Crosswell seismic imaging using reflections
Robert R. Stewart, Giovanni B. Marchisio
The use of reflected as well as transmitted energy in crosswell imaging can provide broad coverage and high-resolution sections between the wells. This paper discusses the general processing flow for the analysis of direct and reflected arrivals. We develop two procedures for crosswell reflection analysis: VSP-type and CDP-type flows.
In the VSP-type procedure, upgoing and downgoing reflections in shot or receiver gathers can be separated and mapped as offset VSP's. These individual maps are then stacked into a final section. On the other hand, more complete shot and receiver coverage may justify the implementation of a CDP-type stacking procedure. This CDP-type procedure includes trace gathering around a common source-receiver mid-depth (CMD gather) as well as a vertical move-out correction (VMO) between the various CMD locations. The VMO velocity is determined by scanning across CMD gathers. The VMO- corrected traces are then sorted into their correct lateral bins and stacked. These novel, yet simple, processing concepts are effective with the crosswell geometry.
These procedures are applied to ultrasonic data acquired in a physical modeling facility. The physical model and survey geometry were designed to simulate a crosswell field survey of an enhanced oil recovery (EOR) monitoring experiment. We find that the CDP-type procedure produces a better image in the zone of interest than does the VSP-type flow. The CDP-type procedure used also requires less operator intervention.