GeoCh5

Well Control11

CONTENTS

1. WHY IS GEOPHYSICS IMPORTANT ?

2. TYPES OF GEOPHYSICAL INVESTIGATIONS

3. BASICS OF WAVE PROPAGATION

4. REFLECTION SEISMOLOGY PRACTICES

5. INTERPRETATION OF REFLECTIONSECTIONS

6. SUMMARY

7. EXERCISES

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LEARNING OBJECTIVES

The aims of this Chapter are to give the Petroleum Engineer:

An understanding of the basic geophysical concepts as used in the petroleumindustry

An overview of the applications of seismic data in reservoir description

After studying this Chapter the student should be able to:

1. Explain the rationale for geophysical investigations

2. List the characteristics of the main geophysical methods

3. Describe the propagation of P- and S-waves

4. Explain how waves are altered at a material interface

5. Describe the nature of seismic reflections

6. Define: acquisition, source, receiver, processing, interpretation and migration

7. Discuss the process of interpreting seismic data

8. List the roles of seismic in reservoir imaging

9. List what properties may be determined with seismic data

10. Describe the controls on, and limits of, seismic resolution

Department of Petroleum Engineering, Heriot-Watt University 3

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1. WHY IS GEOPHYSICS IMPORTANT?

Petroleum Engineers will find themselves working alongside Geophysicists, and theywill make use of geophysical data. A basic understanding of geophysical data - itsstrengths and weaknesses - and how the data are used for interpreting the subsurface,are therefore useful preparations for those experiences.

What can an Engineer gain from geophysical data? Geophysical data can provide theEngineer with vital information concerning the planning of drilling activities, and theplanning for field development. This information includes:

The geometry of the rock bodies that comprise the petroleum system, including theover-burden, the under-burden, the side-burden, and the reservoir itself (theboundary of the hydrocarbon-bearing region can be called the reservoir envelope),along with the characteristics of the petroleum kitchen and its associated migrationroutes

The distribution of reservoir properties (its internal architecture), and the propertiesof the surrounding rocks

The size and distribution of the aquifer

The presence of internal compartments or disruptions of the reservoir (such asfaulting)

Possibly, direct indications of hydrocarbons

Sometimes, the orientation of fractures

In some cases, indications of high pore pressures

However, for reasons that will be discussed in this Chapter, these benefits are notalways realised in practice. The un-fulfilled potential of Geophysics is largely relatedto problems with data quality, but sometimes the reduced benefits are related toinappropriate techniques and limited interpretation skills. Understanding the basiccontrols on data quality (resolution) will help the Engineer appreciate the limits of thedata - although these limits are being consistently pushed back as technology developsin all areas of the geophysical method (acquisition, processing, interpretation andvisualisation).

The main motivation for undertaking geophysical surveys is to develop an image ofthe subsurface geology. The secondary motivation is to determine the properties ofthe rocks and to detect their contained fluids. These goals are met in practice by closeintegration throughout the entire Geoscience team, and between it and the Engineer-ing team.

The rationale underpinning all geophysical methods is that rocks are not all alike, andthat these differences are manifest by differences in the physical properties of therocks. In Chapter 1, you discovered that there are many kinds of rocks, with a wide

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range of compositions. In Chapter 3, we saw that sedimentary rocks are deposited byprocesses that result in spatial heterogeneity of composition, grain size, sorting,packing, etc, and that these processes also result in the stacking of layers with differingcharacteristics. In Chapter 4 it was shown that natural processes that are active in theEarth disrupt the original rock-body distributions, producing translations, rotations,and distortions of the original simple layered arrangements. Other natural events, suchas fluid flows that can produce variations in the cementation of the grains, also add tothe heterogeneous distribution of rock properties. Even the accumulation of hydro-carbons has an effect on the bulk physical properties of the reservoir rock.

Each of these changes in physical character can produce changes in the physicalproperties. For the purpose of developing an image of the subsurface, some of theimportant physical properties of rocks are:

The sonic velocity (speed of passage of acoustic waves)

The density

The resistivity (or its converse, the conductivity)

The magnetic compliance, and/or the natural magnetism

The premise of Geophysics is that these physical properties have effects at adistance. In other words, their variations can be detected in locations that areremoved from the site of the actual property variation - such as at the ground surface.The challenge, and success, of Geophysics is to develop and improve the methods bywhich these detected variations can be related to the spatial locations from which theeffects arise. Useful interpretations of the data depend on both careful calculations andvisualisation methods that create geological displays that correspond to our mentalimage of the subsurface geology.

Scope of this ChapterIn this Chapter, we will briefly consider the range of geophysical methods, but themajority of the Chapter focuses on seismic techniques. We describe the way thatacoustic waves travel through solids, and especially the way that these waves arealtered as they pass from one material to another. We then focus exclusively on activeseismic investigations, in which sonic energy is deliberately propagated into thesubsurface, and where the reflected energy is collected and processed to develop animage of the subsurface configuration of the rocks. A final short section discusses howseismic methods are being used in a new area of activity known as ReservoirGeophysics.

2. TYPES OF GEOPHYSICAL INVESTIGATIONS

An appreciation of the physical properties of rocks has been used to create the fourprimary geophysical investigation techniques:


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Gravity

Electrical

Magnetic

Sonic

Gravity methods are based on the fact that changes in density of parts of the Earth (e.g.a body of rock that has a different density than the surrounding rocks) can be detectedbecause of their effect on the gravity field. Of course, the Earth itself is responsiblefor the majority of the observed gravitational field (it produces an acceleration at thesurface of approximately 9.8 m/sec2), while perturbations associated with changes in1-10 mgal (1 mgal = 103 gal; 1 gal = 1cm/sec2, so gravitational accelaration 980 gal).So, measurements require accuracy of better than one part in a million. Gravitationaleffects associated with density (mass) changes decline with distance. Small densitychanges that occur near the observation point (usually, the Earths surface) can haveas much impact on the observed gravity field as a much larger density (mass) changethat occurs at a larger distance. In practice, gravity surveys are used primarily fordetecting large rock bodies that have a significant density anomaly (e.g. a salt diapir).

Electrical methods are based on the fact that rocks have varying resistivities. In mostcases, the resistivity is actually a bulk value representing the resistive rockframework and the (often lower) resistivity of the contained porefluids. In practice,an electrical potential (voltage) is applied across widely-separated electrodes locatedat the ground surface, and another electrode pair is used to determine the electricalpotential at intervening sites. Source rocks, because of their high carbon content, canhave lower resistivities. Sometimes a survey in undertaken to assess the regionalextent of a source rock layer. However, electrical techniques are not commonly usedfor reservoir studies.

Magnetic methods are based on the fact that rocks can alter the observed magneticfield of the Earth. We all know about the main field of the Earth (e.g. compass needlespointing to the magnetic poles). Rocks that have their own magnetic fields (these canbe produced by some forms of iron- and titanium-containing minerals) can add orsubtract from the main field. If these changes can be detected, the location of themagnetised rocks can be deduced. Strongly-magnetised rocks are not very common,but weakly-magnetised rocks occur in many places (red-beds, basalts, etc). All rocksaffect the local magnetic field through the way that the magnetic forces aretransmitted through them. This effect is called the magnetic compliance, and it toocan be used to infer the spatial distribution of rock units. The spatial resolution ofmagnetic investigations is usually very coarse, and the uncertainties render thistechnique of only limited value. Magnetic methods are not widely applied inPetroleum Engineering.

In contrast to the above methods, seismic (for seismos - the Greek word for shaking)investigations play a major role in Petroleum Geophysics. The money spent onseismic accounts for perhaps 70-80% of the Geoscience budget of most companies.Because of this strong economic emphasis, it is important for the Engineer to

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appreciate both the value and limitations of this method relative to the effort tounderstand the subsurface operation of the Petroleum System.

Seismic waves in rocks are periodic, oscillatory motions; they are a form of acousticwave. Seismic waves can be created naturally (e.g. by an earthquake), or they may beintentionally induced as part of an investigation (this is our primary emphasis in thisChapter). Because the sonic properties of rocks vary from place to place (and perhapsover time as a consequence of producing a reservoir), the passage of seismic wavesis locally affected by the rock pattern and the distribution of fluids. These effects areused by Geophysicists to develop an image of the subsurface distribution of rocks andtheir contained fluids.

There are two main categories of seismic techniques. One is termed (seismic)refraction. In this method, the goal is to determine the position, and velocity anddensity, of rock bodies. It is effective at seeing situations where the lateraldimension is very much greater than the depth. Its depth resolution/accuracy is ratherpoor (at least for problems of interest to Petroleum Engineering). This approach hasonly limited applications to reservoir problems (but note that refraction can be usedto image the extent of major basalt flows, and it is important in making staticcorrections; see below), and we will not discuss it further here.

The other seismic method is called reflection seismology, or reflection seismic, oroften just seismic. In this method, sonic (vibrational) energy is deliberatelypropagated into the ground, and the approach is to detect the sonic energy that isreflected back to the surface, and to use these data to create an interpretation of thesubsurface. Using the ideas developed in the remainder of this Chapter, the returnedseismic data is processed and presented in a form suitable for interpretation. The usualapproach is to display the processed data in a way that looks similar to a geologicalcross section (Fig. 1).

The remainder of this Chapter will concentrate on the seismic reflection method, sinceit is the primary geophysical technique that will be encountered by a practisingPetroleum Engineer.

Figure 1A seismic section from thesouthern North Sea,demonstrating how seismicdata can image geologicalstructures. In this example,a salt diapir has deformedthe overlying sedimentaryrocks, and we can also seepackages of sedimentaryrocks deposited into localpaleo-depo-centres thatevolved as the salt moved.


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3. BASICS OF WAVE PROPAGATION

Above, we said that a seismic wave is an acoustic wave passing through a rockmedium. Such waves are called body waves. They are considered to be elastic waves(see Rock Mechanics Appendix) because there is no permanent distortion caused bytheir passage. There are two types of body waves:

P-waves - ComPressional, longitudinal, Pressure, or Primary waves (Push-Pull) S-waves - Shear, transverse, tangential, or Secondary waves (Side-to-Side)

These body waves consist of particle motions that involve oscillation (periodicmotion, hence the term wave) about a fixed point. In other words, the body itself doesnot change its position in space, but its constituent particles vibrate about theirpositions. In the normal situation that applies to the Earth, rock bodies are notvibrating, because any earlier acoustic waves that may have existed will havedissipated (due to attenuation; see below). The initiation of vibration at a point,especially the way that such initiation of movement sweeps through the rock mass, iscalled the propagation of a wave.

3.1 P-wavesA P-wave is an elastic body wave in which the particle motion involves oscillationsin the direction of propagation of the wave (Fig. 2). Perhaps a simple thought-experiment will clarify this process. Imagine several students standing in a line, allequally spaced apart. Between each student, there is a spring that allows bothshortening and elongation of the distance between the students. To start the acousticwave, one of the students (assume it is the one at the end of the line) moves towardsthe student next to her (and away from the wall). Because of the springs, the studentahead of the initial student must move forward, and the one ahead of him must thenmove forward in turn. To someone watching this process, there is an advancingdisturbance that alters the positions of the students. This disturbance represents thepropagation of the wave.

Time = 0.0 Spring Compressed

Spring Lengthened

Time = 0.0 + tSprings Compressed

Spring Lengthened

Time = 0.0 + 2t

Springs Lengthened

Springs Compressed

Figure 2A one-dimensionaldemonstration of thepropagation of acompressional wave, usingstudents as particles

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Now let us consider what happens after the initial movement has passed a particularstudent. Because of the springs behind that student, the student is pulled backtowards his/her original position. In fact, the stretch of the spring will cause an over-recovery that induces him/her to move further back than where he/she started. Thus,once a vibration is initiated, there is a tendency to continue the oscillation to and fro.

If we adopt a sign convention such that student spacings that are close together areconsidered positive, we can draw a curve representing the density of the students atany particular time (Fig. 3). This curve shows places where the students are bunchedclose together, and places where they are spread apart. The curve has a sinusoidalform. The places where there is a bunching-together are called compression, andplaces where the students are separated are called rarefaction (dilation; sometimeswritten as dilatation). Importantly, the position of the compressional and dilationalspacings is not fixed. Instead, each student is alternately (indeed, periodically) partof the compressional or dilational state. (You may have seen a wave at a sportsevent: spectators stand and then sit down in such a way that an observer sees thisprocess sweep around the stadium. If we let this standing/sitting process represent thecompression/rarefaction of particles, then it is clear that a waveform moves past anyparticular point.)

Wavelength

Position

Dila

tion

Com

pres

sion

Now let us consider how the speed of propagation of the wave is related to the distancebetween successive compressional maxima. This distance is known as the wave-length. Depending on the springiness of the springs, there is a characteristicvelocity of propagation. The velocity and wavelength are related by the frequencyof the waves (the number of waveforms passing an observation point in a given timeinterval). This relationship is:

=

=

=

=

V

f

V

f

where

wavelength

velocity

frequency

Of course, our interest lies with rocks, instead of with thought-experiments involvinglines of students. The principles outlined above apply equally to the propagation ofacoustic waves in rocks, and the students and springs can be thought of as symbolsfor the atoms, and their associated bonds, that make up the mineral constituents of therocks.

Figure 3Plot of compression/rarefaction of studentspacings (at someparticular time) as afunction of distance

Equation 5.1Relationship betweenfrequency, velocity, andwavelength


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The preceding narrative treated a single line of particles (e.g. the students). This one-dimensional conceptual model must be extended to higher dimensions in order toaddress other important aspects of wave propagation. Now, imagine - not just a lineof particles, but a two-dimensional array (Fig. 4). If an acoustic wave propagatesacross this array in a fashion so that the initiation of the oscillations (the wavefront)excites vibrations in particles along a (moving) straight line, this mode of propagationis called a plane wave. Although this situation can be readily understood as a concept,plane waves are somewhat rare in seismic investigations. Instead, most inducedseismic waves propagate away from their source point with a (nearly) sphericalwavefront (Fig. 5). This is known as spherical radiation. The reason for the non-sphericity actually observed is that rock velocities are not constant everywhere.

wav

efro

nt

propagationdirection

compressed zone dilated zone

patrticles arespaced together particles arespaced apart

Compression

Dilation

Compression

Dilation

seismic sourcein shothole

Figure 4Example of a plane waveaffecting a regular array ofparticles

Figure 5Spherical radiation of acompressional wave fromits point source

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3.2 S-wavesAn S-wave is an elastic body wave in which the particle motion involves oscillationin planes that are perpendicular to the direction of propagation of the wave. If wereturn to the image of a line of students (Fig. 6), this mode of propagation involves thestudents having springs attached to their heads and feet. The connections between thestudents are not springs, but rigid bars, so the students do not move forward andbackward, but up and down, as the wave propagates along the line. This type of motionis known as shear. In this example, the initiation of the wave is associated with a sheardisturbance. Another way to initiate shear might be where a P-wave impinges on theline at an angle from the side. We will see below that this process occurs at interfacesbetween materials that have different properties.

Spring Compressed

Spring LengthenedSpring Compressed

Spring Lengthened

Although we imagined a vertical shear in our example, the students might alterna-tively be caused to move from side to side (if the springs were to either side of them)as the wave propagates (Fig. 7). The plane containing the shear movements is calledthe polarisation plane. In fact, its orientation is not restricted to the vertical orhorizontal, but it can be at any inclination. The direction of polarisation is importantbecause it relates to the shear movement of the initial disturbance, or to changes inpolarisation induced by rock-property changes. Fluids (like air and water) do nottransmit shear waves (they lack the rigid connecting bars of our analogy).

Spring Compressed

Spring Lengthened

Spring Compressed

Spring Lengthened

Figure 6Vertical polarisation of ashear wave affecting theline of students

Figure 7Horizontal polarisation of ashear wave


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3.3 Some additional characteristics of wavesAs spherical waves propagate, they must use their energy to excite the movements ofever-increasing numbers of particles as the wavefront expands. The surface area ofa sphere is determined by its radius (depending on the radius (r) squared), so the energyof a spherical wave degrades as it propagates proportionally to 1/r2. This cause of theloss of energy is called divergence. Other losses of wave power occur because energyis needed to make particles change their motions (i.e. to overcome their inertia). Thisconverted energy is physically expressed as a small increment of heat energy. (Inpractice, the heat is so small that it is simply lost in the system.) Because theconversion of motion to heat is similar to the energy conversion that occurs asmaterials slide along a frictional surface, this internal loss is sometimes called internalfriction, but the technical term is absorption. Higher-frequency waves lose theirpower more readily over a given distance than do lower-frequency waves; this islargely because there are more oscillations per second for the higher-frequency wave,and each oscillation has a loss attached to it. Additional energy is lost from a givenwave as it is converted into other wave types (at interfaces with different materials;see below). These reflections and scattering reduce the power of an advancing wave.Together, these losses of energy mean that waves eventually attenuate. Theattenuation of waves is why the Earth is normally at rest and not still vibrating fromthe passage of previous waves. (However, there may be some vibrations occurring atalmost any time, caused by many natural and human-related causes, and theserepresent noise that interferes with seismic investigations.)

P-waves travel faster than S-waves. As a rule of thumb, P wave velocities (Vp) areusually about 1.7 times as fast as those of S-waves (Vs). This difference is related tothe mechanics of the process (see Rock Mechanics Appendix). The P-wave velocitiesof some common rock types are given in Table 1. Note that there is a range ofvelocities for any rock type. This range is related to variations in porosity, cementa-tion, composition, and other factors.

Rock Type Vp (km/sec)

Anhydrite 4.1 - 5.0

Basalt 5.0 - 6.4

Chalk 2.1 - 4.2

Dolomite 3.5 - 6.9

Limestone 1,7 - 7.0

Salt 4.4 - 6.5

Sandstone 1.4 - 4.3

Mixed sand/shale 2.1 - 4.5

Shale - Slate 2.3 - 4.7

Because they have a polarisation, S-waves can be significantly affected by anisotropiesof the medium (i.e. a medium whose properties differ in different orientations). If oneorientation is much faster than the other ones, then it may be possible to detect this byobserving different propagation velocities in the different polarisation planes. Thisaspect underpins efforts to identify open fractures through shear wave variations.

Acoustic waves are affected as they cross the interface between different materials(e.g. passing from one rock layer to another that has different properties). Referring

Table 1Compressional wavevelocities for common rocktypes

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to our student analogy, this represents springs of different stiffnesses attached to thestudents at one end of the line versus at the other end of the line. These variations instiffness relate to variations in velocity. If the frequency of the wave is conserved, thewavelength must change as it propagates into the new material.V

fV

V V

1

1

2

2

2 1 2 1

= =

> >

,

,if

If we move away from our one-dimensional analogies, we need to be aware that it iswavefronts that cross material interfaces, and that the change in velocity produces achange in the orientation of the vector representing the wavefront movement (Fig. 8).The energy of the incoming wave is partitioned, with some of it possibly continuingacross the interface, some being reflected, and some may move along the interface.There is also a potential that waves can be converted from one type to another (eg P-waves converting to S-waves, and vice versa). This process is called mode conversion.

Reflected P-wave

Interface

Reflected S-waveIncidentP-wave

TransmittedS-wave

Transmitted P-wave

Let us now consider in greater detail what happens when a propagating acoustic waveencounters an interface (e.g. a change to a separate rock body with differentproperties). The simplest configuration is one in which the interface is horizontal, andthe wave (let us assume a compressional, or P-wave) is propagating downwards (Fig.9). (For this explanation, it is useful to represent the advancing front of the wave byan arrow. The paths indicated by the arrows are called the raypaths of the wave.) Ingeneral, a wave can approach a point from any direction. When the point is on aninterface, we can identify the angle between the waves direction of advance, and theorientation of the interface. This angle is called the incidence angle. When a waveis travelling at right angles to an interface, this is called normal incidence, so the leftexample is a vertical, normal-incidence case. The question is: what will happen as thewave (vibrations of particles) encounters the interface?

time =

10

time =

11

time =

12

time =

13

incidence angle

normal

(perpendicular)incidence

successivecompressionalwavefronts

interface

i i

i

Figure 8Propagation of wavefrontsat an interface

Figure 9Incidence angles of anapproaching wavefront

Equation 5.2Change of wavelengthcaused by change ofvelocity at constantfrequency (material 1 tomaterial 2)


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The first concept that needs to be covered (to enable this question to be answered) isthat of acoustic impedance. This parameter is determined as the product of the densityand the compressional velocity:

=

=

=

=

V

f

V

f

where

wavelength

velocity

frequency

The acoustic impedance can be associated with the ease of transmission of vibrations.In principle, the density and velocity of a rock (which determine the acousticimpedance) are independent properties, but empirical data indicates that they arecorrelated (i.e. if one increases, so does the other). However, density variations areless than velocity variations (expressed as a percentage). Rock densities range fromabout 2.2 kg/m3 (g/cm3) to about 2.8, with most values being nearer 2.5 or 2.6. Thetable of common rock sonic velocities presented above shows a variation of at leasta factor of two. Therefore, the dominant control on acoustic impedance is the velocity.

The second necessary concept is that of the reflectivity of an interface. In our example,the rock above the interface is termed medium number 1, and the rock below it is calledmedium number 2. The reflection coefficient for a wave passing from medium 1 tomedium 2 is:

RV V

V V12

2 2 1 1

2 2 1 1

=-

+

The energy reflected (bounced back) from the interface is determined by multiply-ing the amplitude of the incident wave by the reflection coefficient. The remainingenergy is, in our example, transmitted downwards into the second medium. Thereflection coefficient is greater than 0.3 for the best interfaces, but is generally lessthan 0.1 for typical geological interfaces.

There are conversions of wave energy that occur if the incidence angle is notperpendicular to the interface. (These situations are the norm, and vertical, ornormal, incidence is the rarity.) For each interface, there is a critical angle, determinedby the velocities of the two media, that controls the behaviour (Fig. 10). Snells Lawdictates the angles between the raypaths of the inbound and reflected/transmittedwave. At incidence angles less than the critical angle, an incoming P-wave is partlyreflected as another P-wave, partly reflected as an S-wave, and some (most) of theenergy is transmitted to the next medium as both a P-wave and an S-wave (Fig. 10a).When the incidence angle equals the critical angle, most of the incoming energy isrefracted along the interface (Fig. 10b), but there is also a reflection and transmissionof S-waves. At angles greater than the critical angle, all of the incoming energy isreflected (Fig. 10c). The equations that represent these energy and mode conversionsare more complicated than we need to address in this Chapter.

Equation 5.3Definition of AcousticImpedance

Equation 5.4Definition of ReflectionCoefficient betweenmaterials 1 and 2

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Incident P-wave

Incident P-wave

Incident P-wave

Reflected Waves

Refracted Waves

Reflected P-wave

Reflected P-wave

Transmitted WavesS

S

P

P

i i

i

c c

i < c i = c i > c

(A) (B) (C)

When waves are reflected, refracted, or converted at an interface, there are importantchanges that can be induced. For example, their sign can be changed, and for shearwaves, their polarisation direction can be altered. If the acoustic impedance of thesecond medium is greater than that of the first, the reflection coefficient is positive(refer to Equation 5.4), and any reflected wave has the same sign as the incomingwave. In other words, an initial compressional waveform is reflected as a compres-sional waveform. On the other hand, if the second medium has a lower acousticimpedance, the reflection coefficient is negative, and an incoming compression isreflected as a rarefaction (dilational waveform). If the acoustic impedances areidentical, there is no reflection.

3.4 Imaging ReflectionsThe principle of reflection of (some) wave energy from an interface is the basis for theentire seismic industry. Modern advances are now also capitalising on other aspectssuch as mode conversions, and additional opportunities exist as a consequence of theway that seismic data are acquired and processed. Subsequent sections of this Chapterwill address how these approaches are implemented in the modern seismic industry.First, however, we will see how the basic knowledge outlined above translates intoimages of reflections.

Seismic sources (see below) produce an acoustic wave that contains many frequen-cies. Therefore, a real reflection consists of many different waveforms (each a perfectsinusoid, but not in phase with each other because their different velocities will haveshifted them in time). If these waveforms are all added together (as they would be ina real case), a new, non-sinusoidal waveform is created. (In reverse, this process iswhat happens when we de-compose an arbitrary waveform into its Fourier compo-nents.) The non-sinusoidal reflection waveform is called a wavelet, and it representsthe net time-varying amplitude of the returned energy.

In the perfect vertical, normal-incidence example that we introduced above, thisprinciple can be developed further to produce what we call a seismic trace. A trace

Figure 10Consequences ofwavesapproaching an interface atdiffering incidence angles.a) Incidence angle less thancritical angle; b) Incidenceangle equal to criticalangle; c) Incidence anglegreater than critical angle.


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is the amplitude versus time plot of the signal that is received at the surface (in realpractice, it is the composite signal of many receivers; see below). Seismic traces, fromadjacent receivers, are plotted next to one another to produce the reflection seismicsection (as illustrated in Fig. 1).

Let us consider the simplest case: a two-layer Earth, with layer 1 being horizontal andextending from the surface down to some uniform depth (say 1000 m), and layer 2occupying the region below that depth (Fig. 11). A source and receiver are located atthe surface. At time = 0.0 sec, a seismic pulse is generated at the source. The resultingacoustic wave propagates (radially) downwards (and laterally, to other locations, butwe are going to ignore that for the moment). The compressional wavefront moves atthe speed determined by the compressional velocity of the material (Vp) (here, let ussay 2500 m/sec). Given this velocity, at time = 0.4 sec (400 ms), the compressionalwave reaches the interface. The physical properties indicate that a positive reflectioncoefficient characterises the interface, so the downgoing incident compressionalwaveform is reflected (but at lesser amplitude) as an upgoing compressional wave-form. The reflected waveform reaches the surface 0.4 sec later. The total time of travelof the wave, from the source, down to the reflector, and back to the receiver, is 0.8 sec(800 ms). This is called the two-way (travel) time, representing the total time to traveldown and back up. This parameter is ALWAYS abbreviated as TWT. The singleseismic trace that would be observed is shown in Figure 12. This has a waveletcentered on the TWT of 0.8 sec. The crucial rule for seismic reflection traces is thatthey ALWAYS depict the TWT. The other rule is that they always depict compres-sional waveforms as a deflection of the curve to the right.

1000

m

surface

interface

source + receiver location

down-going and up-going waves

transmitted wave

Model Configuration

Layer 1: Vp = 2500 m/sec, = 2.5 g/cc

Layer 2: Vp = 3000 m/sec, = 2.8 g/cc

Figure 11Simple, two-layer modelillustrating normal-incidence wave reflectionfrom an interface

116

0.0

0.5

1.0

TWT

(sec)

Seismic Trace

singlewavelet

A more-involved example is shown in Figure 13. Here we have several layers, eachhaving its own thickness and sonic properties. Note, however, that we are stillconsidering each layer to be perfectly horizontal. Based on the argument above(concerning the relative importance of velocity and density in determining reflectiv-ity), we can estimate the positive or negative reflection characteristics of the interfacesby means of the compressional velocities only. For this problem, we need to calculatethe TWT for each layer (each velocity interval), and from these values, determine thecumulative TWT for the deeper reflections. These numbers are shown in Table 2.From the tabulated results, we can construct a seismic trace (Fig. 14), placing anappropriately signed wavelet at each reflection time (TWT).

Vp = 3500 m/sec

Vp = 2500 m/sec

Vp = 2000 m/sec

Vp = 3000 m/sec

600

m85

0 m

350

m

Layer 1

Layer 2

Layer 3

Layer 4

R negative

R positive

R positive

12

23

34

Multi-layer Model

Figure 13Multi-layer model

Figure 12Ideal seismic traceproduced by configurationof Fig. 11


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Layer Interval Thickness Interval Velocity Interval TWT Cumulative TWT

1 600 2500 0.480 0.480

2 850 2000 0.850 1.330

3 350 3000 0.233 1.563

0.0

0.5

1.0TWT

(sec)

Seismic Trace

1.5

This procedure can be used to construct a synthetic seismic trace at several locationsif the subsurface configuration is not horizontally-layered. For example, depths andvelocities may be known from wells spaced some distance apart, and these locationscan be used to construct such traces. The traces can be mentally connected to gainan impression of the form of the seismic reflection section that would be produced bythe subsurface configuration (see Exercises). When traces are spaced closelytogether, the positive and negative waveforms appear to merge (visually). This effectis enhanced by the common practice of colouring-in the positive wavelets. Theresulting bands of dark and light colour are called peaks and troughs, representing thecompressional and dilational portions of the seismic waveforms.

Synthetic traces can be used to address a very important issue: the ability (or not) toimage a thin bed. Consider an otherwise-uniform Earth that has a single, horizontalthin layer of contrasting velocity (Fig. 15). Let us assume that the velocity of themajority of the Earth is 1500 m/sec, and that the velocity of the thin bed (1 m thick)is much greater at 10,000 m/sec. Surely, this extreme contrast of velocity will generatea major reflection?

Figure 14 Seismic traceproduced by modelillustrated in Figure 13

Table 2Determination of TWT forreflections expected inmodel of figure 13

118

Vp = 1500 m/sec

Vp = 1500 m/sec

Vp = 10,000 m/sec

750

m Layer 1

Layer 3

Layer 2

R negative

R positive12

23

Thin-Bed Model

1 m 0.2 ms

apart+ =

wavelets "cancel"

This situation can be treated as a three-layer problem. The reflection coefficient forthe top of the thin bed will be large and positive, and the reflection coefficient for thebottom of the thin bed will also be large, but negative. Given the depth to the top ofthe thin bed (750 m), and the velocity of Layer 1 (1500 m/sec), the TWT to thisreflector is 1.0 sec. Assuming a dominant frequency of 25 Hz for the seismic signal,the wavelength in Layer 1 is 60 m. The interval TWT for Layer 2 is very small: 0.0002sec (0.2 ms). (Note that the wavelength in Layer 2 is 250 m.) Thus, the reflection fromthe base of Layer 2 occurs at the TWT of 1.0002 sec (1.0 + 0.0002). The wavelet forthe reflection at the base of Layer 2 is an exact mirror image of the wavelet for the topof that layer. The two opposite wavelets essentially cancel each other when summedtogether. (They are offset only by the 0.0004 sec of cumulative TWT.) Therefore,no reflection is observed, even for this extreme velocity contrast. For more realisticvelocity contrasts, even-smaller-amplitude signals would be created, and the likeli-hood of imaging the thin bed is essentially nil (any hint of a signal would be lost in thenoise).

By this argument, individual beds do not (normally) produce reflections. But seismicsections (e.g. Fig. 1) clearly show patterns of reflections that correspond to our viewof the subsurface Geology. So, what do seismic reflections represent? The detailedanswer is more involved than this Chapter can address, but we can say that there ismore to the problem than merely the reflectivity of an ideal interface and raypathvectors. The acoustic energy of a seismic signal is a wave, and full wave theory isneeded to determine how the vibrational energy moves through rocks, and how someof it is returned to the surface as a received signal. Interferences arising from multiplelayers (real rock sequences are not single, ideal layers), and tuning effects, are part ofthe story. Although the simple-interface approach that we have taken thus far does notfully represent reality, it is a reasonable approximation, and we can continue using itto gain further understanding of the seismic investigation process.

The thin-bed case outlined above illustrates the principles of tuning and interferencethat are important for understanding how well seismic techniques can image a rocklayer that is being truncated. This situation is known as the wedge problem (Fig. 16).

Figure 15 The thin-bedimaging problem


5Geophysics5

In the example shown here, an upper horizontal layer overlies two layers below thatare tilted. The middle layer is truncated beneath the top layer, forming a wedge shape(this situation might be an angular unconformity, or a depositional on-lap). A seriesof synthetic seismic traces are created along the profile, showing clear images of thetop and bottom of Layer 2 away from its truncation point, but these distinctionsbecome lost as the truncation point is approached. Although the ability to map (seebelow) the top and base of the middle layer is lost, it is nevertheless possible to detectthe disturbance of the seismic reflections and to infer the truncation position. Thus,there is a difference between the ability to resolve a geometry, and the ability to detectthat a geometric change has occurred.

Layer 1: Vp = 2000 m/sec, 2.0 g/cc



The Wedge Model Input

?The Two Events Actually Observed

Note; Not Planar

0.55

0.56

0.57

0.58

0.59

0.60

TWT

(sec)

DistanceSynthetic Seismic Traces

Figure 16The wedge problem

120

4. REFLECTION SEISMOLOGY PRACTICES

A seismic investigation can be described in simple terms as follows:

1. Mechanical energy is injected into the ground (by means of a seismic source)

2. The acoustic waves that are generated travel through the rocks

3. Some of the energy is reflected from a variety of interfaces and returns to thesurface at varying times

4. There, the incoming waves are detected by sensors (geophones or hydrophones)and the data recorded

5. Sophisticated computer algorithms manipulate these data and produce imagesthat convey the spatial distribution of physical properties of the rocks. Thesedistributions of properties are inferred to be directly related to the distributionof the rocks themselves.

The first four steps are called acquisition, and the fifth step is called processing (andthis merges with a task called interpretation).

4.1 Seismic AcquisitionWe might imagine an unrealistic Earth in which all rock layers are horizontal, withhorizontal interfaces. In that idealistic situation, we could place a seismic source anda receiver at the same point on the surface, and these devices would propagate anacoustic wave and then sense its reflections. The reflected wave energy would arriveback to the receiver at times determined by the velocities of the rock layers and theirthicknesses (as in the example above).

In reality, the Earth is more complicated than this simple situation. (If it were not, thenwe would have little need to conduct seismic surveys in the first place!) Acousticwaves generated by a seismic source (often called a seismic shot) propagate in aroughly spherical fashion, and they reflect off of a variety of surfaces that may not belocated directly beneath the source point. The returning energy is, therefore, scatteredover a wide area of the surface. In order to capture more of this returning energy, andfor other reasons that we describe below, multiple receivers are placed across an areaof the surface. Thus, the energy of a single seismic shot is usually recorded by multiplereceivers. A schematic drawing of this arrangement illustrates the main elements ofthe acquisition process (Fig. 17).


5Geophysics5

Figure 17Schematic illustration ofseismic acquisition

ReceiverSource

Geophones / HydrophonesSource

The Earth

Filter

Rock Interface

SurfaceDynamite

Vibroseis

Air Gun

Sea Bed

Transmitted Energy

Reflected Energy

On land, seismic sources can consist of:

explosives (often called dynamite, even if other explosive materials are actuallyused). These are usually buried in shallow - approximately 5 m deep - boreholesthat are called shotholes, or

vibroseis (a special, very heavy truck that has a vibrating plate which is placedagainst the ground).

In the marine environment, sources tend to be air guns (sometimes water guns). Theseimpart a short-lived mechanical pulse to the water, and the resulting shock wavetravels to the sea (or lake) bed, and then into the sediments and rocks below. Energyreflecting from deep rock interfaces must also re-cross the water column (unless sea-bottom receivers are in use; see below). Because the returning signal passes throughthe water column, no shear waves can be recorded.

Each source location is called a shotpoint, regardless of the source type, andindependent of whether the survey is on land or under a body of water.

Air guns and explosives produce a seismic signal that contains many differentfrequencies (from 10 Hz to more than 100 Hz). A vibroseis source also produces arange of frequencies, although in this case, the truck vibrates the ground in such a wayas to sweep through a set of frequencies (perhaps from 20 Hz to 75 Hz) over a finitetime interval. It is the range of frequencies that exist in a seismic source that causesa sharp return signal from a reflection interface (see discussion above).

Seismic receivers in the marine environment are called hydrophones (these detectpressure changes caused by compressional waves, converting them to electrical

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signals; Fig. 18). A single streamer of 16-24 equally-spaced hydrophones (channels)is common, but nowadays, multiple parallel streamers are used, producing many100s of channels of data. In the onshore environment, seismic receivers are calledgeophones. Geophones are clamped to the ground and detect motion throughvibrations of a coil of wire moving through a magnetic field, producing time-varyingelectrical signals (similar to the operation of standard microphones, as used intelephones etc). Geophones are also placed in arrays, and one of the costs of onshoreseismic acquisition is the careful surveying of geophone and shotpoint locations.

Hydrophones (channels)1 16

Interface

SourceStreamer

(plus additional paths to each hydrophone)

Sea level

Refractions

Reflections

The configuration of geophone/hydrophone arrays is dependent on the design of theparticular survey. Two-dimensional seismic surveys (2-D seismic) are acquired bywidely spaced (100 m apart, and often much more) lines of receivers that are parallel(or sub-parallel) to each other. The in-line spacing of receivers is, however, muchsmaller (often 25 metres). Nowadays, it is more usual (for both development andexploration) to acquire three-dimensional (3-D) surveys. In these, the spacingbetween lines could be of the same order as the spacing of the in-line receiver array(25 meters, or, increasingly, 12.5 meters). This arrangement allows the data to beprocessed along the acquisition line orientation (in-line), and normal to it (cross-line),providing high-resolution coverage of the subsurface (this is particularly useful inresolving faults, Fig. 19). Horizontal sections (time slices) can also be generated from3-D data cubes.

Figure 19Example of improvedinterpretation enabled by 3-Dseismic surveys andimproved processing

Figure 18Marine seismic acquisition


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Seismic waves that are detected by each geophone or hydrophone are converted toelectrical signals. These signals are transmitted by wires (along what are calledseismic cables), or sometimes by radio, to a central control facility where they arerecorded. Occasionally, data are recorded onto a local tape or disk sited with thereceiver, and later assembled into a larger dataset. The start time for recording is thetime of the shot, and the duration of recording is anywhere from one or two seconds,to perhaps as much as six or eight seconds. In order to ensure fidelity of the signal,recording is universally digital (making use of analog-to-digital signal converters).The digital sample interval is usually either 2 ms or 4 ms (two or four one-thousandthsof a second). At 4 ms, there are 250 data points per second, producing some 1500 datapoints for a six-second recording. Each data point may be assigned two bytes ofstorage (16 bits to record 65,536 different amplitude levels).

Because each receiver eventually records returning waves from many differentshotpoint locations (perhaps several hundred), and because there are many receiversin a typical survey (at 25 m spacing, there are 40 each way per square kilometre, timesseveral tens of km2 in a typical seismic survey), the quantity of data is enormous. Inaddition, vibroseis methods involve a source sweep that takes perhaps 20 seconds.The receiver must record for this amount of time, PLUS the desired imaging time (e.g.the six seconds in the above example). The costs of managing such huge data volumesrepresent a significant factor in the decision to acquire seismic data, and they impactthe design of every survey that is undertaken.

Seismic acquisition is conducted by service contractors. Occasionally, a survey isconducted by these companies on spec, where the contractor expects to be able tosell the data at a later time to oil company purchasers. More often, an oil companyarranges for a contractor to conduct a survey for a specific purpose (upcoming leasesale, development decision, production monitoring, etc). In all cases, the surveys areconducted either with oil company representatives or consultants monitoring everyaspect of the operation.

Seismic surveys entail sophisticated logistical support and often are associated withhigh mobilisation costs. Surveys need to be co-ordinated with other activities (in themarine environment, this entails other seismic surveys, associated marine operations,shipping, platform installations) that may be active within the area. The weather isoften a major consideration. Safety (in remote tropical or desert areas, and in thedeeper oceans) and environmental issues (environmentally sensitive zones such asbreeding grounds, locations where there is a risk of ground water contamination,potential impact on fishing, etc) have to be considered in seismic surveying.

4.2 Seismic ProcessingThe aim of processing is to emphasise primary reflections (signal) and to reducespurious received energy (noise). (A view that is gaining popularity is that noiseis also signal - but a signal that we just dont yet understand.) The energy of the seismicwaves that return to receivers is very small in comparison to the source energy. Thisis because energy was lost during spherical radiation, and because only small fractionsof THAT energy were reflected back, and because THOSE reflections also radiatedspherically, and some of their reflected energy was reflected back down when theycrossed the interfaces in the rocks above.

124

The strategy that has been developed to address the weak signals is to add together (tostack) separate received signals (from different receivers), since this should(hopefully) reinforce the true signal, and the noise would cancel out (being random).

Modern acquisition techniques are designed to capitalise on this notion. Consider thegeometry of shots and receivers illustrated in Figure 20. Multiple pairs of shot/receiver locations generate reflections from the same subsurface point. This point,midway between the source and receiver, is called the common mid-point (CMP).

Source Points Receiver Locations

Surface

Interface

Common Mid Point

7 6 5 4 3 2 1 2 3 4 5 6 7

Although each of the source/receiver pairs images the same subsurface point, thepaths of the waves for each pair have different lengths, which means that they traveldifferent distances. The longer distances represent greater times of travel, so thenotion of stacking the data together requires that an additional numerical manipulationbe undertaken to account for this effect.

Another example of the way that distance of travel impacts the time of a reflection canbe seen if we consider a single source point and multiple receivers arrayed beside it(Fig. 21). Here, a shot sends a wave into the subsurface. The paths taken by thewavefronts that return to the receivers are as shown. The source/receiver paths arelonger for greater distances on the surface between the shotpoint and receiver. (Thesedistances between source and receiver are referred to as the offset). In fact, thereis a well-defined relationship (known as moveout) between offset and path length,and if the velocity is considered, between offset and TWT. This relationship plots asa hyperbola whose form is a function of the velocity of the upper layer and the depthof the reflecting interface. (A plot from a single source point shooting into multiplereceiver locations is known as a gather.) Because the form of this plot is sensitiveto these parameters, moveout plots are used to deduce subsurface velocities. In

Figure 20Illustration of CommonMid Point geometry ofsource/receiver locations


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practice, this is done by assuming a range of possible velocities, and comparing theresulting observed moveout to the expected form, thus enabling a constraint to beplaced on the actual velocity of the overlying layer.

15 16 17 18 19 20 21 22 23 24 25 26 27

25m

Reflector

Layer 1: Vp = 1800 m/sec


Ray Paths from Shotpoint 21 into Array of Recievers

100m

21 23 25 271917150.00

0.10

0.20

TWT

(sec)

Shotpoint Number

source point

Gather for Shotpoint 21

The understanding of the moveout phenomenon gained for horizontal reflectors canbe extended to circumstances where the reflector is dipping (not horizontal). Thegeometry of the wavefronts in this case (Fig. 22) depend additionally on the magnitudeof the tilting, and there is no simple plot to use for estimating velocities (because thetilt is also not known). The complex wave-path geometry illustrated by this situationnecessitates a more-involved processing effort. In simple terms, the underlying goalis to relocate the reflections to a position that is more nearly where the should be ina distance-time plot - i.e. a normal reflection section.

Figure 21Example gather illustratingMoveout for a horizontalreflector

126

Dipping Reflector

100m

95 96 97 98 99 100 101 102 103 104 105 106 107 Surface



25 mRay paths from shotpoint 101 into array of receivers

101 103 105 1079997950.00

0.10

0.20

TWT

(sec)

Shotpoint Number

source point

Gather for Shotpoint 101

The processing methods that are intended to accomplish such relocations of thereturned signals can be grouped under the heading of migration. Migrationalgorithms require that velocity distributions be given as input data. Since velocitiesare usually believed to be directly associated with the rock bodies, this task requiresthat the answer be known before the answer can be derived. In practice, additionalinformation on velocities (such as from well logs), and other techniques that make useof other aspects of the reflection data, can be combined in an iterative way to developgood estimates of the velocity distribution. The end goal of a migration effort is toproduce a depth section - that is, one where the time axis is replaced by a depth scale.

One of the major successes of migration is to remove (actually, move to its correctlocation) the diffracted data that arises at subsurface velocity discontinuities (such as

Figure 22Example gather illustratingMoveout for a dippingreflector


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where beds are truncated by faults). The diffractions have the same hyperbolic formas the moveout gathers, and they can represent a severe interpretation problem if theyhave high amplitudes and thus obscure other data.

Above, we have emphasised the fact that the amplitude of seismic reflections will bevery small in comparison with the amplitude of the seismic source. Indeed, theamplitude of later arrivals (reflections from deeper in the TWT section) is much lessthan the amplitude of early (shallow) arrivals. In order to make interpretation easier,seismic data are subjected to a gain adjustment (during processing) such that thelargest peak-to-trough amplitudes of each region are scaled to appear equal on thefinal plot.

Another processing modification that is usually applied to the data is filtering. In thisactivity, the recorded data are subjected to a frequency filter. The objective of this stepis similar to that of the gain adjustment: to make interpretation easier. Recall thathigher-frequency signals are degraded over distance more than lower-frequencysignals. Thus, data from deep in the section will (naturally) have a lower frequencycontent. It is sometimes helpful to remove high-frequency returns from the shallowerportions of the dataset so that peaks and troughs have similar appearances regardlessof their location in time. Other reasons for performing filtering do exist, but we do notneed to address these specialist issues here.

Additional processing steps are needed to make other corrections to the data. Foronshore surveys, the locations of the shotpoints and receivers are never exactly asplanned. The actual positions need to be used to move the data to reflect reality. Inparticular, the elevations of the sources and receivers are extremely critical. Suchalterations of the data are called static corrections. Direct arrivals (seismic energytravelling along the surface, or through the water, from source to receiver) must beremoved, since these do not contribute useful information regarding the deepsubsurface, and their presence will detract from interpretation. In the marineenvironment, water-bottom multiples (acoustic waves bouncing off the sea/airinterface, and then again off the water bottom) can be a serious interpretation problem(because their amplitude can remain large). These various processing issues are notedhere to indicate some of the complexity that occurs behind the scenes.

5. INTERPRETATION OF REFLECTION SECTIONS

The main objectives of a seismic interpretation are to determine:

two-way travel times and/or depths to rock interfaces

the dip of interfaces

the location of discontinuities in rock interfaces (faults)

changes of stratigraphy (due to lateral facies changes) or unconformities

rock and/or fluid properties from seismic velocities or impedances within layers

128

These objectives can only be met if the acquisition and processing activities have beenperformed in a way that produces a result that is suited to the task at hand. In reality,interpretation often reveals aspects that can be used to return to the processing stageto gain an improvement in the quality of the image. Such iterative loops can be highlyrewarding in terms of their resulting enhancements to the understanding of thesubsurface.

The usual end product of a seismic interpretation effort is a map, or series of maps, thatdepict the shape(s) of geological horizons or the distribution of some characteristic(such as high- versus low-amplitude reflectivity). Increasingly, these maps are beingused to develop a full 3-D representation of the subsurface (this is often called a geo-model, or shared Earth model, when the regions between the surfaces are filled withinformation about the rocks that are there). Such a model can be used to depict thespatial variations in rock characteristics, including petrophysical properties (thesemight be derived from seismic attributes; see below). The information from a modelof this sort can be used to undertake reservoir simulations, or basin-process simulations(basin modelling).

Most interpretation work relies on the classical seismic reflection section (Fig. 23). Inthis display, the vertical axis is TWT, while the horizontal axis is distance. (Theindividual shotpoint locations shown on this axis can be related to a map.) Notice thepresence of white-black bands dipping to the left, away from the well location. Thesebands are termed events, and they are composed of the troughs and peaks,respectively, of adjacent seismic traces in which the reflections are at different TWT.The close spacing of the traces (on the print) enhances the clarity of the events. Betterdata quality, and improved processing, make interpretation easier (Fig. 24).

Figure 24An example of modernseismic data from the NorthSea. This line is taken froma 3D survey conducted in1986. It is in the samelocation as the line shownin Figure 23

Figure 23An example of a North Sea2D line from 1966 showingthe quality of the data uponwhich early, largediscoveries were made


5Geophysics5

The basic task is to pick events on the seismic section (Fig. 25). At each shotpoint,the TWT of these events is noted, and transferred to a map. These data are thencontoured to produce a time map (refer to Chapter 7). In modern practice, theseismic events are usually picked with the assistance of a seismic workstation. Thiscomputer has special software that understands seismic rules (basic interpretationprocedures are programmed in), and hence can perform what is called an auto-pickof events (once the interpreter has made initial picks). The picks are followedthroughout the data volume (either manually or with the assistance of the workstation).Discontinuities of the seismic events are usually interpreted as faults. The faults areoften mapped as separate surfaces, although sometimes they are merely treated asgaps or breaks in the mapped horizon.

AB C

In order to be useful for planning wells, or for calculating reserves, time maps needto be converted to depth maps (actually, elevation maps; see Fig. 26). This taskrequires that the velocities be known. As noted above, velocities are usually estimatedduring seismic data processing. Additional information can be gained from othersources. Well logs (in particular, the sonic log) measure the thickness and velocity ofrock layers. The interval velocities can be integrated to give an average velocity toany depth in the well. However, there are uncertainties in this technique due to avariety of small problems. A better technique that is available after a well is drilledis called a check-shot survey (or sometimes, a velocity survey). In this technique, asonic source is placed at the surface, and a geophone is progressively lowered alongthe wellbore to determine the actual average compressional velocity from the surfaceto any depth.

Figure 25Events that have beenpicked on the seismic lineshown in Figure 24

130

A B CFigs 24 -26

Points A, B, C

refer to Fig 25

U n c o n f o r m i t y

Dunlin

Statfjord Fault

Brent

Structural Cross Section

In the earlier stages of exploration and development, it may not be clear what thevarious observed seismic events mean. In other words, we may not know if an eventrepresents a reservoir unit or a seal. If a well is near the seismic line, and its rocksuccession has been interpreted, the sonic data from the well-log suite can be used tocreate a synthetic seismogram (something like a sophisticated seismic trace). Thisplot can be used to decide which events on the seismic section relate to which rockunits.

There are a few common pitfalls associated with converting time images to depth.Unexpected lateral lithological variations (facies change from shale to limestone),missed compaction trends (due to differential burial), or unidentified high-velocitymaterials (e.g., salt, volcanics), or low-velocity materials (gas chimneys abovereservoirs), particularly in areas of sparse well control, can all lead to significant errorsin depth prediction. These, often understandable or unavoidable errors, can lead to

Figure 26Structural contour map ofthe field over which theseismic lines shown in thepreceding figures wereobtained


5Geophysics5

depth discrepancies - particularly in exploration - and are the reason for many changesin drilling programmes and the outcome of wells. Luckily (or unluckily!) the targetcan be both shallower, or deeper, than prognosed.

5.1 Other Seismic Methods

Direct Hydrocarbon Indicators (DHI)Under certain (often optimal) conditions, the seismic data can give direct indicationsof the presence of hydrocarbons. Free gas in the reservoir can give rise to largeimpedance contrasts (bright spots) by reducing the impedance in the reservoir.These, when recognised (e.g. Troll Field), can lead to very high licence bids by theoperators as they are evidence (pre-drilling) of significant hydrocarbon accumula-tions. However, bright spots can be related to other phenomena - such as coals, tuningeffects, non-hydrocarbon gases or non-recognised, spurious noise - and thereby leadthe industry astray. A bright spot associated with a gas-water contact should be flat.The effect of free gas on seismic is large and non-linear - small gas saturations mayhave very large effects. Where fields are blessed with a DHI, caused by the gas-watercontact, these can be used to monitor production performance if subsequent surveyscan detect position changes of the DHI (e.g., Frigg Field).

Time-Lapse seismicRepeat 3-D surveys after a passage of time are called 4-D, or time-lapse, seismic.Time-lapse seismic is becoming increasingly used for reservoir monitoring. In somefields, where there are DHIs (e.g. Foinaven Field in the Atlantic Margins), OceanBottom Cables (OBC) are being laid on the sea-floor to allow for repeat seismicmonitoring of the production. In this case, the reservoir sands are relatively shallow,and the oil-bearing sand is clearly distinguishable from the water-bearing sand. Thefixed receivers eliminate some of the repeatability problems (resulting from differentprocessing and navigation schemes) between successive seismic surveys. They alsoallow for the recording of shear-wave data. However, at this stage, OBC is asignificant investment with unproven, but potentially high, rewards.

Wellbore seismicThe Vertical Seismic Profile (VSP) is an extension of the velocity (or check-shot)survey. The data are recorded in a similar way as the velocity survey except that anarray of geophones is used to record at many depths simultaneously, and the fullwaveform is recorded. Arrivals can be separated as upgoing (from below the receiver)and downgoing (from above it). The direct arrival is a downgoing wave.

The main uses of the VSP are:

velocity calibration (as in a velocity survey)

distinguishing primary reflections from multiple events

prediction ahead of the bit (in detailed planning of complex high-angle wells, it canbe useful to further define the well path)

imaging near-well-bore discontinuities - faults, salt wall margins - which mayinfluence sidetrack locations or production performance

132

When the source is placed in one well and the receiver array is placed in another well,cross-well seismic can be acquired. Tomography is an established medical techniqueto reconstruct a section through a body from measurements around the body. Inseismic cross-well tomography, we can use a combination of well-to well, surface-to-well and surface-to-surface techniques on reflection or refraction energy to image theinter-well reservoir structure and properties. At a smaller scale, full waveform soniclogging within a borehole (a horizontal borehole) can be used to image bed boundariesand assess bed continuity.

Amplitude versus Offset (AVO), and AnisotropySeismic reflection data is normally presented as a stacked section. Any variation inamplitude with offset (amplitude versus offset, AVO) is normally lost in theprocessing (recall, this is the moveout correction). AVO anomalies can be caused bythe changes in Poissons ratio in the rocks on either side of an interface (see RockMechanics Appendix). (The Vp/Vs ratio varies with the ratio of compressional toshear velocity of the rock.) These variations are due to changes in the rock matrix orthe fluid content. AVO effects are recognised on the CMP gathers. AVO can be usedto detect fluid changes during production. This is a specialised technique with its owncosts and benefits, along with pitfalls.

Seismic wave velocity depends on the direction of propagation in anisotropic media.An S-wave travelling through such a rock will split into two waves. Fractures produceanisotropy, and shear-wave splitting can be used to determine fracture orientation andintensity. There are additional techniques involving P-wave anisotropies, but these aremore specialised than we can cover here.

Seismic resolutionSeismic resolution is a function of the wavelength of the acoustic energy. At usualseismic frequencies, the wavelength varies from about 30 m (shallow depths) to 300 m(deep in basins) in typical hydrocarbon systems. Seismic resolution (the ability todetermine the top and base of a bed by observing distinct peaks or troughs - the events)is usually taken to be equal to a quarter of a wavelength. Therefore, resolution rangesfrom about 8 to 60 m. The resolution range of a variety of subsurface acoustic toolsis given in Figure 27. Seismic detection (of a bed, or changes between surveys in 4-D seismic) does not require imaging of the top and base of beds, and therefore can beexpected at thicknesses up to one thirtieth the wavelength (if the impedance contrastis sufficiently large). If this rule holds, then detection might be possible for bedsranging from 2 to 10 m in thickness (refer to discussion above regarding the wedgeproblem).

AttributesAbove, we have commented on the way that various anomalies affect the qualityof seismic data. We have noted that changes in fluid content (as might happen in oneplace during the production history of a reservoir, or differences between places, suchas the hydrocarbon and water legs of a reservoir) can affect the seismic signal. We alsonoted that variations in cementation and porosity can have subtle impacts on theseismic data. Historically, the goal of processing has been to eliminate these effects,so as to produce a cleaner section for interpretation. Increasingly, methods are beingdeveloped that capitalise on these changes in an effort to identify the spatial locations


5Geophysics5

Figure 27Comparison of scale andresolution of variousacoustic techniques

of different rock and fluid types. The characteristics of the seismic data that can beused for making these distinctions are grouped together under the name attributes.It is now common to see seismic data being processed to emphasise such variations,and for maps to be created showing their spatial patterns. The premise of this workis that there is a direct correlation between an attribute and the properties orcharacteristics of interest in the rocks. The term attribute can also be applied to derivedparameters (e.g. attribute 1, plus attribute 2, divided by attribute 3). The growingsignificance of seismic attributes proves the adage that noise is merely signal wedont understand.

SURF

ACE

BORE

HO

LE

Earthquake seismology

Seismic refraction

Vertical seismic profiling

Crosswell tomography

Wireline sonic logs

Acoustic image logs

Seismic reflection

Range of Investigation (metres)

TypicalFrequency (Hz)

VerticalResolution (mertes)

10000000

25000

5000

2500

500

1

0.05

2

10

30

60

1000

15000

1000000

1000

100

20

10

1

0.1

0.001

6. SUMMARY

In this Chapter the Engineer will have learned that:

the seismic method is the study, by means of imposed acoustic waves, of interfacesbetween beds or formations of contrasting impedance, and that the reflection andrefraction of energy from these interfaces allows for the mapping of these surfaces

seismic data allow the interpretation of surfaces in time. Well data are needed foraccurate depth conversion

seismic resolution is a function of frequency bandwidth (wavelength)

there are a variety of surface and borehole techniques and, with time-lapseoperation, these can provide cost-effective reservoir monitoring

134

In the old days (many decades ago), seismic methods were very direct: source andreceiver were located together, and each receiver trace was simply plotted next to itsneighbours. The resulting sections were not very good, but the interpreter could beconfident of how the data were collected. Now, with the stacking of data frommultiple receivers, and the associated requirement for sophisticated processing,seismic sections can seem slightly mysterious, and rather removed from any potentialfor a reality check. Some people have turned this situation into an opportunity forhumour (Fig. 28), implying that seismic data are purely imaginary. The amount ofIndustry money expended on seismic, and the successes that have resulted from itsuse, indicate that this technique is of crucial importance for exploration anddevelopment. Petroleum Engineers should expect to be involved with Geophysicistsat an increasing level throughout their career.

Acknowledgements

The authors acknowledge Mike Cox, Philip Ringrose, Robin Westerman and ColinMacBeth who provided assistance with this Chapter.

7. EXERCISES

1. This exercise expands on the understanding developed in the Chapter regarding thecreation of synthetic seismic traces. The subsurface configuration of a hypotheticalregion is given by Figure 29. Note that the layers are horizontal, but that they havevelocities that vary from one side of the section to the other. The task is to create atrace for each location, A and B. This requires that you create a table for each locationin which the depth, velocity, interval TWT, and cumulative TWT are calculated. Thenassess the reflection characteristics of the interfaces (positive or negative). Place awavelet at the correct TWT for each trace, and, draw correlation lines between thewavelets.

Figure 28Tongue-in-cheek view ofthe seismic method


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A B

Vp = 1800 m/s

Vp = 2000 m/s

Vp = 1700 m/s

Vp = 1900 m/sVp = 2200 m/s

Vp = 1900 m/s

Vp = 2100 m/s

Vp = 1750 m/s

Vp = 1950 m/sVp = 2300 m/s

500

m60

0 m

700

m20

0 m

several km

2. In this exercise, you will determine depths from a time section (simplified). Youare given a seismic section in Figure 30. This section is, as usual, in TWT. The mainreflectors are both peaks and troughs, representing positive and negative reflectioncoefficients, respectively. Estimated P-wave velocities are given for each interval forlocations X and Y (assume that these were derived from moveout gathers). Your taskis to generate a depth cross section. This requires that TWT be converted to intervalTWT, and this converted to interval thickness. Use the interval thicknesses toconstruct the cross section.

0.4

0.6

0.8

1.0

TWT

(sec)

X Y

Vp = 1900 m/s

Vp = 2100 m/s

Vp = 1750 m/s

Vp = 1950 m/s

Vp = 2300 m/s

Vp = 2400 m/s

Vp = 1800 m/s

Vp = 2000 m/s

Vp = 1700 m/s

Vp = 1900 m/s

Vp = 2200 m/s

Vp = 2300 m/s

5 km

Major Reflection Events, and Velocities, as Given

Figure 30Cross section informationfor Exercise 2

Figure 29Simplified seismic sectionfor Exercise 1

136

So

luti

on

to

Ch

ap

ter

5

Exerc

ise 1

(Giv

en d

ata

in b

old

)

Well A

Inte

rval T

hic

kn

ess (

m)

Inte

rval V

elo

cit

y (

m/s

)T

ime inte

rval (o

ne w

ay)

Tim

e inte

rval (t

wo w

ay)

Depth

(m)

Tim

e (

s)

500

1800

0.2

77777778

0.5

55555556

500

0.5

55556

600

2000

0.3

0.6

1100

1.1

55556

700

1700

0.4

11764706

0.8

23529412

1800

1.9

79085

200

1900

0.1

05263158

0.2

10526316

2000

2.1

89611

Well B

Inte

rval T

hic

kn

ess (

m)

Inte

rval V

elo

cit

y (

m/s

)T

ime inte

rval (o

ne w

ay)

Tim

e inte

rval (t

wo w

ay)

Depth

(m)

Tim

e (

s)

500

1900

0.2

63157895

0.5

26315789

500

0.5

26316

600

2100

0.2

85714286

0.5

71428571

1100

1.0

97744

700

1750

0.4

0.8

1800

1.8

97744

200

1950

0.1

02564103

0.2

05128205

2000

2.1

02873

Exerc

ise 2

Well X

Inte

rval T

hic

kness (

m)

Inte

rval

Velo

cit

y (

m/s

)T

ime inte

rval (o

ne w

ay)

Tim

e inte

rval (t

wo w

ay)

Depth

(m)

Tim

e (

s)

432

1800

0.2

40.4

8432

0.4

8

110

2000

0.0

55

0.1

1542

0.5

9

93.5

1700

0.0

55

0.1

1635.5

0.7

57

1900

0.0

30.0

6692.5

0.7

6

110

2200

0.0

50.1

802.5

0.8

6

Well Y

Inte

rval T

hic

kness (

m)

Inte

rval V

elo

city (

m/s

)T

ime inte

rval (o

ne w

ay)

Tim

e inte

rval (t

wo w

ay)

Depth

(m)

Tim

e (

s)

541.5

1900

0.2

85

0.5

7541.5

0.5

7

115.5

2100

0.0

55

0.1

1657

0.6

8

87.5

1750

0.0

50.1

744.5

0.7

8

68.2

51950

0.0

35

0.0

7812.7

50.8

5

115

2300

0.0

50.1

927.7

50.9

5


5Geophysics5

1.0

250

500

750

1000

2.0

Well A Well B

X Y

TWT(Secs)

Depth(M)

Q2

Q1

GeoCh5

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Transcript of GeoCh5