Comparison between surface and VSP reflection imaging of shallow sites
- Univ. of California, Berkeley, CA (United States)
We compared surface geophone shot records with multi-offset VSP hydrophone records at a bay margin depositional environment site, characterized by a complex interleaving of largely unconsolidated clay, sand, gravel and mud layers. The aim being to determine which seismic method was better suited to produce a reflection image between 3m (water table) and the basement at 35m, suitable for structural interpretation. We discovered that the VSP data had at least three times the bandwidth of the surface data, with lower levels of cultural noise. One of the most important advantages of VSP is the ability to resolve reflections from shallower depths than is possible with surface reflection techniques, and we determined that at this site we would not be able to image reflections above 20m. Additional advantages of using offset VSP imaging included accurate time-to-depth conversion, deterministic statics, and with reliable velocity control. We used inter-hydrophone baffles to attenuate and slow down tube waves, which facilitated reflection wavefield separation, and increased the recorded signal bandwidth. The VSP-CDP mapped image ties in signal bandwidth. The VSP-CDP mapped image ties in well with cone penetrometer logs, and clearly shows reflections from thin gravel and sand units between 5m and 35m, with an inter-bed resolution of 0.5 m, out to a distance 9 m from the well. The mapped image also clearly shows lateral bed discontinuities, interpreted to be sand/gravel lenses and pinch-outs.
- OSTI ID:
- 542992
- Report Number(s):
- CONF-951013--; CNN: Grant B283607
- Country of Publication:
- United States
- Language:
- English
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