Introduction

Abstract

We know that two types of elastic waves (P and S) can travel through the body of an elastic medium. The particle motion and velocities of these waves are distinctly different. During propagation the particle motion of the longitudinal wave (or P-wave) is along the direction of the raypath, and of the shear wave (or S wave) is perpendicular to the raypath. Figure 1.1 a is a depiction of a simple reflection path geometry for near normal incidence. It assumes a horizontal reflector, and shows raypaths of a P-wave from an explosion and of S-waves from a horizontal vibrator. G is the detector, which may be sensitive to vertical or horizontal vibration. For a near normal raypath, the reflection coefficient for P- and S waves depend on compressional (P) and shear (S) wave velocities and densities on two sides of the reflector. It is the acoustic impedance contrast (i.e. (ρ ₂ α ₂ − ρ _l α _l) for a P-wave and ρ ₂ β ₂ − ρ _l β _l for the S-wave), which controls the absolute value of the normal incidence reflection coefficient.