For soil exploration in the vadose zone, a high-frequency multi-channel analysis of surface waves (HF-MASW) method has been developed. In the study, several practical techniques were applied to enhance the overtone image of the HF-MASW method. They included (1) the self-adaptive MASW method using a variable sensor spacing configuration, (2) the phase-only MASW method, and (3) a nonlinear acoustic technique. To demonstrate the capability of these techniques, a HF-MASW test was conducted, which consisted of a small shaker operated in chirp mode with three gapped frequency bands and a geophone array with a variable spacing configuration. The overtone images obtained from (1) full spreadlength, (2) self-adaptive spreadlength, (3) full-spectra analysis including magnitude and phase spectra, and (4) phase-only spectrum analysis MASWs, respectively, were compared in each frequency bands. The results revealed the enhanced dispersive images by the HF-MASW using the self-adaptive and phase-only with the variable spacing configuration technique. The study also demonstrated the nonlinear acoustic technique by means of harmonic generation to increase the measurable frequency range of dispersion curves. It is worthy to mention that these techniques can also be applied to the conventional MASW method using controlled seismic vibrators, such as vibroseis.

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