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03 VSP Processing

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Borehole Seismic Survey

Borehole Seismic Survey

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1 Borehole Seismic IntroductionBorehole Seismic Introduction 2

2 Borehole Seismic Tool and AcquisitionBorehole Seismic Tool and Acquisition 3

3 VSP ProcessingVSP Processing

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4 Sonic Calibration and Synthetic SeismogramSonic Calibration and Synthetic Seismogram 5

5 VSP ExamplesVSP Examples

Kieu Nguyen Binh Kieu Nguyen Binh HCMC-2010

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Borehole Seismic Survey

Borehole Seismic Survey

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1 Borehole Seismic IntroductionBorehole Seismic Introduction 2

2 Borehole Seismic Tool and AcquisitionBorehole Seismic Tool and Acquisition 3

3 VSP ProcessingVSP Processing

4

4 Sonic Calibration and Synthetic SeismogramSonic Calibration and Synthetic Seismogram 5

5 VSP ExamplesVSP Examples

Kieu Nguyen Binh Kieu Nguyen Binh HCMC-2010

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#3

#3

VSP Processing

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One-Way Time vs. Two Way Time

OWT

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50 % overlap

100 % overlap

200 % overlap

Trace display parameters – Trace Overlap

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-Trace-by-trace normalisation - 100% overlap - One Way Time

- Gather normalisation - 1000% overlap

- One Way Time

VSP display options – Trace Normalisation

A VSP display can be normalised individually trace-by-trace, or by a single

normalisation value (gather normalisation) for the whole data set. Gather normalisation show the real amplitudes of the data.

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-Trace-by-trace normalisation - 1000% overlap - One Way Time

- Trace-by-trace normalisation - 1000% overlap - Zero Aligned Time

- Trace-by-trace normalisation - 1000% overlap - Two Way Time

VSP display options – OWT, TWT

and Aligned

TWT – traces are shifted  by the transit time pick at

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VSP display options – Trace separation

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Wavefield Separation Deconvolution

Processing Sequence

BPF, NRM, TAR Static correction Corridor  Stack Median Stack Data Edit Upgoing Wavefield Downgoing Wavefield Field

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Reference sensors

Time break sensors, there is also a hydrophone hanging ~ 5 metres  below the gun

The hydrophone is the red device – it will hang about 5 metres below the gun when deployed

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Raw shots

Hydrophone

At the surface near the airgun

Geophone

Downhole in tool

3 or 5 shots per level.

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Mean stack  Hydrophone

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Median stack  Hydrophone

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Transit Time

Picking

(3215 metres)

Inflection Point Tangent

Peak  Zero crossing

Trough

Inflection Point Time varying from

IPT = 1209.1 msec

IP = 1212.9 msec

T =1216.8 msec

ZC = 1222.1 msec P =1229.3 msec

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Transit Time pick

(Shallow level at 744 metres depth)

Inflection Point Tangent = 392.5 msec Trough = 396.2 msec

3.7 msec difference … deeper levels give 7.7 msec difference

Shallow depths -> More high frequency

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Transit Time Picking

(Hydrophone)

Inflection Point Tangent = 28.6 msec Trough = 31.2 msec

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Filtering and

Transit time

picking

 No Filtering

4-120 zero phase filter 

4-60 zero phase filter  4-90 zero phase filter 

Level at 744 metre.

The effect of filtering on the time  picks is most severe at shallower 

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Earth Filter 

Stacked Data Stacked Data

Aligned on time pick  Expanded time scale

2 msec drift in the trough

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Pre-processing after Stacking

 Spectral Analysis  Band pass filter 

- to remove noise outside of signal range

 Trace normalization

- to equalize downgoing waves of the same amplitude arrive for all receivers

 Geometrical spreading correction

- to recover amplitude of later arrival

 Static correction to SRD

- Correct reference time to Seismic datum

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Frequency Spectrum

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Bandpass Filter 

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Normalization

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 Amplitude Recovery

 Amplitude Recovery

where t is time and t

where t is time and t00is break timeis break time

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Wavefield Separation Deconvolution

Processing Sequence

BPF, NRM, TAR Static Correction Corridor  Stack Median Stack Data Edit Upgoing Wavefield Downgoing Wavefield Field Data Data Processing

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Wavefield Separation - Velocity Filtering

A VSP is made up of two distinct wave types

The upgoing waves - the primary interest

• The complete downgoing waves being reflected at each acoustic reflector  • A whole suite of events generated by multiple reflections

The downgoing waves

• The direct compressional signal • A whole suite of events generated by multiple reflections

• It can be quite long and reverberatory in character 

• Masks the other type, the upgoing waves

Upgoing Downgoing

One Way Time

     D    e     p      t      h

Velocity filtering separates these two signals which have different apparent velocities across the data array. Velocity filtering is done in 3 main stages

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1. Estimate Downgoing Energy

Subtract transit time to vertically align all downgoing energy

 Apply median filter to enhance in-phase downgoing energy and suppress all out of phase energy Shift each trace back to its

original one-way time

One Way Time

   D  e  p    t    h

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Median Stack Traces Aligned to First

Break

Estimation of Downgoing Energy

Vertical Geophone (Z) Aligned Enhanced Downgoing Wavefield Time       D     e      p       t       h    D  o   w   n   g   o    i  n  g    W  a   v   e    f    i  e    l    d

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Subtraction of Downgoing Energy

Upgoing Downgoing

One Way Time

   D  e  p    t    h

One Way Time

   D  e   p    t    h

By subtracting the downgoing energy from the total wavefield, a residual wavefield is left, which contains background noise and the desired upgoing wavefield

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Enhance Upgoing Energy

Upgoing One Way Time

   D  e  p    t    h

 Add first break transit time to vertically align all upgoing energy at

it’s two-way time

      D     e

     p       t       h

Residual Wavefield after Subtraction of  Downgoing Wavefield

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   U  p   g   o    i  n  g    W  a   v   e    f    i  e    l    d

 Add TT - Median Stack

Two Way Time

   D  e   p    t    h

 Apply median filter to enhance in-phase upgoing energy and

suppress all out of  phase energy

Enhance Upgoing Energy

      D     e      p       t       h Two-Way Time Enhanced Upgoing Wavefield

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Deconvolution

The function of deconvolution is to precisely improve the resolution capabilities of the upgoing wavetrain:

It removes the near surface multiples & the bubble effects It optimizes the resolution characteristics of the source signature

Deconvolution filters are computed on the downgoing wavetrain and applied to both the downgoing and upgoing waves

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Deconvolution

Long Signal Mixed Reflections Short Signal Well Separated Reflections Reflector 1 Reflector 2 Original Signals Deconvolved Signals 1 2 1 2 1 2 2 1

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Depth Time Depth Time Depth Time Depth Time TWT

Deconvolution

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Corridor Stack

Reasons for corridor stack - Shortest raypath

- Least effect from formation dip - Deconvolution is most accurate

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VSP – Surface Seismic merge

Good match at 1300 msec. Not so good deeper down.

VSP is 8-75 Hz. Using lower frequency VSP decon does not improve the match VSP is the correct answer. This can be confirmed with a synthetic seismogram

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Why Triaxial Geophones ?

Needs of Triaxial Geophones in VSPs

* Related to Survey Geometry (OVSP, WVSP,…)

* Related to Geophysical Phenomena (Mode Converted Wavefields, out of plane energy)

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Near vertical well

Z contains most of the downgoing compressional

X and Y are rotating in the borehole as the tool moves up

X

Z

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X, Y and Z

X & Y projected to max and min

0.01 sec 0.02 sec 0.04 sec 0.06 sec 0.05 sec 0.03 sec

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x

y

Particle motion cross plot to determine

Horizontal MaXimum component

HMX

HMX=X. COS

+ Y.SIN

HMN=Y. COS

X SIN

X geophone response

 Y geophone response

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Projections on X and Y

Can repeat this procedure using HMX and Z as input. Outputs are TRY and NRY (Tangent and Normal). Not too relevant in vertical well

HMX

HMN

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Horizontal Component

(HMX)

Vertical Component

(TRY)

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Horizontal

component

Vertical

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HMX

Z

VS = (2500-800)/(2.15-1.0) = 1478 m/sec F = 60 hz VP = (2500-800)/(0.88-0.32) = 3035 m/sec F = 80 hz

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Compressional and Shear acquisition

In a vertical well, Z geophone is up-down orientation.

Z will see compressional X and Y will see shear 

Particle Motion Particle Motion Z geophone X & Y geophone

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Wavefield projection

 –

simple angle based

Assumptions:

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TRY angle vs deviation for GAC 50 55 60 65 70 75 80 85 90 95 100    3   5   7   9   3   5   2   2   3  4   6  4   3  4   0   7   3   3   5   0   3   2   9   2   3   2   3  4   3  1   7   7   3  1   2   0   3   0   6   3   3   0   0   5   2   9  4   8   2   8   9   0   2   8   3   3   2   7   7   6   2   7  1   9   2   6   6  1   2   6   0  4   2   5  4   7   2  4   8   9   2  4   3   2   2   3   7  4   2   3  1   7   2   2   6   0   2   2   0   2   2  1  4   5 depth        d      e       g       r       e       e       s deviation TRY angle

TRY angle in deviated well

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Rig Source & VI Source VSP

Rig Source & Vertical well

VI-source & Deviated well Rig Source & Deviated well

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Rig Source & VI Source VSP

    O     W     T     T     W     T

Rig Source & Vertical well

Rig Source & Deviated well

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Rig Source & VI-source VSP

Transit Times corrected to Vertical

Rig VSP Deviated well has 4 msec OWT error at TD

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Pro’s and Cons or Rig source / VI source VSP

Rig Source (+’s)

 Can deploy the airgun from the rig crane.

 Easy logistics.

 Cheaper to do the survey. Rig Source (-’s)

 Sonic log and seismic raypath not necessarily the same.

 Seismic raypaths affected by refraction.

 Seismic travel times affected by anisotropy.

 VSP image requires migration.

VI-VSP (+’s)

 Get the true vertical transit time at each geophone level.

 No migration required of VSP image for  horizontal layered formation.

VI-VSP (-’s)

 Require a boat to deploy the crane.

 Require offset shooting equipment to fire airgun.

 Require Navigation to location airgun position.

 Sonic log and seismic raypath are not the same – assume no lateral velocity

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Review - Rig Source VSP

Shifting each trace by the transit time pick, gives the correct TWT Rig Source VSP Upgoing OWT Rig Source VSP Upgoing TWT correction Rig Source VSP Downgoing OWT

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Offset Source VSP

Offset Source VSP Upgoing OWT Offset Source VSP Upgoing TWT correction Offset Source VSP Downgoing OWT

Shifting each trace by the transit time pick, no longer gives the correct TWT. The time is too long, and gets

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NMO correction at first arrival for Offset VSP

Rig Source VSP TWT Offset Source VSP TWT correction Offset Source VSP

 NMO correction (Simple) Normal move-out correctionshifts each trace, such that the first

break is at the correct TWT value, but using a simple geometrical

relationship.

 A narrow window corridor stack, would give the seismic trace at the wellbore The data deeper in the trace has not

been corrected properly.

The spatial offset traces from the wellbore for the data deeper in the trace is not shown.

NMO correction is OK for small offset, but not good for large offsets.

 A more complicated NMO algorithm can be used that shifts every point in the trace correctly…. However …. Better  to…. Need migration

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Migration for Offset VSP

Rig Source VSP TWT

Offset Source VSP TWT correction

Offset Source VSP Migration Horizontal axis is now in metres offset from the well

To locate the reflection point at the correct time

To locate the reflection at the correct spatial offset

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Walkaway VSP

One level with walkaway can give an image, but need at least 5 levels to do up-down wavefield

separation.

Typically use 8 or more simultaneous levels

Common shot gather  Common receiver gather 

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Walkaway VSP after Migration

Same as for Offset VSP

To locate the reflection points at the correct spatial and time positions

Is model based.

Rig Source VSP TWT

Gather 5 –  bottom geophone Gather 1 – top geophone

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Non-vertical incidence VSP’s

Summary

Three component (X, Y &Z) acquisition and

processing techniques essential for Offset and Walkaway VSP’s

 A rig source VSP in a deviated well with flat formations, requires Offset VSP processing technique.

 A rig source VSP in a vertical well with dipping formations, requires Offset VSP processing technique.

Migration is required for non-vertical incidence. (NMO can be used for a first approximation.)

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Borehole Multiples

References

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