Acoustic-to-seismic transfer function at the surface of a layered outdoor ground

Abstract

The ratio of the surface soil particle velocity to the surface acoustic pressure is termed the acoustic to seismic transfer function. Measurements of this transfer function typically show several maximum and minimum in the frequency range between 50-500 Hz. The magnitude of this transfer function can be explained in light of the porous nature of the ground surface .The ground is modeled as a poro-elastic layer overlying a non-porous substrate. The boundary conditions at the air/porous soil and the porous soil/non- porous substrate interfaces are applied to setup the acoustic-to-seismic coupling problem. In the porous layer, up an downing going Biot Type I, II compressional and shear plane waves are allowed. In the non-porous elastic substrate down going compressional and shear plane waves are allowed. Using the Biot characteristics equations and these boundary conditions the steady state frequency dependent acoustic to seismic transfer function is calculated. Layer depths, Type I, and shear wave speeds are determined from a shallow seismic refraction survey. Soil density, air porosity and permeability are determined from other measurements. The calculated transfer functions are compared to that measured on several outdoor grounds.