Amplitude ``statics' in shallow refraction seismology

Abstract

Increases in refracted amplitudes not related to changes in the head coefficient are usually associated with increases in traveltimes in the near-surface layers, while decreases in amplitudes are associated with decreases in traveltimes. These correlations demonstrate that the amplitude variations are related the variations in the near surface geology, rather than to variations in the coupling of the detectors with the ground. The change in amplitude can be described with the transmission coefficient of the Zoeppritz equations. Correction factors can be applied for those surface conditions which are sufficiently extensive to permit the measurement of the wavespeed. Where this is not possible, then the lowest amplitude or amplitude product is representative of the head coefficient for the main refractor.