Lee M. Liberty Associate Research Professor Boise State University

Surface refraction Surface reflection VSP Cross well

 

Check Shot

: geophone in hole, source at surface. Record first arrival time and calculate velocity Advanced version is VSP (Vertical Seismic Profile) with many geophone locations and process all reflections

Source 1-Way Time Reflection Recorded by VSP Receiver in wellbore

      Logs record frequencies in kilohertz, seismic usually <100 Hz Logs only measure several feet around well bore Synthetics do not include attenuation, dispersion, multiples, etc.

Synthetics assume 1-D (Zero-offset) response Seismic data includes range dependent effects (AVO) Sonic and density logs can be inaccurate ◦ ◦ poor hole conditions such as washouts Recording or editing problems or errors Why VSPs usually have better match with seismic • VSP frequencies similar to seismic data (~5-100 Hz) • Measures larger area around well bore (10s to 100 feet) • VSPs include attenuation, dispersion, multiples, etc. like seismic • Zero-offset VSP in straight hole produces 1-D response – unlike seismic data with multiple offsets • Offset VSPs include range dependent effects (AVO)

       Direct tie of seismic to well logs Better signal/noise than seismic Image below the bit Higher resolution view than seismic Image laterally away from the well Distinguish primaries from multiples High resolution mini seismic surveys

  Downgoing Upgoing

First arrival times: - to flatten downgoing + to flatten upgoing and convert to two-way time

How to do a mix (running average): For each sample interval, an average of n traces is taken, where n is the mix window length. The average replaces the value of the center trace of the window. The window is moved one trace to the right and the procedure repeated

The median mix rejects noise spikes and data not horizontally aligned

Seismic unix  sumix

Consider a step function Mean Mix Mean values smears changes laterally Median Mix Median filter preserves changes and eliminates inconsistent data

Hinds, et al., 1996 Seismic Unix  sumedian

Stack of selected VSP traces closest to the well bore (yellow) Where the geophone is closest to the reflector and least contaminated by multiples

     Downgoing Upgoing Corridor stack Interval velocities Tie to lithologic logs

     Downgoing Upgoing Corridor stack Interval velocities Tie to lithologic logs

 Attenuation analysis

     Downgoing Upgoing Corridor stack Interval velocities Tie to lithologic logs

Steeper the dip, the farther updip the imaging Cannot tell updip direction from a zero offset VSP

Hard to image down dip Can’t always get what you want.

Need to plan ahead using modeling Hardage, 1983

Mini seismic survey. Can also do in 3-D Produces multi-fold data (CMPs)

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Walkaway VSP at a range of geophone depths for both horizontal (S1 and S2) and vertical (V) receivers. The horizontal energy decreases in amplitude (relative to the vertical component) with depth, related to converted wave partitioning at greater raypath angles (bottom) Static effects from the walkaway survey showing two-way travel time delay times from a range of receiver depths. These data were calculated by subtracting the average direct wave velocity calculated at each receiver depth.

1.6 km

~ 11 km

4.0 km

3 km

(He et al., 2007)

1500 Reflection wavefield Depth (m) 3500 0 Offset (m) 1000 0 Time (s)

(He , 2006)

3

120 shots Overburden

Velocity Tomogram

C4-X4 Distance - 12.7m

Porosity Logs 3 6 9 12 15 18 0 5 10 15 Receiver Depth (m) C4 X4 0 4 8 12 16 1500 20 Velocity (m/s) 2500 3500

C4-X4 Level-run velocity model 0 4 .2

.3

Porosity Seismic Slowness vs.

porosity 8 12 Velocity (m/s) 2000 16 3000 .34

.38

.42

Slowness (ms/m)

Crosswell Shot Gather

Receiver Depth (m) Unprocessed Shot 21 m 3 6 9 12 15 18 0 5 10 Receiver Depth (m) Shot Depth 21 m 21 m 15 3 6 9 12 15 18

C4 12.7 m X4 s hot depth - 21 m in well X4 downgoing energy 2 4 6 8 LS 0 4 8 12 16 20 Distance (m) from C4

Crosswell Shot Gather

Receiver Depth (m) Unprocessed Shot 2.4 m 3 6 9 12 15 18 0 5 10 15 Receiver Depth (m) Filtered Shot 2.4 m 3 6 9 12 15 18

C4 12.7 m X4 s hot depth - 2.4 m in well X4 upgoing energy 2 4 6 0 4 8 12 16 20 24 Distance (m) from C4