Inversion techniques to enhance images of the subsurface

Abstract

Seismic surveys are commonly used in the exploration for oil and hydrocarbons. However, in some regions of Australia, strong lateral velocity variations in the subsurface material tend to obscure the seismic images. Nowadays, the major problem associated in the production of seismic images of the subsurface is the availability of reliable subsurface velocities that can be inverted from routinely collected reflection data. This paper presents a technique for inverting two-way travel times and stacking velocities from such data sets to model the subsurface velocities. We model the subsurface by a number of layers of material separated by laterally varying boundaries; the velocities within each layer also vary laterally. We seek the smoothest possible functions describing the lateral variation in interval velocities and boundary location which also fit the observations to a specified tolerance. That is, we permit a trade-off between the smoothness of the model and goodness of fit to the data. Data are weighted according to the accuracy with which they are acquired; this causes the residual at each data point to be compared with its expected error. We represent the interval velocity and boundary location for each layer as a sum of B-splines, and develop a non-linear inversion technique to model subsurface velocities. This algorithm is applied to reflection data from the Gippsland Basin using various interpretations of the stacking velocity data. It is found that the velocity profiles obtained have some significant differences compared with those predicted by the Dix formula, and yet generally seem to be more consistent with well data than previous velocity models. In particular, the method presented here seems able to handle velocity inversions better than earlier techniques. Furthermore, this technique provides a more objective way of determining an appropriate coarse-scale velocity model to be used in the migration of seismic data than traditional methods of "smoothing" Dix results.