Characterization in three dimensions of a low-velocity layer using crosswell seismic travel times

Three crosswell surveys conducted in a five-spot at the Gypsy Test Site in NE Oklahoma give a three-dimensional picture of the top and bottom boundaries of the Pennsylvanian Gypsy formation. The seismic expression of the channel sand sequence comprising the Gypsy formation is that of a low-velocity unit encased by higher velocity sands and sandy shales. The existence of reflections and headwaves due to these large velocity contrasts at the boundaries of the Gypsy formation is confirmed by raytracing in a velocity model using a hybrid approach based on the method of Andersen and Kak. This one dimensional model is established using the horizontal path log. Multiple stages of median filtering greatly enhance reflection amplitudes. Clearcut reflections from within the Gypsy formation suggest that a horizontal stacking scheme would successfully image individual channel sands.;The origin depths of the reflections from selected common shot and common receiver gathers establish formation boundary depths. A simple analysis of the extent of headwave and reflection events produces a boundary map in the vicinity of the receiver wells. Depths derived using the crosswell data alone agree well with density logs.;Continuity mapping using headwaves is feasible and presents an inexpensive alternative to fullaperture crosswell surveying for structural objectives.