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