Refining Higher Modes of Rayleigh Waves Using Seismoelectric Signals Excited by a Weight-Drop Source: Study From Numerical Simulation Aspect

摘要

Effective extraction of higher mode dispersion information has been a research hotspot in the study of surface waves. In this study, we introduce a novel approach of refining Rayleigh wave higher modes by using seismoelectric signals, namely, electromagnetic (EM) signals originating from seismoelectric conversion. Adopting a weight-drop source, we simulate seismic and EM signals for several porous models. An extended version of the frequency-Bessel transform method is introduced to extract dispersion information from seismic and EM signals. Results suggest that the evanescent seismoelectric waves, which are induced at subsurface interfaces penetrated by Rayleigh waves and finally are recorded nearby the ground surface after undergoing amplitude decay, can reflect dispersion characteristics of Rayleigh waves and can provide extra higher-mode dispersion information that seismic waves cannot. We find the vadose zone with higher water saturation will cause the seismoelectric signals to have stronger strengths and change their frequency coverage of dispersion information. Electric fields obtained from potential differences between longer dipole electrodes suffer more severe distortion. In an electrically resistive environment, predicted seismoelectric signal amplitudes in the frequency band with dominant energy are well above expected background noise levels. We also find seismoelectric signals have a stronger antialiasing ability than seismic waves and their dispersion energy values are sensitive to salinity and fluid viscosity. Generally speaking, our study suggests Rayleigh wave dispersion information with abundant higher modes and good frequency-range coverage can be obtained by using seismic and seismoelectric signals together due to their complementarity.