in some cases may tend to act as semisolids in the
rock pores. These rocks obviously exhibit relatively higher seismic properties.
c. Pressure and temperature: A rock usually face two types of
pressure. The first one is the geostatic lithostatic pressure which
tends to downwards due to gravity. The other one is fluid
pressure which tends to upward direction due to the buoyancy of
the fluid in the rock pores. The overburden pressure tends to
close the pore space, whereas, the fluid pressure tries to retain
the voids. The difference between these two pressures, known as
the effective pressure or differential pressure is an important
factor in determining seismic properties. In a normal hydrostatic
pressure situation where squeezed water escapes to the surface
effective pressure increases with depth and seismic velocity
increases with the raising density and elasticity. The overpressured rocks have lower effective pressure, decreased elasticity and density and exhibit lower
seismic properties.
A rise in temperature leads to changes in pore fluid properties such as viscosity and elasticity.
Seismic properties, with increase in temperature, marginally decrease in water and gas saturated
rocks but may decrease significantly in oil saturated rocks.
Seismic Reflection Method
Seismic reflection is the most widely used geophysical technique. It can be used to derive important
details about the geometry of structures and their physical properties. Major fields of application
of Seismic reflection include:

Hydrocarbon exploration,

Research into crustal structure with several kilometers of depths of penetration,

Engineering and environmental investigations (depth <200m),

Mapping structural features such as shallow faults, buried valleys and Quaternary deposits,

Hydrological studies of aquifers.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
The basic principle of the seismic reflection technique application is to measure the time taken for a seismic
wave that travels from a source down into the ground where it is reflected back to the surface where it can
be detected by a receiver (geophone):

The measured time is known as the two-way time (TWT).

The basic issue in seismic reflection interpretation is the conversion of the measured two-way time
into depth. Although the two-way time (TWT) is known (measured), still there are two unknown
parameters; these are: depth and velocity. Velocity is considered as the parameter that most affects
the conversion of the two-way time into depth.
Reflection and Transmission of seismic waves in layered media

At an interface between two rock layers there is generally a change in propagation velocity resulting
from the difference in physical properties of the two layers. At such an interface, the energy within
an incident seismic wave is partitioned into transmitted and reflected waves.

The relative amplitudes of the transmitted and reflected waves depend on the velocities (V),
densities (ρ) and the angle of incidence.

The total energy of the transmitted and reflected waves must be equal to the energy of the incident
ray.

The amount of energy transmitted through the interface is inversely proportional to the acoustic
impedance defined by:
Z= ρV
Where,
Z= Acoustic Impedance
V= Velocity
ρ= Density
This means that the smaller the contrast in acoustic impedance across the rock interface the greater is the
portion of the transmitted energy.

The more energy is reflected, the greater is the contrast. This is expressed by the Reflection
Coefficient, R, given by:
R= A1/A0
𝑹=
𝝆₂𝑽₂−𝝆₁𝑽₁
𝝆₂𝑽₂+𝝆₁𝑽₁
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
𝑹=
𝒁₂−𝒁₁
𝒁₂+𝒁₁
-1<R<1
Where,
R = Reflection coefficient
A1 = Amplitude of the reflected wave
A0 = Amplitude of the incident wave
Z= Acoustic impedance
V = Velocity
ρ = Density
Negative values of the reflection coefficient indicate a phase change of 1800 in the reflected wave.

The transmission coefficient, T is given by:
T = A2/A0
T = 2 {Z1/(Z2+Z1)}
Where,
T= Transmission coefficient
A2= Amplitude of transmitted wave
A0= Amplitude of incident wave
Z= Acoustic impedance
The case of single horizontal reflector
The travel time equation of a reflected wave from a shot point to a receiver
(geophone) located at a horizontal offset X can be derived as follows:
4S2 = 4h2+x2 = t2v2
t2 = (4h2+x2) /V2
t = (4h2+x2)1/2 /V
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Field procedure of seismic reflection
Seismic reflection profiling obtains a cross-section through the subsurface by recording data continuously
along a surface profile.
1. Single-channel Seismic Profiling (constant offset): In the case of shallow reflection profiles, the
simplest survey is to use a source and a single geophone (receiver). In this procedure:

source and receiver are both moved along the profile by the same amount between shots

plotting successive shots side by side creates a reflection profile of the subsurface.

used in engineering and hydrogeological surveys.
2. Multichannel Seismic Profiling (single fold): Single-channel reflection surveys are subject to
noise. Combining multiple reflections from single subsurface location allows attenuation of this
noise.
Seismic data gathering
Large volumes of seismic data can be recorded and be organized in different ways. A GATHER is the
name for a collection of seismic traces. There are several trace gathers, these are:

Common Shot Gather: A collection of seismic traces recorded at several receivers (geophones)
from single shot. This is the configuration in which seismic data are acquired in the field.

Common Receiver Gather: A collection of seismic traces corresponding to several shots recorded
at a single receivers (geophone).
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography

Common Midpoint Gather: A collection of seismic traces in which the shot and the receiver are
symmetrically distributed about the same midpoint location.
The common Depth Point (CDP) is the point on a plane horizontal interface from which all the
reflections in a Common Midpoint (CMP) Gather are generated.

The Common Midpoint (CMP) Gather is fundamental in seismic reflection data processing for
two reasons:
o
The variation of travel time with offset, the moveout will depend only on the velocity of
the subsurface layers (horizontal uniform layers).
o
The reflected seismic energy is usually very weak. It is imperative to increase the signal
noise ratio of most data.
Processing of Seismic Data
1. Preprocessing:
i.
Demultiplexing: Demultiplexing in seismic reflection data refers to the process of separating
and organizing the recorded seismic traces acquired during seismic data acquisition. During the
seismic survey, multiple traces are often recorded simultaneously, each corresponding to a
specific source-receiver pair. Demultiplexing is essential for isolating individual traces
associated with distinct shot points or source-receiver pairs, allowing for subsequent processing
and analysis.
ii.
Trace editing: Trace editing in seismic reflection data involves the identification and correction
of anomalies, noise, or unwanted features in individual seismic traces. The goal is to enhance
the quality of the seismic data before further processing or interpretation.
iii. Gain recovery: Seismic waves decrease in amplitude as they propagate further into the Earth.
Reflections recorded at late times have lower amplitude than those at early times. Gain recovery
in seismic reflection refers to the process of restoring the amplitude characteristics of seismic
data that may have been altered during acquisition or processing.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
2. Deconvolution: Removes the effects of the seismic wavelet (the shape of the seismic pulse
generated by the seismic source) to enhance the resolution and clarify the subsurface geological
features from the recorded seismic data.
3. CMP Sorting
4. Velocity analysis
5. NMO Correction: Correction for the differences in arrival times of seismic reflections caused by
variations in subsurface velocity. The objective of NMO correction is to transform seismic traces
from the time domain to a semblance-corrected hyperbolic moveout in the offset domain, enabling
more accurate subsurface imaging.
6. Stacking: Is the process to combine multiple seismic traces recorded at the same location but with
different source-receiver offsets. By stacking these traces, coherent signals are reinforced, while
random noise tends to cancel out, resulting in a clearer and more interpretable seismic section. It
involves summing and averaging the seismic traces within a CMP gather to enhance the amplitude
of coherent signals while reducing the amplitude of random noise.
7. Migration: Correcting the effects of velocity variations in the subsurface, improves the spatial
resolution and geological interpretation of seismic image.
8. Display
Noise in seismic data
The “noise” can be defined as all kind of events or amplitudes appearing on the seismic data other than
genuine reflections. Recorded noise is sometimes generated by the seismic system components themselves
(e.g., air gun bubble, bird noise, etc.), and sometimes it arises because of the specifications of the working
environment (e.g., marine mammals, propellers, marine traffic, etc.). The first noise type is termed source
generated noise whereas the second type is known as ambient noise.
The noise in the seismic data is classified either as coherent noise, which has trace-by-trace consistency and
can be traced over several seismic traces, or random noise, which does not have a systematic consistency
from one trace to another.
Coherent noise, such as multiple reflections or power line harmonic noise, can be modeled and subtracted
from the data by a suitable match filter application. Random noise can be attenuated if it can be separated
from signal by one of its specific characteristics, such as frequency band. Today, stacking of multichannel
seismic data is the most effective noise attenuation technique both for random and coherent noise types.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Signal-to-Noise Ratio (S/N)
Noise is an inherent part of the seismic recording and processing system present due to ambient, geological
or geophysical causes. This noise cannot be wished away, but can be effectively reduced by conscious
efforts during data acquisition and processing. Noise, though usually unwanted, can be occasionally helpful
in interpretation.
For examplei.
Remnant diffraction noises despite processing may indicate clues to the presence of sharp
edges, such as faults and other subtle stratigraphic objects.
ii.
The presence of scattering noise may give an idea about the order of heterogeneity of the
reflector, leading to an indication of highly tectonised zones, crushed with faults and fractures.
iii.
Processing of recorded noise as scatters from fractures and fractured zones can also be used as
a technique for delineating naturally fractured carbonate and basement fractured reservoirs.
Since noise severely affects seismic clarity in portraying the subsurface image, it is desirable to record good
and clean signals with minimum noise. It is a common practice to benchmark the quality of data in terms
of a measure of a ratio between signals and noise (S/N). The Signal-to-Noise Ratio (S/N ratio) in seismic
data refers to the ratio of the amplitude of the desired signal (seismic reflections from subsurface structures)
to the amplitude of background noise. A higher S/N ratio indicates a clearer and more reliable seismic
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
signal, making it easier to interpret and analyze subsurface geological features. Improving the S/N ratio is
crucial in seismic data processing to enhance the quality of the seismic images and increase the accuracy
of geological interpretations. Improved data acquisition techniques including meticulous survey layout
plans, field experimentations and strict on-field execution ensure good quality of data. In this context, the
common depth point (CDP) seismic data acquisition is a unique technique. A standard technique practiced
all over the world, it achieves signal enhancement at the cost of noise, via a summation process of several
traces reflected from the same subsurface depth point but with different offsets known as CDP folds.
Though summation of higher number of traces in a fixed offset range generally provides better S/N ratio,
there may be a limit beyond which it may not be desirable, as adding additional traces (folds) may cost
more money without improvement in the seismic images. Also, summation is an integration process, which
affects resolution, especially in cases where large far offset traces are included for summing. In areas where
the geology promotes good quality seismic reflections, the interpreter may still prefer to look at less-fold
CDP data which is likely to offer better resolution and at a lower cost. It may also be noted that data with
high S/N ratio does not necessarily assure higher resolution, as the resolution depends on other factors such
as source signal frequency, sampling interval and subsurface wave propagation effects, besides noise.
Seismic velocity
Velocity information is the main parameter to define the depth, dip, and the lateral locations of the reflectors
in the seismic data. It is also crucial information for some of the processing methods, such as normal
moveout (NMO) correction and migration. Seismic wave velocities change over a wide range in nature,
even for the same rock type. Several factors control the velocity of a specified medium
Lithology

Saturation and fluid type

Porosity

Cementation, grain size and pore shape

Age of the rock

Pressure/compaction or depth

Density of the medium

Temperature

Frequency of the seismic signal

Anisotropy and fractures

Clay content

Consolidation
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Types of seismic velocity
a. Interval velocity: Interval velocity is the average velocity between two reflective interfaces. If two
reflectors located at depths h2 and h1 give one-way travel times t2 and t1, respectively, then the
interval velocity of the layer between the depths h2 and h1 is given by
Vint= (h2-h1 )/ (t2-t1)
b. Average velocity: Average velocity is the average of the seismic velocity between sea surface and
a specific reflector, and is used for the conversion of seismic data from time to depth.
Vave = ⅀hi / ⅀ti
c. Instantaneous velocity: If the wave velocity varies continuously with depth, then the (h2-h1)
difference becomes ∂h while the (t2-t1) difference turns into ∂t. Instantaneous velocity is the
differentiation of the depth with respect to time, and its time integral gives the average velocity
between source and reflector. Instantaneous velocity can be defined as-
Vins = ∂h / ∂t
d. Root-Mean-square velocity: If the subsurface consists of n number of layers, and if their
corresponding interval velocities are V1, V2, …,Vn, the RMS velocity can be defined as-
Vrms = (⅀Vi2ti) / ⅀ ti
e. NMO or stacking velocity: For a specific reflection, the time difference between the recording
time of that reflection at any offset from the shot and zero-offset time is defined as normal moveout,
which arises only due to the offset between source and recording channel. NMO velocity is used to
remove normal moveout times from CDPs before stacking, and therefore accurate determination
of NMO velocity is extremely important for the quality of final stack sections. It is defined as-
Vnmo = [x2 / (t(x)2 – t(0)2]1/2
Components of Marine Seismic Acquisition
Acquisition of marine seismic data requires several different systems and software packages to compatibly
work simultaneously, which are in communication with each other in real-time during the acquisition.
Block diagram of a marine seismic acquisition system-
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Typically, the following equipment is mobilized for a 2D seismic survey•
Seismic vessel
•
Seismic recorder
•
Onboard processing/QC system
•
Streamer
•
Seismic sources
•
Gun controller
•
Streamer depth controller
•
Air compressors
•
Integrated navigation system
•
DGPS/rGPS systems
•
Tail buoy
•
Lead-in and deck cables
The following additional equipment is also required for 3D surveys•
Paravanes
•
Dilt floats
•
Lateral streamer positioning equipment
•
Acoustic rangers
•
Other equipment (velocity meters, separation ropes, etc.)
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Seismic sources
The ideal source would produce a single, high energy spike that is easily detectable in the presence of noise
after travelling to the deepest part of the earth. Seismic sources for offshore investigations fall into two
groups-
i.
Impulsive- Provides a short-lived burst of elastic wave energy
ii.
Swept frequency- Produce a low amplitude signal
i. Impulsive sourcesa. Chemical explosives
b. Piezoelectric and magnetostrictive sources
c. Sparker and boomers
d. Airguns
e. Water guns
f.
Free-falling weights
Seismic detectors
i.
Hydrophones: Hydrophones for detecting water-borne seismic signals are generally made of
piezoelectric materials such as barium titanate, lead zirconate, or lead metaniobate. Pressure
changes in the water generate small currents in the transducer circuit which are amplified and
transmitted by conventional conductor cable, fiber optic link, or radio to data storage and
display units. Transducers of deep submergence are either pressure-compensated or free
flooding. Hydrophones are usually designed as matched pairs so their outputs add for pressure
variations but cancel for voltages produced by translational acceleration. For seismic work at
sea hydrophones are grouped into arrays which have important noise-reducing properties.
ii.
Seabed Geophones: Geophones are often used with self-contained seabed instruments, in
boreholes, for seismic work in shallow coastal zones where obstacles prevent the deployment
of a continuously towed receiver and for detailed three-dimensional seismic survey. A common
design consists of a cylindrical permanent magnet with a circular slot containing a coil
suspended by leaf spring. The whole unit is mounted in a pressure housing. When the seabed
is displaced vertically the magnet follows the ground motion, while the coil tends to remain
fixed because of its inertia. The signal is then transmitted in analogue or digital form to a
recorder by cable or radio link.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Multichannel seismic acquisition system
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Flowchart for three-dimensional seismic processing
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
DHI (Direct Hydrocarbon Indicator)
Seismic direct hydrocarbon indicators (DHIs) are anomalous seismic responses caused by the presence of
hydrocarbons. DHIs occur when a change in pore fluids causes a change in the elastic properties of the bulk
rock which is seismically detectable.
A hydrocarbon indicator or direct hydrocarbon indicator (DHI) is an anomalous seismic attribute value or
pattern that could be explained by the presence of hydrocarbons in an oil or gas reservoir.
1. Bright Spots: Bright spots are strong amplitude anomalies with negative reflection coefficient and
are mostly linked to gas in sand reservoirs, capped by shale. In geologic scenarios where watersaturated rocks have velocities and densities, (e.g. 2300 m/s, 2.2 g/cm3) close to that of overlying
shale (e.g. 2100 m/s, 2.3 g/cm3), reflections will show weak amplitude due to the marginal positive
impedance contrast. When the sand is saturated
with gas, the velocity is considerably reduced
(e.g., to 1600 m/s), causing a significant
negative contrast at the interface. This results in
creating bright amplitude at the top of the gassand.
2. Dim Spots: Dim spots are weak amplitudes with a positive reflection coefficient, linked to mostly
gas in carbonate reservoirs. Water-saturated limestone generally demonstrates much higher
velocity (~3400 m/s) than that of the capping
shale (~2600 m/s) and shows high reflection
amplitudes with positive polarity. Impregnated
with gas, the velocity of the carbonate rocks is
greatly reduced (e.g. 2900 m/s), which decreases
impedance contrast and creates reflections at
carbonate reservoir top with weak amplitudes.
3. Flat Spots: Flat spots are moderate to high
amplitude,
horizontal
reflections
that
are
associated with gas water contacts and show
reflections with positive polarity. This is a unique
case where the reflection is not related to
lithology but to fluid contacts, and the impedance
contrast being influenced by fluid density of gas
and water.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography
Sag Effect: Velocity lowering due to gas in a reservoir can create artefacts below it. In case of a thick
gas reservoir, arrival times of reflectors below will be delayed and may indicate a local depression,
vertically below the gas zone. This is called ‘sag’ or ‘pulldown’ effect which is exemplified by synthetic
response and actual field seismic.
Ocean-409: Marine Geophysics
Md. Zahid Hasan Nahid
Lecturer, Department of Oceanography