Advantages and Disadvantages of Passive Multichannel Analysis of Surface Waves (MASW) for Observing Subsurface Discontinuities

Abstract:

The multichannel analysis of surface waves (MASW) method has been widely accepted as a quick and efficient technique for quantifying shear-wave (Vs) velocity to help resolve different subsurface geologic features. However, depth of imaging is directly related to frequency and active seismic sources may not generate surface waves with long enough wavelengths to sufficiently sample with the target. The passive MASW method, similar to the active method, utilizes multiple receivers to construct 2D velocity profiles, but harnesses ambient, coherent noise energy, which can provide lower frequencies and thus greater investigation depth compared to active or controlled source energy. Active source energy is controlled during a survey by the operator, whereas passive or ambient energy can originate from local sources at various azimuths. However, multi-azimuthal sources observed on a single record can enhance or interfere on the dispersion curve image (i.e., frequency – velocity spectrum) depending on the propagation direction of the seismic wavefront. Given this azimuthal factor and experience with active MASW, the quality of the overtone image is greatly dependent on the singularity of the signal. Over three years, a series of passive MASW surveys were acquired in south-central Kansas designed to observe and understand these varying effects of wide azimuthal train noise sources on overtone images. From these acquisitions, we discovered that no two train source signals are the same for the presented location. Picking the optimal source azimuth is critical to harnessing useful dispersion energy from the appropriate Rayleigh wave inducing source. Time window-splitting aided preliminary dispersion curve analysis, which allowed us to differentiate modes from passive sources at various azimuths that are otherwise compiled into a single image and result in degradation of the overtone image. Through a series of analyses in conjunction with time window-splitting, we have been able to better understand multi-azimuthal passive source energy and implications for dispersion curve images.