Reverse Vsps And Crosshole Seismic Tomography While Coring

At the Boise Hydrogeophysical Research Site, numerous 20 m deep wells were continuously<br>cored with a split spoon sampler to detail geologic changes beneath the site. During the coring of<br>one of these wells, we recorded the energy radiating from the drill stem using a combination of<br>downhole hydrophones and surface geophones. The hydrophone strings were located in a pair of<br>adjacent wells and the geophones were located along the surface between these wells and the<br>well being cored. The experiment’s geometry provides a series of reverse VSPs and crosshole<br>tomographic data sets that can help distinguish P-wave velocity changes in the subsurface.<br>Energy from the core tip is not necessarily the first arrival. However, the near continuous<br>recording of the source as it penetrates the ground enables us to locate the energy from the core<br>tip. We also record energy when the source is in the vadose zone. Comparing the seismic while<br>coring results with a similar experiment using a borehole sparker source confirms the reliability<br>of recording seismic energy generated by the split spoon sampler. Core tip recording is an<br>inexpensive seismic method that has great potential for borehole-to-surface and crosshole<br>tomographic imaging at relatively large offsets in near-surface environments.