VSP Measurement

7/30/2019 VSP Measurement

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The Vertical Seismic Profiling

Measurement Procedure


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The recording geometry

involved in a typicalonshore VSP is essentiallythe same, except that wecan use a wider variety ofenergy sources, including

shear-wave sources.For onshore vertical

seismic profiling, we canrecord VSP data in a casedwell after the drilling rig

has moved away


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By contrast, a drilling rig (or production platform) is alwayspresent at the wellsite during a marine VSP survey.

The objectives of vertical seismic profiling differ from thoseof velocity surveying in the following ways:

We record VSP data at vertical depth intervals that aremuch smaller than those recorded through velocitycheckshot data. Successive VSP geophone stations

should be no farther apart vertically than 1/2, where 1isthe shortest spatial wavelength contained in therecorded seismic wavelet.

The only information we need from a velocity survey isthe time-depth coordinates of the first-break wavelets.In vertical seismic profiling, however, the weakdowngoing and upgoing events that follow the first-arrival wavelets are just as important as the first-breaktimes.


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Thus, the dynamic-range, gain-accuracy, and noise-rejection properties of the downhole geophone, thelogging cable, and the surface recording system must besuperior to those we use in velocity surveys. Otherwise,these important weak-amplitude events may not beproperly recorded.

The value of vertical seismic profiling is contingent upon the

quality of the VSP data recorded. Specifically, VSP data must contain both recognizable

reflection signals and minimal noise.

Consequently, the field procedures used to record VSP dataneed to be carefully planned and executed so that a properenergy source and adequate recording system are onsite,and the recording geometry correctly images the desiredtarget.


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However, geological interpretations made from surface-recorded seismic data are limited because we often cannot

determine the exact frequency changes, amplitude losses,and phase shifts that have been imposed on the seismicwavefield as it travels through the earth.

We could enhance the interpretive value of conventionalseismic data by examining the seismic wavefield in detail asit moves through the earth, rather than sampling it only atthe earth's surface.


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When VSP

Should Be Considered


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Good-quality VSP data are invaluable measurements whichprovide an important link between surface seismic data andsubsurface geology, enabling seismic interpreters to correlatesubsurface stratigraphic and lithological conditions to the reflectedseismic waves measured at the surface.

The insight provided by in situ wavefield measurements is themajor reason for interest in vertical seismic profiling.

VSP measurements provide at least three important contributionsto seismic interpretation:

i. VSP data calibrate seismic signals in terms of the geologythat exists at the depths at which the wavefields areobserved.

ii. VSP data can provide an additional, and usually improved,image of the subsurface geology near a VSP well.

iii. VSP can represent the optimal seismic image at the welllocation.


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The following are specific examples of ways we can use VSPdata to calibrate seismic data:

We can establish precise depth-to-time conversion functions,which enable us to translate geological data, measured asfunctions of depth, into functions of traveltime. We can thendirectly correlate geological data with surface-recordedseismic data.

We can measure the amplitude decay of the expanding VSPwavefield to check the validity of the amplitude restorationfunction used in processing surface-recorded seismic datanear the VSP well.

We can use the frequency losses measured as a seismicwavelet propagates through a stratigraphic section to judgewhich portions of the seismic frequency band are mostdiagnostic of subsurface geological conditions.


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By measuring seismic wavelet amplitudes above andbelow subsurface impedance contrasts, we can verify the

accuracy of reflection coefficients derived from log data.

We can identify which surface-arriving signals are primaryreflection events and which are potentially misleadingmultiples.

We can use in situ measurements of upgoing reflectionwavelets to specify which surface-recorded waveshapescorrespond to which stratigraphic relationships penetratedby a VSP well.

We can use the VSP as a control for many surface seismicphases such as feasibility, survey design, processingparameter QC and interpretation.


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Common Applications of VSP Data in

Exploration and Development


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Establish depth-to-time conversion functions

Identify primary seismic reflectors and interbed

multiples

Adjust or verify log-derived reflection coefficients

Provide an alternative/complement to the syntheticseismogram

Provide parameters for processing seismic data

Amplitude decay functions

Deconvolution operators

Q compensation filters

Anisotropy parameters

Demultiple models

Estimate structural dip


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Predict conditions ahead of the bit

Locate faults near a well

Create high-resolution images of the subsurface Provide detailed data on compressional-and shear-

velocity behavior, including VP/VS velocity ratios as afunction of offset, and compressional and shear-waveimages of the subsurface

Fracture orientation analysis

While service companies might prefer to see their clientsrecord VSP data in the majority of wells they drill, the Oilcompanies must carefully weigh the cost of any VSP survey

against the potential benefit of having the data. The Oil companies often judge the cost to be greater than

the potential gain, so may prefer to scale back their plansfrom the VSP to a less expensive checkshot survey.


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The following list summarizes some of the commonsituations where VSP data would be useful, and may offersome guidance to novice VSP users.

We should record VSP data when:

It is important to establish a rigorous tie betweensubsurface geology and surface seismic data;

The surface seismic data in a prospect area aresuspect for any reason -for example, wheneverinterbed multiples can not be properly attenuated orrecognized;

It is impossible to record the sonic log data needed to

make a synthetic seismogram. This condition can existin highly deviated, uncased wells if pipe-conveyed ortubing-conveyed logging services are unavailable. Insuch a case, we can run a VSP after the well is casedto provide a substitute synthetic seismogram;


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The stratigraphic anomalies we need to define aresmaller than the resolution limits of our surface-recorded seismic data;

It is important to acquire improved imaging of thestratigraphy near a well;

Pre-survey modeling indicates that the VSP should beable to image the geological objective;

It is important to tie compressional and shear-waveseismic reflectors;

We need a comprehensive database to provide asmuch reservoir information as possible, perhaps forfield unitization negotiations between competingcompanies.

We need data for surface seismic feasibility studies,acquisition design, processing parameter selection anddata interpretation;


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We wish to extract the embedded wavelet in the

surface seismic data, e.g. for optimal amplitudeinversion to acoustic impedance.

We need quantitative information on anisotropy andattenuation for optimal p and p-s imaging andevent correlation.

Subsequent sections of this presentation will analyze anddescribe the value of VSP in these situations.

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VSP Measurement