Critical Depths For Higher Modes By Minimally-Invasive Seismic Profiling: Simulations And Field Test

To measure more complete multi-mode dispersion data and thus improve the accuracy of inversion profiles, the authors recently developed a minimally-invasive hybrid surface-and-borehole method. The new method employs a borehole geophone at selected depths to record seismic waves from a moving source on the soil surface. The goal of this paper is to develop a procedure for estimating the ranges of optimum geophone depths for capturing the higher modes. Stiffness matrix and finite element-based numerical simulations of the hybrid testing method are performed to identify the relationships between critical geophone depths and cut-off frequencies. A preliminary field test is conducted using a vertical geophone placed at five depths while impacts are applied to the soil surface over a range of offsets. Dispersion images from the five geophone depths were superimposed to produce a dispersion image having three modes with improved clarity relative to the surface-only MASW method. A comparison of the experimental and theoretical cut-off frequencies of higher modes is used to validate the prediction of critical depths by the stiffness matrix method. Matching of such experimental and theoretical cut-off frequencies is proposed as additional optimization constraints to reduce the uncertainty of final inversion profiles.