Is It Time To Modernize Near-Surface Refraction Seismology With Full Waveform Methods?

Abstract

Historically, near surface refraction seismology has focused almost exclusively on inverting first arrival traveltimes to generate spatially varying models of the seismic velocities in the weathered and sub-weathered regions. This study describes two approaches to full waveform near surface refraction seismology, using common offset gathers (COG) and the refraction convolution section (RCS). Full waveform refraction methods can improve the resolution and characterization of the routine mapping of the base of the weathering, through stacking, flattening and spectral analysis. Full waveform refraction methods can usually reveal first and later events wherever reflection events are recorded within the refraction Fresnel zone. In most cases, full waveform refraction methods can provide more detailed images of the sub-surface structure than can be obtained with low resolution 1D refraction traveltime tomography. The amplitudes of first and later events are related to the head coefficient, which in turn, is a simple ratio of the specific acoustic impedances. Both the density and the P-wave modulus models of the near surface, which are derived from the head coefficient and the seismic velocities, can be employed for more comprehensive characterization of the regolith for geotechnical and groundwater investigations, as well as for starting models for full waveform inversion. Full waveform methods represent a new frontier for the modernization of near surface refraction seismology. They offer the opportunity for more effective implementation of exploration refraction seismology through extracting greater value from the data.