Relationship between head wave amplitudes and seismic refraction velocities to detect lateral variation in the refractor

Abstract

Refractor ambiguities are big problem in seismic refraction method especially in seismic engineering. There can be hidden subsurface geological phenomena such as hidden faults and shear zones which are not simply predicted by the travel-time graph or some geophysical methods. Head wave amplitudes are used to show the resolution of refractor ambiguities and the existence of anisotropy in complex geological area. Wave amplitude is proportional to the square root of energy density; it decays as 1/r. In practice, velocity usually increases with depth, and causes further divergence of the wave front and a more rapid decay in amplitudes with distance. Amplitudes measured from first peak to first trough and corrected for geometric spreading, can be demonstrated some subsurface information such as anisotropy. Meanwhile, amplitudes are not commonly study by researchers in seismic refraction studies, because of being the very large geometric spreading components due to variations related to wave speeds in the undulated refractor. The variations in amplitudes are described with the transmission coefficient of the Zoeppritz equations. This variation in velocity and density produces head wave amplitude and head coefficient changes in refractor, even with refractors exhibiting large variations in depth and wave speeds. The head coefficient can be approximately calculated by the ratio of the specific acoustic impedance in the overburden layer in the refractor. This study shows that there is a relationship between the amplitude and the seismic velocity which the lower the contrast in seismic velocity and/or density, the higher the amplitude and vice versa.