processing of reflection seismic data

7/28/2019 Processing of Reflection Seismic Data

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PROCESSING OF REFLECTION SEISMIC DATA

GENERAL REMARKS

1. Conventional processing of reflection seismic data yields an earth imagerepresented by a seismic section.2. Common mid-point (CMP) recording is the most widely used seismic dataacquisition technique. By providing redundancy, measured by the fold of

coverage in the seismic experiment, it improves signal quality.

3. Seismic data processing strategies and results are strongly affected by fieldacquisition parameters. Additionally, surface conditions (i.e. presence of near

surface weathering layer) have a significant impact on the quality of data

collected in the field.

4. Surface conditions also have an influence on how much energy from a givensource type can penetrate into the subsurface.

5. Besides surface conditions, environmental and demographic restrictions (i.e.,those related to existence of populated areas) can have a significant impact onfield data quality.

6. Other factors such as weather conditions, care taken during recording, and thecondition of the recording equipment, also influence data quality. Almost

always, seismic data are collected often in less-than-ideal conditions. Hence,attenuation of noise and enhancement of the signal during processing of data

is dependent on the quality of seismic data during recording in the field.

7. In addition to field acquisition parameters, seismic data processing results alsodepend on the techniques used in processing. A conventional processing

sequence almost always includes the three principal processes: deconvolution,

CMP stackingand migration.

8. Deconvolution often improves temporal resolution by collapsing the seismicwavelet to approximately a spike and suppressing reverberations on marine

data. The problem with deconvolution is that the accuracy of its output may

not be self-evident unless it can be compared with well data. The reason forthis is that the model for deconvolution is non-deterministic in character.

9. Common midpoint stacking can attenuate uncorrelated noise significantly,thereby increasing the S/N ratio. It can also attenuate a large part of thecoherent noise in the data, such as guided waves and multiples. The NMO

correction before stacking is done using the primary velocity function.

Because multiples have higher moveout then primaries, they are under-

corrected and hence, attenuated during stacking. The main problem withCMP stacking is that it is based on the hyperbolic moveout assumption.

Although it may be violated in areas with severe structural complexities,

seismic data acquired in most cases seem to justify this assumption reasonably

well.10. Data acquired over land must be corrected for elevation differences at shot

and receiver locations and travel time distortions caused by a near-surface

weathering layer. The corrections usually are in the form of vertical traveltime shifts to a flat datum level (statics corrections). Because of uncertainties


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in nearsurface model estimation, there always remains residual statics whichneed to be removed from data before stacking.

11. Migration collapses diffractions and moves dipping events to their supposedlytrue subsurface locations. In other words, migration is an imaging process.

Because it is based on the wave equation, migration also is a deterministic

process. The migration output often is self-evident. When it is not so, this canoften be traced to the imprecision of the velocity information available forinput to the migration program. Other factors that influence migration results

include type of input data2-D or 3-D, migration strategies time or depth,post- or pre-stack, and algorithms and associated parameters. Two-dimensional migration does not correctly position events with 3-D orientation

in the subsurface.

12. Events with conflicting dips require an additional step dip moceout (DMO)correction, prior to CMP stacking. Conflicting dips with different stackingvelocities often are associated with fault blocks and salt flanks. Specifically,

the moveout associated with steeply dipping fault-plane reflections or

reflections off a salt flank is in conflict with reflections with gently dippingstrata. Following NMO correction, DMO correction is applied to data so as to

preserve events with conflicting dips during stacking. Migration of a DMO

stack then yields an improved mage of fault blocks and salt flanks. The

rigorous solution to the problem of conflicting dips with different stackingvelocities is migration before stack.

13. Even when starting with the same raw data, the results of processing by oneorganization seem to be different from that of another organization due todifferences in the choice of parameters (e.g., handling of correlation window,

selecting the traces used for cross correlation with the pilot trace, etc.) and the

detailed aspects of processing algorithms

14. One other aspect of seismic data processing is the generation of artifacts whiletrying to enhance signal due to the quality of the software used for processing.

WAVE TYPES

1. Field records contain (a) reflections, (b) coherent noise, and (c) randomambient noise. One important aspect of data processing is to uncover genuine

reflections by suppressing noise of various types. At best, signal processing

suppresses whatever noise is present in the field data and enhances the

reflection energy that is buried in the noise.2. Reflections on shot records are recognized by their hyperbolic traveltimes. If

the reflecting interface is horizontal, then the apex of the reflection hyperbola

is situated at zero offset. On the other hand, if it is a dipping interface, then the

reflection hyperbola is skewed in the up-dip direction. There are several wavetypes under the coherent noise category.

3. Ground rollis recognized by low frequency, strong amplitude, and low groupvelocity. It is the vertical component of dispersed surface waves. In the field,receiver arrays are used to eliminate ground roll. Ground roll can have strong


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backscattered components because of lateral inhomogeneities in the near-

surface layer.

4. Guided waves are persistent, especially in shallow marine records in areaswith hard water bottom. The water layer makes a strong velocity contrast with

the substratum, which causes most of the energy to be trapped within and

guided laterally through the water layer. These waves are dispersive incharacter. These waves also make up the early arrivals. The stronger thevelocity contrast between the water layer and the substratum, the smaller the

critical angle; thus more guided wave energy is trapped in the supercritical

region. When there is a strong velocity contrast, refraction energy propagatesin the form of a head wave. Guided waves are also found on land records.

These waves are largely attenuated by CMP stacking. These waves have

prominently a linear moveout and so can be suppressed by dip filtering

techniques or slant stacking.5. Side-scattered noise commonly occurs at the water bottom, where there is no

flat, smooth topography. Irregularities of varying size act as point scatterers,

which cause diffracted waves. They can be on or off the vertical plane of therecording cable. These arrivals typically exhibit a large range of moveouts,

depending on the spatial position of the scatterers in the subsurface.

6. Cable noise is linear and low in amplitude and frequency. It primarily appearson shot records as late arrivals.

7. The air wave with a speed of 300 m/s can be a serious problem when shootingwith surface charges such as land air gun. Perhaps the only effective way to

remove air waves is to zero out the data on shot gathers along a narrowcorridor containing this energy (called notch muting)..

8. Power lines also cause noisy traces in the form of a mono-frequency wave. Amono-frequency wave may be 50 to 60 Hz, depending on where the field

survey was conducted. Notch filters are used in the field to suppress suchenergy.

9. Multiples are secondary reflections with interbed or intrabed raypaths. Guidedwaves include supercritical multiple energy. Multiples are suppressed bymethods which are based on moveout discrimination, and prediction theory,

which uses the periodic behaviour of multiples. The most effective moveout

based suppression technique often is CMP stack with inside trace mute.10. Random noise has various sources. A poorly planted geophone, wind motion,

transient movements in the vicinity of the recording cable, wave motion in the

water that caused the cable to vibrate, and electrical noise from the instrument

all can cause ambient noise. The net result from scattered noise from manyscatterers on the subsurface also contributes to random noise. CMP stack

suppresses a significant part of the random noise uncorrelated from trace to

trace.


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GAIN APPLICATION

1. Seismic data often require application of a gain function time variant scalingof amplitudes, for various reasons. The scaling function is commonly derived

from the data. At an early stage in processing, gain is applied to data to correct

for wavefront divergencedecay in amplitude caused by geometric spreadingof seismic waves. Seismic data are often gained for display purposes: forinstance, by applying automatic gain control (AGC) which brings up weak

signals.2. Unlike a gain function, trace balancing is a time-invariant scaling of

amplitudes. Trace balancing usually is based on rms-amplitude criterion.

Specifically, each trace in a group of traces is scaled so that they all have the

same desired rms amplitude level.3. A field record represents a wavefield that is generated by a single shot.

Conceptually, a single shot is thought of as a point source that generates a

spherical wavefield. Wave amplitude decays as 1/r where r is the radius of the

spherical wavefront. In practice, velocity usually decreases with depth, whichcauses further divergence of the wavefront and a more rapid decay in

amplitude with distance. Velocity dependent scaling functions have been used

to compensate for spherical divergence.4. The frequency content of the initial source signal changes in a time-variant

manner as it propagates. In particular, high frequencies are absorbed more

rapidly than low frequencies. This is because of the intrinsic attenuation in

rocks. One plausible mechanism for attenuation is related to pore fluids. Asthe waves propagate through rocks, the fluids that are present in the pores are

disturbed. The disturbance is grater in partially saturated rocks. Pore fluids

consume part of the energy of the propagating waves, which causes a

frequency dependent decay.5. The effect of attenuation is removed by modifying the amplitude spectrum of

the signal, thereby making it broader. Deconvolution is one process that is

used to achieve this goal. Time-variant spectral whitening and inverse Q-filtering are other methods to compensate for frequency dependent

attenuation.6. One undesirable effect of any gain application is boosting up of noise

components in the data while bringing up the strength of reflections. Besides

ambient noise, coherent noise in the data may also be boosted. To prevent this

velocity-independent scaling functions have been used.7. Various types of gain criteria are used in practice. Based on a desired

criterion, a gain function is derived from the data and multiplied with trace

amplitudes at each time sample. The gain function may be estimated from (i)

envelope of the ungained trace, (ii) rms amplitude within a specified time gate

on an input trace, or (iii) instantaneous AGC.


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BASIC DATA PROCESSING SEQUENCE

1. There are three primary steps in processing seismic data deconvolution,stacking, and migration, in their usual order of application. A seismic data

volume can be represented in processing corrdinatesmidpoint, offset, and

time. Deconvolution acts along the time axis. It removes the basic seismicwavelet (the source time function modified by various effects of the earthand recording system) from the recorded seismic trace, thereby increasing

temporal resolution. Deconvolution achieves this goal by compressing the

wavelet. Stacking is also a process of compression. The data volume isreduced is reduced to a plane of mid-point time at zero offset first by

applying normal moveout correction to traces from each CMP gather, then

by summing them along the offset axis. The result is a stacked section.

OFFSET STACKING

MIDPOINT

MIGRATION

T D

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M C

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Finally, migration is applied to stacked data. It is a process that

collapses diffractions and maps dipping events on a stacked section to their

supposedly true subsurface locations. In this respect, migration is a spatialdeconvolution process that improves spatial resolution.

2. All other processing techniques may be considered secondary in that theyhelp improve the effectiveness of the primary processes. Many of thesesecondary processes are designed to make data compatible with the

assumptions of the three primary processes. Deconvolution assumes a

stationary, vertically incident, minimum phase source wavelet and white

reflectivity series that is free of noise. Stacking assumes hyperbolicmoveout, while migration is based on a zero-offset (primaries only)

wavefield assumption. Strictly speaking, none of these assumptions is valid.

However, when applied to field data, these techniques do provide resultsthat close to the true subsurface image. Success of a process depends not


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only on the proper choice of parameters pertinent to that particular process,

but also on the effectiveness of the previous processing steps.

PREPROCESSING

3.

Field data are recorded in a multiplexed mode, i.e., samples at the sametime at consecutive channels. The data first are demultiplexed, i.e., all thetime samples in one channel followed by those in the next channel. The

demultiplexed data consists of seismic traces at different offsets with a

common shot point.4. Preprocessing also involves trace editing. Noisy traces, traces with transient

glitches, or mono-frequency signals are deleted. Polarity reversals are

corrected. In case of very shallow marine data, guided waves are muted

since they travel horizontally within the water layer and do not containreflections from the substratum.

5. Following the trace editing and pre-filtering (to remove certain types ofnoise in marine data), a gain recovery function is applied to the data tocorrect for the amplitude effects of spherical wavefront divergence. This

amounts to applying a geometric spreading function which depends on

traveltime. While primary reflection amplitudes are corrected for

wavefront divergence, energy associated with multiple reflections andrandom noise also is invariably boosted by geometrical spreading

correction.

6. Finally, field geometry is merged with the seismic data, i.e., each seismictrace is associated with the particular geophone group and shot location.

This precedes any gain correction that is offset dependent.

7. For land data, elevation statics are applied at this stage to reduce traveltimesto a common datum level. This usually requires correction for the nearsurface weathering layer in addition to differences in elevation of source

and receiver stations. Estimation and correction for the near-surface effects

usually are performed using refracted arrivals associated with the base ofthe weathering layer.

DECONVOLUTION

8. Prestack deconvolution is aimed at improving temporal resolution bycompressing the effective source wavelet contained in the trace to a spike,

(spiking deconvolution). Predictive deconvolution also is used commonlyto remove reverberations in marine seismic data. Deconvolution is applied

to prestack data trace by trace. Sometimes, however, a single deconvolution

operator is designed and applied to all the traces on a shot record.

Deconvolution techniques used in conventional processing are based onoptimum Wiener filtering. Because both high- and low-frequency noise and

signal are boosted, the data often need filtering with a wide band-pass filter

after deconvolution.


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CMP SORTING

9. Seismic data acquisition with multifold coverage is done in shot receivercoordinates. Seismic data processing, on the other hand conventionally is

done in midpoint-offset coordinates. The required coordinatetransformation is achieved by the sorting data into Common Midpoint(CMP) gathers. Based on field geometry information, each individual

trace is assigned to the mid point between the shot and receiver locations

associated with that trace. Those traces with the same mid point location are

S R1 R2 R3 R4 R5

DATA ACQUISITION GEOMETRY

S1 S2 S3 S4 M R4 R3 R2 R1

CDP M: CMP

DATA PROCESSING GEOMETRY

grouped together, making up a CMP gather. CDP (Common Depth Point)

gather is equivalent to a CMP gather only when reflectors are horizontal and

velocities do not vary horizontally. For dipping reflectors, only the termCMP gather is used. Commonly used types of gathers are: (1) common shot


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gather (one shot, many receivers),(2) common-receiver gather (many shots,

one receiver), (3) common mid point gather (many shots, many receivers,

one common mid point), (4) common offset section (many shots, manyreceivers, same offset), (5) CMP stacked section (zero offset section).

10.

For mst recording geometries , the fold of coverage fn, for CMP stacking isgiven by

fn = nS/(2s)

where n is number of recording channels, S is reciever-group spacing and s

is shot interval.

VELOCITY ANALYSIS

11.

Velocity analysis is performed on selected CMP gathers or groups ofgathers. This analysis yields velocity as a function of two-way zero-offset

time. The velocity functions picked at analysis locations then are spatially

interpolated between the analysis locations to create a velocity field. This is

used to supply a velocity function for each CMP gather along the profile.

NORMAL MOVEOUT CORRECTION

12. The velocity field is used in normal moveout (NMO) correction of CMPgathers. This is based on the assumption that, in a CMP gather, reflection

travel times as a function of offset follow hyperbolic trajectories. The NMO

correction removes the moveout effect on the travel items. Traces in eachCMP gather are then summed to form a stacked trace at each mid point

location. The stacked section comprises the stacked traces at all mid point

locations along the line traverse.13. The CMP recording technique uses redundant recording to improve the

signal to noise ratio during stacking. To achieve redundancy, multiple

sources per trace (ns), multiple receivers per trace (nr) and multiple offsetcoverage of the same subsurface point (nf), are used in the field. Give the

total number of elements in the recording system, N = ns x nr x nf, the

signal-to-noise ratio theoretically is improved by a factor of N. Thisimprovement factor is based on the assumption that the reflection signal ontraces of a CMP gather is identical and the random noise is mutually

uncorrelated from trace to trace. Because these assumptions do not strictly

hold in practice, signal-to-noise ratio improvement gained by stacking is

somewhat less than N CMP stacking also attenuates coherent noise suchas multiples, guided waves, and ground roll. This is because reflected signal

and coherent noise have different stacking velocities.


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MULTIPLE ATTENUATION

14. Multiple attenuation and reverberations are attenuated using techniquesbased on their periodicity or differences in moveout velocity between

multiples and primaries. These techniques are applied to data in various

domains, including the CMP domain, to best exploit the periodicity andvelocity determination criteria. Deconvolution is one method of multipleattenuation that exploits the periodicity criterion. Despite theoretical

limitations, deconvolution can remove a significant amount of the energy

associated with short-period multiples and reverberations.

DIP-MOVEOUT CORRECTION

15. The NMO correction applied to the CMP gathers is optimum for flat events.Stacking velocities, however, are dip dependent. Dip-moveout (DMO) is

needed to correct for the dip effect on stacking velocities and thus preserveevents with conflicting dips during CMP stacking. Dip-moveout correction

is applied to data following the NMO correction using flat-event velocities.

This then is followed by inverse moveout correction (i.e., restoring the

NMO applied earlier) and subsequent velocity analysis at closely spacedintervals. This gives a new velocity field following DMO correction.

CMP STACKING

16. The new velocity field obtained after DMO correction is now used to applyNMO correction to the CMP gathers. Finally a CMP stack is obtained by

summing over the offset axis.

POSTSTACK PROCESSING

17. A typical post stack sequence includes: (a) Deconvolution after stack torestore high frequencies attenuated by CMP stacking. It also is effective in

suppressing reverberations and short period multiples; (b) time variantspectral whitening to further flatten the spectrum; (c) time variant band pass

filtering to remove noise at the high- and low-frequency end of the signal

spectrum; (d) attenuation of random noise uncorrelated from trace to trace;

and (e) application of some type of display gain to the stacked data.

MIGRATION

18. Migration involves moving the dipping events to their supposedly truesubsurface positions and collapsing the diffractions. Migrated stacked

section is displayed in time. Structural complexities in the subsurface,

caused by folding and faulting generally give rise to problems in stacking


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and imaging the subsurface, for which various solutions have been

proposed.

RESIDUAL STATICS CORRECTIONS

19.

Residual statics correction is an additional step in conventional processingof land and shallow-water seismic data before stacking. Sometimesfollowing features are observed during conventional processing: (a) events

in some CMP gathers are not as flat as they are in other gathers; (b) the

moveout in CMP gathers does not always conform to a perfect hyperbolictrajectory; (c) a reflection event arrives on long-offset traces before it

arrives on short offset traces; and (c) difficulties in velocity analysis. These

are caused by near surface velocity irregularities that cause a static or

dynamic problem. Lateral velocity variations are caused by a complexoverburden. To improve stacking quality, residual statics corrections are

performed on the moveout corrected gathers. The estimated residual

corrections are applied to the original CMP gathers with no NMOcorrection. Velocity analyses are then often repeated to improve the quality

of velocity determinations. With the improved velocity field, the CMP

gathers are NMO-corrected and stacked.


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BASIC PROCESSING SEQUENCEField Tapes and Operators Log

(1) Preprocessing(a)Demultiplexing(b)Reformatting(c)Editing(d)Geometric Spreading Correction(e)Setup of Field Geometry(f) Application of Field Statics(2) Deconvolution and Trace Balancing(3) CMP Sorting(4) Velocity Analysis(5) Residual Statics Correction(6) Velocity Analysis(7) NMO Correction(8) DMO Correction(9) Inverse NMO Correction

(10) Velocity Analysis(11) NMO Correction, Muting and Stacking(12) Deconvolution(13) Time-variant Spectral Whitening(14) Time variant Filtering(15) Migration(16) Gain Application

processing of reflection seismic data

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