Report 9 Crosswell Imaging with Surface Sources and Tube Waves

Conventional crosswell surveys are expensive because a source must be placed downhole, which means that the hole must be prepped and placed off-line for a number of days. We suggest an interesting but unproven alternative: place a source on the surface, drop a simple obstruction-like device downhole that scatters tube wave arrivals from the surface source, and use the scattered tube waves energy as a seismic source in that borehole. We test this idea by computing the tube wave responses from a synthetic crosswell data set and use these tube-wave responses to construct a well-resolved velocity image between the wells. The tube wave responses are also computed from field data recorded with a conventional crosswell survey. The processing method is based on stacking of coherent seismic energy that emanates from the location of tube wave scattering. Results show that the arrival time of both the transmitted arrivals and reflection events in the tube wave responses agree well with those in the real common shot gathers. However, there are fewer reflection events in the tube wave responses than in the conventional shot gathers and their bandwidth is not as wide. Nevertheless, this result suggests the possibility that more economical crosswell seismic surveys can be performed using surface seismic sources and tube wave scatterers in the source well. The remaining question is whether or not the tube wave scatterer is strong enough to generate reflections with a sufficiently high signal-to-noise ratio.