DETECTION OF LATERAL VELOCITY CONTRASTS BY CROSSWELL TRAVEL-TIME TOMOGRAPHY

Crosswell traveltime tomography is a common technique in the oil indus try for determining the velocity function in the plane between two bor eholes. However, the method suffers from the well-known problem that t he lateral resolution is far less than the vertical resolution because of the unfavorable illumination conditions for survey geometries comp rising vertical wells. Consequently, it is very difficult to image sud den lateral changes in the velocity function accurately using this tec hnique. We propose a method for determining such changes, which severe ly constrains the solution space by inverting for the position of a la teral velocity contrast only. The velocity model on each side of the c ontrast is derived from the well logs. The potential of the method is first demonstrated in two synthetic examples in which its properties a re compared with those of an unconstrained 2-D tomographic inversion. As expected, the constrained method has much better convergence proper ties than the unconstrained one in these examples. We also compare the two methods in a real setting, in which a sudden lateral velocity cha nge, associated with a geological transition, is expected. It turns ou t that a lateral contrast can be imaged with both methods in this real data example. However, the image obtained with the new method better explains the observed facts.