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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
#4 Sonic Calibration and Synthetic Seismogram
Sonic Calibration What is it? Sonic times are corrected with borehole seismic times Drift = (Vertical Seismic time - Integrated Sonic time)
Seismic & Sonic measurements: 1. Measure at different frequency range 2. Have different lateral investigation 3. Are direction dependent measurements
Sonic Calibration Why do we need it? Upscale borehole time measurements from cm to Km Absolute reference Creation of: (1) A continuous calibrated time/depth relation at the well (2) A calibrated synthetic seismic answer at the well
What are the causes for drift? Sonic times too long: • •
Noise, stretch,cycle skipping, hole conditions such as rugosity, borehole enlargement Formation alteration
Sonic times too short: • •
Noise, negative stretch, cycle skipping, velocity inversion due to gas, High dips Dispersion
Checkshot times too long: • •
Time picking precision Different raypath
Checkshot times too short: Different raypath (Anisotropy, high dip formations relative to borehole, lateral formation changes) •
Synthetic Seismogram (Geogram) Processing • Editing of logs • Drift computation • Adjustment of drift (sonic calibration) • Time to depth conversion • Reflection coefficients • Attenuation coefficients • Convolution
Interval Velocity computed from VSP
INTV = delta Depth / delta Time
Interval Velocity computed from Sonic Log INTV (ft/sec) = 1,000,000 / DT (usec/ft) INTV from VSP (black) INTV from DT (red)
DT
INTV
Positive and Negative Drift Positive Drift: VSPTT > ITT Positive drift is usually associated with the dispersion effects between the higher frequency bandwidth of the sonic 10kHz as compared to the lower frequency seismic bandwidth below 100Hz Correction: block shift method was applied to the sonic
Negative Drift: ITT > VSPTT Negative drift is usually associated with formation alteration or damage. As formation alteration generally affects mechanically weak formations, and cause slower sonic velocity Correction: ∆T minimum method was applied to the sonic
Drift = VSPTT – ITT (blue) is 5 msecs at TD
VSP transit time (black)
Integrated Sonic Transit Time, ITT (red)
Sonic Calibration = adjust the sonic log, so that the drift curve is zero. After adjustment, the ITT is the same as the VSP-TT The ITT from the calibrated sonic log provides a high resolution depth-time curve
Sonic Calibration Original drift before calibration (scale is -20 to +20 msec) Drift after calibration (scale is -10 to +10 msec)
After calibration, all the log data is converted to a time index, using the time-depth relation from the calibrated sonic log
Time index data
Z=V*Rh
onvolution - from reflectivity to synthetic seismi Reflectivity Coefficient
Zero Phase Wavelet
Multiply each wavelet by the size of the reflection coefficient
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 seismiogram
VSP & Synthetic C-Stk
Apparent dipping reflection seen away from the corridor stack window
Synth
with attenuation
The corridor stack and Synthetic are independent measurements. If they agree, then this confirms they are both correct.
VSP verses Checkshot survey ~ 20 m level spacing
If Sonic Calibration is the only objective of a borehole seismic survey, then the levels can be spaced further apart. The sonic log supplies the “higher resolution” part of the interval velocity.
~ 100 m level spacing and key depth points
Drift
Wireline
–
ISONIC
Formation Alteration
ISONIC / Wireline differences in shales come from formation damage (time-lapse sonic effects characterize mechanical state of rock)
Anisotropy Formation lateral variations Ray bending
Sonic log
anhydrite
Vertical Incidence
Rig source
Tie Geological Markers Surface Seismic
SYN
Surface Seismic
SYN A.I.
Vel.
h t e p m Den. e i d t
R
m m m R Are they really multiples? Could it rather be related to a lateral investigation issue? A successful synthetic match clearly establishes the relationship between a change visible on the log data (at which a marker can be interpreted) and the response to that change on the seismic data - it clearly establish what the events on the seismic data represent in geological terms
Tie Geological Markers Surface Seismic
VSP SYN
Surface Seismic
R
Are they really multiples? m
m
m
R
Yes, it is confirmed by the VSP which has larger lateral investigation than other openhole logs
