Abstract

In a stratified earth, seismic waves tend to propagate faster parallel to bedding than across layer boundaries. In this context, a boundary is an interface between two zones with different acoustic impedance. During sediment deposition, clay minerals in shales settle in a preferential direction, and also form plate-like crystals during diagenesis, causing similar behavior to seismic wave propagation. This phenomenon causes velocity anisotropy, defined as the dependence of the velocity of a rock on the direction of wave propagation through the rock. Other causes include aligned cracks and fractures, and stress due to the weight of overburden