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Borehole Seismic Survey 1
Borehole Seismic Introduction
2
Borehole Seismic Tool and Acquisition
3
VSP Processing
4
Sonic Calibration and Synthetic Seismogram
5
VSP Examples
Kieu Nguyen Binh HCMC-2010
Borehole Seismic Survey 1
Borehole Seismic Introduction
2
Borehole Seismic Tool and Acquisition
3
VSP Processing
4
Sonic Calibration and Synthetic Seismogram
5
VSP Examples
Kieu Nguyen Binh HCMC-2010
#5 VSP Examples
VSP match to the Surface Seismic
VSP is good quality and zero phase. After a gross static shift, the match to surface seismic is OK near the top, but less good deeper down.
The residual mismatch is caused by time and phase shifts in the surface seismic, that result from the low Q.
What can be done: – From the VSP, we can estimate the effects of the Q and attempt to remove these effects from the surface seismic.
VSP match to the Surface Seismic XCOR from 1.2 to 2.4 sec
VSP result
Surface Seismic and VSP corridor stack
VSP – Surface Seismic merge Surface Seismic
VSP Corridor Stack
Good match at 1300 ms. Not so good deeper down. VSP is zero phase along the entire well interval Surface seismic is not zero phase and changes with depth?.
VSP-SS match with non-ZP VSP corridor stack Surface Seismic
VSP Corridor Stack
By processing the VSP to be non-zero phase at the bottom, we can get a good match to the surface seismic
Before Decon
Zero Phase Deconvolution
After Decon
Before Decon
ZP Decon – Operator design on top trace
After Decon
The VSP is now not zero phase
VSP Resolution
Vibroseis sweep 8-90 hz The zero offset VSP has coherent energy from 8-90 hz. The corridor stack is filtered to 8-45 hz to match the surface seismic. Surface seismic 8-90 hz VSP corridor stack 8-45 hz VSP corridor stack
Multi-pathing Multi-pathing may be defined as the phenomena where downgoing arrivals on a VSP level are not all following the same path. Multipathing is usually characterized by higher than expected interval velocities on a VSP. The higher velocity may be seen as high negative drift on the sonic in good hole conditions or even physically unreal high velocities. Direct arrival may also appear to bifurcate even though the source signature seen on the reference surface hydrophone remains stable
Y
Y
MULTI-PATH? NO MULTI-PATH?
Top Basement Slotted Liner Casing Open Hole
Basement Fault
Higher VSP Velocities
Multi-pathing #1
Q- estimation
The deeper traces in a VSP have less high frequency information Q is a measure of the loss of higher frequencies as a function of OWT
Q-analysis from VSP VSP waveform
Frequency spectrum
High gradient = low Q = high attenuation 1500-2350 m Q=25
3200-3950 m Q=81
Q is a measure of the loss of the higher frequency signal with depth. It is caused by formation layering.
VSP can directly measure Q
A low value of Q means high attenuation.
A low value of Q also means the surface seismic is less likely to be zero phase.
Downgoing after upgoing removal Direct downgoing compressional Need to remove upgoing before Q estimation Loss of frequency with depth => low Q
Multi-spectral Ratio Method Multi-Spectral Ratios method is a more statistical method that estimates the Q for every possible receiver pair. Red-yellow dots indicate greater confidence in spectral slope. The confidenceweighted average Qp=52. The estimates with greatest confidence lie between Qp=45-65 Q estimates versus receiver pair midpoint
Q filtering No Q
After Q=200 filter
After Q=100 filter
Q filtering compensates the data for the frequency dependant attenuation. Although it restores the balance of frequencies, it can also introduce phase rotation to the data,
VSP Inversion • Inversion is the inverse procedure to synthetic seismogram • Result is not unique, since the input data is band-limited • Corridor stack inverted to give acoustic impedance curve • Need low frequency content • Flat layer below TD
Acoustic Impedance Inversion
VSP Inversion used to assist with overpressure prediction.
Inversion shows drop-off in velocity below TD.
For best results a full mechanical earth model (MEM) should be constructed before drilling the well TD
VSP Inversion needs flat Sonic Velocity structure. Geophones with a low frequency response are essential.
Processed VSP data
Final VSP Inversions on 18-11-96 (Intermediate TD 3807)
VSP inversion lookahead 140 msec TWT, or 200 metres
VSP summary for Look-ahead VSP requires high frequency data to be get best resolution when identifying a target boundary. VSP also requires low frequency data to get velocity trend, and pore pressure below TD. The frequency content is dependent on: the VSP tool. the near surface ground conditions (loose sand, hard rock etc), and formation type of energy source Obtain nearby well logs and VSP, to know the expected VSP response.
VSP Limitation
VSP data is band-limited. Typically from 5-80 hertz ?
VSP has no information below 5 hz.
Standard seismic tools are limited to10hz.
VSP inversion is good at extracting “relative changes” in velocity.
It is not good at extracting “absolute values” in velocity.
Extracting long period velocity variations is not possible with VSP. Walkaway VSP can overcome this limitation. Both VSP and WVSP inversion assume a 1D earth velocity model.
Depth (ft)
Depth Prediction below TD from VSP
e m i T ) y c a e s ( W o w T
Extend T/D curve to meet the peak extrapolation: Depth = 16082ft TVD
China Example VSP Corridor
Synthetic from Inversion
Reflectivity
TD is about 200 metres above the coal beds Run VSP Inversion on the VSP to predict distance ahead Acoustic Impedance
