HEAD WAVES FROM A BED OF FINITE THICKNESS

Abstract

The behavior of the head wave from a high‐speed layer embedded in a low‐speed half‐space has been investigated with two‐dimensional seismic models. Twelve layer thicknesses ranging from four wavelengths to one‐tenth wavelength were used. A simple theory based on interference between the head wave and the reflections from the bottom of the layer gave amplitude‐distance values which agreed with the observations for layer thicknesses down to about one‐third of a wavelength. For thick layers, the experimental amplitude dependence on distance was different from the theoretical law (−3/2 power of the distance). The velocity minimum for thin layers discovered by Lavergne was confirmed and a possible slight velocity maximum at intermediate layer thicknesses noted. The velocity for zero layer thickness appeared to be greater than the free bar velocity. Systematic variations of head‐wave spectra with layer thickness occurred.