Shear-wave splitting in compliant rocks

Abstract

Shear-wave splitting is a phenomenon that has received a lot of attention primarily because of its connection with vertically aligned cracks or fractures within reservoirs. However, in most cases the largest amount of shear-wave splitting is observed to occur in the near-surface layers where the rocks are the least consolidated, and so are least likely to be stiff enough to support cracks. Ironically, we have observed during the processing of many multicomponent data sets in western Canada that the largest amounts of shear-wave splitting occur in an area where we least expect to see it— in the highly unconsolidated sediments that comprise the heavy oil plays in the northwestern part of Alberta. At first we thought that the rocks in this area were surely too soft to support cracks and therefore that shear-wave splitting would be smaller than observed elsewhere. The data have taught us that the opposite is true.