the seismic method

8/8/2019 The Seismic Method

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Basic Seismology

ForPetroleum Industry


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INDEX

1 INTRODUCTION..................................................................................................................1

2 PRINCIPLES OF THEORY..................................................................................................2

2.1 Energy Propagation and Elastic Waves........................................................................................................................2

2.2 Ray Theory.......................................................................................................................................................................5

3 SEISMIC DATA ACQUISITION.........................................................................................10

3.1 Land Data Acquisition..................................................................................................................................................12

3.2 Marine Data Acquisition...............................................................................................................................................14

4 PROCESSING...................................................................................................................16

4.1 Preprocessing.................................................................................................................................................................16

4.2 Static Corrections..........................................................................................................................................................17

4.3 Deconvolution................................................................................................................................................................17

4.4 Velocity Analysis, NMO correction and Stack...........................................................................................................17

4.5 Time Migration..............................................................................................................................................................18

4.6 Special processing......................................................................................................................................................... .19

4.6.1 A.V.O.......................................................................................................................................................................19

4.6.2 Depth Imaging..........................................................................................................................................................20

5 POTENTIAL METHODS....................................................................................................21


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1 Introduction

This report is aimed at non-geophysicist students involved in the petroleum industry and deals briefly with seismic

acquisition and processing and other geophysics methodologies.

Hydrocarbon accumulations are located under the surface of the earth, at depths of between 500m and 5000m; for this

reason it is impossible to collect a large amount of direct information. Geophysics is useful because it is an indirect

method of studying the sub-surface and, as a consequence, represents a very important tool in the petroleum industry for

localizing and evaluating possible targets. It represents the first step in exploration and can be divided into planning,

acquisition, processing, interpretation and mapping. The method is successfully applied in almost all environments: sea,

land, transitional, etc

Of all the geophysics methods, seismic is the most important and most widely used. The underlying concept of seismic

exploration is simple. In nature acoustic waves are generated by earthquakes; in the petroleum industry they are generated

(with frequencies typically ranging from about 5 Hz to just over 100 Hz) using specific energy sources. As these soundwaves leave the seismic source and travel downward into the Earth, they encounter changes in the Earth's geological

layering, which cause reflections, refractions and diffractions. The reflections travel upward to the surface where

electromechanical transducers (geophones or hydrophones) detect the echoes arriving on the surface and convert them

into electrical signals, which are then amplified, filtered, digitized, and recorded. The recorder consequently measures the

travel times from source to reflector and from reflector to receiver (Two Way Time) and the characteristics of the signal

(frequency, amplitude). Different Two Way Times are due to different reflection points. The recorded seismic data

usually undergoes elaborate processing using digital computers. The result is a vertical section (2D method) or a 3D

volume (3D) representing the earth's structure.

After data has been acquired and processed, the next two steps involve interpretation and mapping; experienced

geophysicists are needed to evaluate whether the rocks might contain valuable resources and if it is opportune to drill a

well.

A brief description of non-seismic methods is also given (chapter 5).

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2 Principles of Theory

2.1 Energy Propagation and Elastic Waves

The application of an external force to a solid body causes the warping of the body itself: if it returns to its original shape

when the force ceases it is defined an elastic body.

The Earth's crust can be considered as completely elastic (except in the immediate vicinity of the shot), therefore the

name given to this type of acoustic wave transmission is elastic wave propagation.

Several kinds of wave phenomena can occur in elastic solid and are classified according to how the particles making up

the solid move as the wave travels through the material.

P-WAVES (COMPRESSIONAL WAVES)

The most commonly used waves in exploration seismic are P-waves. When the particle motion is in the direction of the

wave propagation we have aP-wave (Undae Primae), also called pressure wave or compressional wave.

The introduction of energy into the earths surface using a seismic source generates a compressional force that causes an

initial decrease in the volume of the medium on which the force acts. The elastic nature of the rock then causes an

immediate rebound or expansion, followed by a dilation force. This response of the medium constitutes a primary

compressional wave or P-wave. If we were able to put a finger against the rock in line with the P-wave arrival, our finger

would move back and forth in the direction of the wave propagation, just like the particles that make up that rock. The P-

wave velocity is a function of the rigidity and density of the medium. Typical P-wave velocity in the soil is:

Dense rock from 2500 to 7000 m/s,

Spongy sand from 300 to 500 m/s

S-WAVES (SHEAR OR LONGITUDINAL WAVES)

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Elastic waves where particle motion is at right angles to the direction of propagation are called Shear waves, S-waves

(Undae Secundae), or Longitudinal waves.

A shear strain occurs when a sideways force is applied to a medium; a shear wave may be generated that travels

perpendicularly to the direction of the applied force. The velocity of a shear wave is a function of the resistance to shear

stress of the material through which the wave is traveling and is often approximately half of the material's compressional

velocity.

In liquids (i.e. water), no shear wave is possible because shear stress and strain cannot occur due to the characteristics of

the molecular bond in liquids. For this reason, in marine data, all arrivals are compressional waves so generally the data

appears to have higher signal-to-noise ratios than land registrations (S-wave arrivals are generally considered noise).

An interesting property of S-waves is their sensitivity to the anisotropy of the rocks. In stratified rock where there arefluid-filled cracks and inclusions, there is often greater resistance to shear force than in homogenous rock, as cracks limit

the degree of shear particle movement. The result is that a shear velocity change may occur as a result of layering and

cracking. Compressional waves are not so readily affected by cracks. Comparison of compressional velocities with shear-

wave velocities in such media therefore gives information about the nature of the rock. Obtaining such information is the

goal of the type of seismic surveying called shear-wave exploration.

CONVERTED WAVES

When a P-wave or S-wave arrives at a boundary between two layers a portion of the wave is reflected, a portion is

transmitted and a portion is converted from P to S or vv. The converted waves are not generally used for imagingpurposes in exploration seismic so they are considered noise. Nevertheless, there are particular geological situations

where converted waves can be helpful for reconstructing a subsoil model. Such cases need complex data processing

methodologies.

SURFACE WAVES

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Particular kinds of waves are generated in the near surface. The amplitude of these waves can be much greater than deep

reflections and at times represent a serious noise problem.

The weathering of surface rocks (on land) and the deposition of soft sediment over the years causes a layer of semi-

consolidated surface rock overlying the sedimentary section to be explored. This layer is known as the weathering layer

or low-velocity layer (LVL). The latter term is used because of the low velocity of propagation of P-waves passing

through the layer. The LVL also allows the transmission of surface waves along its air-earth boundary.

RAYLEIGH WAVES

Surface waves spread out from a disturbance like the ripples caused by a stone dropped into a pond. Lord Rayleigh

(1842-1919) developed the physics to explain surface waves; in his honour surface waves are now commonly known as

Rayleigh waves. Dobrin (1951) performed a series of experiments to test Rayleigh's theory. His measurements of the

relative amplitude and direction of particle motion agreed with the theory that Rayleigh waves were low frequency, and

travel horizontally with retrogressive elliptical motion away from the energy source (shot), as shown below. Going deeper

(i.e., down the bore hole), Dobrin found that the particle motion of surface waves decreased in terms of amplitude as the

depth increased, eventually reversing the direction. This point was in the vicinity of the base of the weathering layer.Because the motion of the ground appears to roll, the wave is commonly known as ground roll. In some regions where

the weathering layer is thick, ground roll completely masks useful reflected data. Offshore surveys often observe

Rayleigh wave equivalents (Scholte waves) as long-period, water-bottom, sinusoidal waves, known as bottom roll or mud

roll. This tends to occur only in water depths of 10-20 m.

LOVE OR PSEUDO-RAYLEIGH WAVES

Love waves are surface waves borne within the LVL, with a horizontal motion perpendicular to the direction of

propagation and, theoretically, no vertical motion. Also known as horizontal SH-waves, Q-waves, Lq-waves, or G-waves

in crustal studies (seismology), these waves often propagate through multiple reflection within the LVL, depending on the

LVL material. If these waves undergo mode conversion, a number of noise trains appear across the seismic record,

obscuring the reflected energy content even more. These kinds of waves are useful for calculating static corrections (seeChapter 4.2) for S waves.

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DIRECT AND HEAD WAVES

The expanding energy wave front that moves along the air-surface interface outward from a shot is commonly observed

as the direct wave and has the velocity of the surface layer through which it travels. In offshore cases, direct waves havebeen used to determine the speed of sound in water, which is around 1500 m/s. Head waves are the portions of the initial

wave front transmitted down to the base of the weathering layer or the water bottom and are refracted along the

weathering base. They then return to the surface as refracted energy or refractions. Sometimes the refracted velocity is

higher than the velocity of propagation in the surface layer. In this case, refracted head waves appear in the mid- to far-

offset traces before the direct wave arrives.

2.2 Ray Theory

Seismic waves created by an explosive source spread outward from the shot point in 3 dimensions. To

understand spherically expanding 3-D waves, a simple mechanism is needed to explain how the wave responds on contact

with geological discontinuities. Huygens's principle is commonly used to explain the response of the wave in geometricalterms. Every point on an expanding wave front can be considered as the source point of a secondary wave front. The

envelope of the secondary wave fronts produces the primary wave front after a small increase in time. The trajectory of a

point moving outward is known in optics as a ray, and hence in seismic as a ray path. Thinking of waves in terms of ray

paths allows geophysicists to model waves numerically and visually more simply than if the full wave front were

considered. The ray theory is a good enough level of approximation for geometrical considerations of the propagating

wave field.

The mathematical explanation of 3-D elastic waves found in standard textbooks can fully model the behavior of

a wave field and take into consideration all phenomena related to the waves at the same time. Nevertheless such

modelling is complex and entails calculation difficulties for immediate applications.

Ray-trace modelling is much simpler from the point of view of calculation but the different phenomena related

to the waves behavior (reflection, refraction, diffraction etc.) have to be considered separately. Hereafter, the ray pathconcept is used to explain what happens as a wave front expands. For example, the wave front energy gradually decays in

amplitude as it passes through rock. The phenomena to be modeled, i.e. diffraction, reflection, refraction and so on have

to be introduced one by one. For this reason we will consider these phenomena separately. The next section will discuss

this amplitude decay.

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ENERGY DECAY

There are two main reasons for the amplitude decay of wave front propagation in a medium, namely spreading 1oss and

absorption. As a seismic wave expands outward from a shot, the energy per unit area of wave front is inverselyproportional to the square of the distance from the shot because the total energy, which remains constant if absorption is

ignored, has to spread over an increasingly larger area. This phenomenon is called energy spreading loss. The amplitude

of a wave is proportional to the square root of the energy per unit area. Thus, a wave's amplitude is inversely proportional

to the distance traveled.

As a wave front expands, the spreading loss per unit area is given by:

where r1 and r2 are two radial distances from a shot. Correction to recover this amplitude is applied in data processing.Such processing algorithms are based on equation (l).

Amplitude loss also occurs as a wave front passes through the rock, vibrating the rock particles. The vibrating particles

absorb energy as heat; this form of energy loss is called absorption. Amplitude loss due to of absorption varies

exponentially with distance, so that in amplitude terms:

where A1 is the amplitude at distance r1, A2 is the amplitude at distance r2 and is the absorption coefficient of the

material.

Accounting for both types of losses, we have in amplitude terms:

Assuming a typical value of of 0.25 dB per wavelength , where r1 and r2 are the radial distances from the shot, V is the

velocity of sound through the material, fis the wave front's predominant frequency, and wavelength = V/f, then from

equation (3),

Thus, for a fixed velocity, absorption loss is frequency and distance dependent. For a particular frequency and velocity,

absorption loss may be greater or less than the spreading loss. At short radial distances from a shot (shallow depth), the

spreading loss is greater than the absorption loss because the logarithmic ratio in equation (l) is greater than the linear

ratio in equation (5). At greater depths, the absorption loss tends to be greater than the spreading loss.

According to equation (5), higher frequencies experience greater absorption loss than lower frequencies. The combined

effect of both losses explains why deeper events in shot records generally have lower frequency content.

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REFLECTIONS

Energy incidents on a subsurface discontinuity are both transmitted and reflected. The amplitude and polarity of

reflections depend on the acoustic properties of the material on both sides of the discontinuity. Consider the boundarybetween two layers of sonic velocities V1 and V2, and densities l and 2.

Acoustic impedance is the product of density and velocity. The relationship among incident amplitude Ai, reflected

amplitude Ar, and reflection coefficient Rc is:

Ar = Rc* Ai , (6)

where

Equation (7) shows that Rc ranges from -1 to +1 and is negative when the second layer has lower acoustic impedance thanthe first layer. When a reflection coefficient is negative, the polarity of the reflected wave is the opposite of that of the

incident wave. When velocity is constant, a density contrast will cause a reflection, and vice versa. In other words, any

abrupt change in acoustic impedance causes a reflection to occur. Energy which is not reflected is transmitted. With a

high Rc, less transmission occurs and, hence, the signal-to-noise ratio drops below such an interface. Ideally,

geophysicists would prefer the earths layering to have a small Rc increasing in size gradually with depth to offset the

spreading and absorption losses as depth increases. When an impinging wave arrives at an interface, part of its energy is

reflected back into the same medium. The incident angle i is then equal to the reflection angle i'.

SNELL'S LAW

Snell's Law describes how waves refract. Simply put, Snell's Law states that the sine of the incident angle of a ray, sin i,

divided by the initial medium velocity V1 equals the sine of the refracted angle of a ray, sin r, divided by the lower

medium velocity V2.

That is:

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sin i / V1 = sin r / V2 (8)

Thus, when a wave encounters an abrupt change in elastic properties, part of the energy is reflected, and part of the

energy is transmitted or refracted with a change in the direction of propagation occurring at the interface.

REFRACTIONS

When an impinging wave arrives at such an incidence angle that energy travels horizontally along the interface at the

velocity of the second medium, then critical reflection occurs. The incident angle, I, at which critical reflection occurs,

can be found using Snell's Law:

Sin i = V1 / V2 sin90 = V1 / V2 (9)

The figure below shows a typical ray path when angle i equals or exceeds i. The arrivals at x 1, x2, etc. are refractions.

Refraction data is useful for determining the LVL depth, dip, and velocities. However, when complex LVL bodies exist,

the refraction method for determining weathering information is less accurate. In these cases, deep uphole surveys are

often preferred.

DIFFRACTIONS

Diffractions occur at sharp discontinuities, such as at the edge of a bed, fault, or geologic pillow. Consider a wave front

arrival at a discontinuity point as shown below. When the wave front arrives at the edge, a portion of the energy travels

through into the higher velocity region, but much of it is reflected, as shown. In conventional in-line recording,

diffractions can be collapsed by the migration process but if they arrive from outside the plane of the seismic line's profile

their energy will be recovered. Such diffractions are considered noise and reduce the in-line signal-to-noise ratio.

However, in 3-D recording, where specialized data processing techniques are used, all diffractions are considered as

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useful scattered energy. The 3D migration process performed can transfer the diffracted energy back to the point from

which it came, thereby enhancing the subsurface image. Hence, in 3-D surveying, out-of-plane diffracted events are

considered part of the signal.

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3 Seismic Data Acquisition

The acquisition of seismic data can be summarized as follows: a seismic wave is generated by exploding an energy

source near the surface to cause a shock wave to pass downward towards the underlying rock strata. Some of the shock

wave's energy is reflected off the rocks back to the surface. The geophones vibrate as the reflected seismic wave arrives,

and each generates an electrical signal. This signal is passed along cables to a recording truck, where it is digitized and

recorded on magnetic tape or disk. This experiment is repeated for the entire area of interest.

The location of the energy source is called the shot point, and the location of the receivers is called station. The sub-

surface reflection point (in the case of horizontal layer it is exactly half the source-receiver distance) is called CMP

(Common Mid Point), whereas the source-receiver distance is called offset. Shot points and receiver distances, as well as

maximum offset, acquisition direction, etc differ for each survey, because they depend on the purpose of the survey

and the geologic characteristics of the area (lithology, seismic velocity, depth and dip of reflectors, energy absorption,

etc). Therefore the first step of the acquisition is to define the main parameters.

In general the seismic experiment is strongly affected by different types of noise, which affect the final quality of the

data.

The picture on the left represents a typical onshore noise, called ground rolls (surface waves).

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Another type of noise is the "air blast". On land, the energy source (shot) can generate an airwave known as air blast,

which itself can trigger an air-coupled wave, a secondary wave-front in the surface layer. This wave generally travels at a

slower velocity than the compressional wave. The speed of the airwave, which depends mainly on temperature andhumidity, varies from 300 to 400 m/s.

In order to reduce these effects, modern methodology usesmultiple coverage: the same sub-surface point ( CMP) is

sampled several times (up to 120 times) at different offsets (source-receiver distance). Each raypath is represented by a

single trace in the field data. During processing all this information will be stacked, increasing the signal-to-noise ratio.

Two different types of acquisition are used: 2D and 3D. In 2D acquisition the shots and receivers are aligned along a

profile on the ground and the final result is a bi-dimensional image (vertical profile). In 3D acquisition, sources and

receivers are located in an area and the rays are registered not only along a profile but in an area. The result is a seismic

cube. This method is more expensive than 2D methods, but the amount of information obtained as well as the accuracy of

the result is incomparable.

2D LINE 3D VOLUME

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Modern seismic acquisition uses specific energy sources and receivers. Two main branches can be recognized: marine

and land acquisition. The theory is the same but different instrumentation, techniques and methodologies are used.

Marine registrations are relatively free of noise and are generally characterized by a high signal-to-noise ratio, while landregistrations have a low signal-to-noise ratio. The case where both land and marine operations are carried out together is

known as transition-zone recording. Transition-zone recording takes place in the coast line area where the land line is

terminated by the sea and shallow sea depths restrict the access of standard marine seismic vessels.

3.1 Land Data Acquisition

Energy sources: there are two main types of sources for land acquisition: dynamite and vibrators.

The first one is the oldest and is very common. The charge is placed in a hole (a few meters deep) and when the receivers

are ready it is fired. This is repeated for every planned shot point. In the case of a 3D survey, there can be thousands of

shots.

The other common source is the vibrator; a truck (or more commonly, several trucks) with a mobile plate. When the truck

reaches the planned vibration point, the plate is placed on the ground and starts to vibrate at set frequencies (from low to

high frequency or vice-versa). The ground is energized in this way.

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Receivers: on land receivers are called geophones, and are buried in the earth. Every receiver station groups several

geophones in a single signal, in order to increase the signal-to-noise ratio. In a marine environment, the receivers are

called hydrophones.

2D acquisition: sources and receivers are aligned along a profile. When the shot (shot point no. 1, for example) is fired,

both source and receivers move toward the end of the profile until they reach the new positions (shot point no. 2).

3D acquisition: sources and receivers are not aligned. The receivers are placed in an area (instead of a profile, as in 2d

acquisition) and, as a consequence, the rays come from different azimuths (multi-azimuth acquisition). Several lay outs

are tested and used, depending on geological and environmental constraints, characteristic of the target, etc

The most demanding part of land surveys is moving the line equipment along across fields or through populated

communities. In general, land acquisition is more expensive than marine surveys and is characterized by lower signal-to-

noise ratio.

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ophone


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3.2 Marine Data Acquisition

In marine operations, a special vessel tows one or more energy sources astern parallel to one or more towed seismic

receiver lines. In this case, the receiver lines take the form of cables containing a number of hydrophones. The vesselmoves along and fires a shot, with reflections received by the streamers. If a single streamer and a single source are used,

a single seismic profile may be recorded like in land operations. If a number of parallel sources and/or streamers are

towed at the same time, the result is a number of parallel lines recorded at the same time. If many closely spaced parallel

lines are recorded, a 3-D volume of data is recorded.

Energy source: the most commonly used source in marine acquisitions (both 2D and 3D) is the air gun (air-compressed

gun). The nearly-explosive release of pressurized air directly in the water produces an acoustic wave and the output is a

bubble which transmits energy downward toward the sea bottom and the layers. The amount of energy is defined before

the start of the survey and depends on several factors, such as the depth of the target, transmission loss, level of the noise,

etc. The total energy is normally obtained using several guns (array of guns) fired together.

2d acquisition: the equipment is represented by a single source with the cable containing the hydrophones (streamer). In

this way, the acquisition occurs along a profile. The cable can be up to 10 Km long.

3d acquisition: the equipment is represented by one or 2 sources and a number of cables, called streamers (from 4 to 16).

Compared to 2d acquisition, 3d requires the use of more equipment and larger vessels.

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4 Processing

Acquired seismic data is not directly suitable for exploration activities; it requires accurate processing using specific

computers, algorithms and expert technicians. The introduction of digital recording has improved the quality of the results

and reduced the overall time. There are two main aims of data processing: the elimination of unwanted signals (called

noise), leaving only primary reflections with geological significance, and the conversion of recorded data into sections (or

volumes) which appear to be equivalent to geological structure sections (or volumes).

The processing activity can be divided into the following main steps:

1. Pre-processing

2. Deconvolution

3. Stacking

4. Migration.

Of course, several other steps are applied, but they may be considered secondary in that they help improve the

effectiveness of the primary processes.

The processing activity normally follows a typical sequence, but tests are common before and during each step.

Moreover, the success of processing depends not only on the correct choice of parameters for that particular process, but

also on the effectiveness of the previous processing stages.

4.1 Preprocessing

This first step of data processing starts from the field data and focuses on the preparation of data for the next steps. Itincludes changes of format, removal of bed traces, recovery of the effect of amplitude reduction due to wave frontdivergence and application of the field geometry (coordinates of shots and receivers).

Demultiplexing: Field data is recorded in multiplex mode using a specific format. Mathematically,

demultiplexing is seen as transposing a large matrix so that the columns of the resulting matrix can be read as seismictraces recorded at different offsets with a common shot point. At this stage, data is converted into a convenient formatused throughout processing.

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Spherical divergence recovery: the amplitude of the signal decreases with the distance from the source. This ismainly due to the increase in the diameter of the wave front. The recorder signal, as a consequence, measures lower

amplitude values as the distance from the source increases. The aim of this step is to obtain an amplitude value relatedonly to the acoustic impedance contrast between two adjacent layers. In this way, the amplitude value is a function of thelithological changes and is not related to the distance of the source.

4.2 Static Corrections

An additional step is needed for most land and some shallow-water data. In this case, the elevation of sources andreceivers may be different and shallower layers could also be affected by lateral velocity variations which, producingdifferent travel times for different source-receiver pairs of the same CMP, result in a distortion of the NMO hyperbola.

The first step in static correction is to reduce all sources and receivers to a fixed datum plane, adding or subtractingmilliseconds corresponding to the time between the true position and the datum elevation. This step is called Field StaticApplication and removes the most significant part of travel time distortion from the data. Normally a small error is stillpresent in the data after field static application. For this reason, a second step of statics called Residual Static Applicationis applied. Time shifts are applied in a surface-consistent way, where the shifts depend only on shot and receiverlocations.Typically, this step is not necessary for marine data.

4.3 Deconvolution

The following step is called deconvolution. Reflections are supposed to be recorded as impulses, but this is not true forseveral reasons: the energy source is not exactly impulsive, a geophone is not able to perfectly record the signals as input,and, besides, the presence of multiple reflections twists the signal. The aim of deconvolution is to remove (partially ortotally) the unwanted effects. Deconvolution thus improves temporal resolution by compressing the effective sourcewavelet contained in the seismic trace to a spike (spiking deconvolution).

4.4 Velocity Analysis, NMO correction and StackEstimation of the seismic velocity field is used in normal moveout(NMO) correction. The principle is the following: thesame sub-surface reflection point (CMP, Common Mid Point) is illuminated several times (up to 120 times), changing theoffset (source-receiver distance). As a consequence the travel time is different for each pair of source-receivers and the

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reflected events on a shot record do not appear as straight lines but as curved lines. This effect is called normal moveout(see figure below).As the reflected wave s-Q has a longer travel path than reflected wave s-P, a horizontal bed appears on a record as a

hyperbolic curve. The shape of the curve is a function of the velocity of sound in the Earth and the depth of the reflectinginterface. During data processing NMO is removed from the data by shifting each trace sample upward. The quantity ofthis shift is called theNMO correction. In order to correctly sum the traces, it is fundamental to define the velocity fieldencountered by the rays during the path. This velocity value is applied to flatten the hyperbola ( normal move-outvelocity). Finally, all the traces can be summed at zero-offset position. This operation, called stack, is aimed at improvingthe signal-to-noise ratio and the final trace is the result of several traces stacked together.

4.5 Time MigrationA stack section can represent only flat and horizontal events in their correct position. The greater the dip of the seismicinterface, the greater the error will be in the stack section positioning. Furthermore, there could be diffracted events in thestacks caused by discontinuities in the interfaces (faults, spires, peaks) that do not give any benefit to the understanding ofunderground formations as they are represented.Time migration is the process that, on the basis of the wave propagation theory, moves dipping reflectors into their truesubsurface positions and collapses diffractions, thereby delineating detailed subsurface features. From this point of view,migration can be seen as a form of spatial deconvolution that increases spatial resolution. The migration process can beconsidered as the solution of the inverse problem in relation to wave field propagation.The result of migration is a more realistic image of the sub surface.

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Stacked data


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4.6 Special processing

4.6.1 A.V.O.AVO, which stands for Amplitude Variations with Offset, is a simple technique that looks for direct hydrocarbon

indicators. It uses the amplitude (or more precisely the variation of amplitude) of the seismic data before stack as an

indicator of the fluid content. Most of the time, gas sands have lower impedance than encasing shales and have reflections

that increase in magnitude with offset. Not all gas sands show increasing AVO effects, since the result depends on the

nature of the acoustic impedance change. Three different classes of anomalies can be identified, related to different

variations of amplitude with offset.

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Direct Hydrocarbon Indicators: normally, in a seismic section, lateral and vertical amplitude variations are due

to variations in lithology (which produce variations in acoustic impedance). It is also possible, moreover, to obtain

amplitude variations when the fluid content in the same rock changes; this effect on a seismic section is called a brightspot. In fact, the reflection coefficient may be different for gas-filled sands or water-filled sands. However, there are othergeological situations that create bright spots, such as coal seams.

4.6.2 Depth ImagingSeismic data, as explained before, records the two-way-time needed to cover the source-reflector surface-receiver

distance. As a consequence, all information is recorded in seconds (or more precisely, in milliseconds); the travel time

depends on the length of the ray paths and, on the velocity fields (i.e., the velocity of rays inside the layers). We obtain an

image of the sub-surface in time, whereas the world is in depth. Depth imaging is a method that, by evaluating the

velocity field of the sub-surface, converts seismic data from time to depth. It can be successfully applied in areas affected

by strong vertical and/or lateral variations in seismic velocities (in this case the depth image can be very different from

the same image in time); this occurs in different geologic contexts: salt tectonic contexts, areas with structural

complexity (where correct evaluation of the fault throw is fundamental), etc Calibrating the depth seismic data withexternal information (such as well ties) ensures good reliability of the results.

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5 Potential Methods

Potential Methods are useful in integrating seismic surveys especially in areas where the seismic response does not allow

for correct structural imaging.

The gravity method is used to determine spatial variations in the subsurface rocks density, which cause small

changes in the Earths gravitational field. In gravity surveys very small variations in the force of gravity are

measured in rocks within the earth. Different types of rocks have different densities, and dense rocks have

greater gravitational attraction. A gravimeter, which measures the force of gravity in the earth, is shown on the

left.

The magnetic method is used to determine spatial variations in rock (igneous and metamorphic) magnetisation

properties, which cause small changes in the Earths magnetic (geomagnetic) field. This method provides a

useful contribution in defining the top of basements and in detecting igneous rocks or intrusive bodies (Sills,

Laccolite) that frequently give a questionable response in seismics due to their high amplitude signal. In

Magnetic surveys we look for variations in the magnetic field of the earth. The magnetic field of sedimentary

rocks is usually much smaller than that of igneous or metamorphic rocks. This allows us to measure the

thickness of the sedimentary section of the earths crust.

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The magnetotelluric (MT) method is used to determine the vertical distribution of resistivity of subsurface rocks

and its lateral variations by measuring the Earths natural electric and magnetic fields. The method is used in

reconnaissance studies, at medium-great depths. When used along with the gravity and magnetic methods, MT

can provide a significant contribution in the case of poor seismic imaging due to complex settings causing poor

penetration of acoustic waves (e.g. over-thrust areas, salt tectonics, basalt covers or intra-layers).

MARINE INSTRUMENT

MT APPARENT RESISTIVITY SECTION

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the seismic method

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