Theoretical analysis of non-geometric PS-waves recorded at the water bottom interface

Abstract

When a seismic source is placed in the water at a height less than a wavelength from the water–solid interface, a prominent S-wave arrival can be observed. It travels kinematically as if it was excited at the projection point of the source on the interface. This non-geometric S-wave has been investigated before, mainly for a free-surface configuration. However, as was shown in a field experiment, the non-geometric Swave can also be excited at a fluid–solid configuration if the S-wave speed in the solid is less than the sound speed in the water. The amplitude of this wave exponentially decreases when the source is moved away from the interface revealing its evanescent character in the fluid. In the solid, this particular converted mode is propagating as an ordinary S-wave and can be transmitted and reflected as such. There is a specific region of horizontal slownesses where this non-geometric wave exists, depending on the ratio of the S-wave velocity and the sound speed of water. Only for ratios smaller than 1, this wave appears. Lower ratios result in a wider region of appearance. Due to this property, this particular P-S converted mode can be identified and filtered from other events in the Radon domain.