A novel method of extracting higher modes of Rayleigh waves from seismoelectric signals

Abstract:

The surface wave (e.g., Rayleigh wave) analysis method is an appealing noninvasive tool for estimating the subsurface shear wave velocity structures. It has been widely applied in near-surface exploration geophysics. Surface waves possess the multimodal dispersion information consisting of fundamental mode and higher modes. The higher modes possess greater investigation depth and show higher sensitivity to the shear wave velocity than the fundamental mode. However, it is usually a great challenge to extract high-quality dispersion information of higher modes from real seismic data. Consequently, the extraction and application of higher modes of surface waves have been an extremely crucial subject in the study of surface waves. All the existing researches are based on seismic data. Is there any other kind of data capable of providing dispersion information of higher modes?

In this study, we introduce a novel method extracting higher modes of Rayleigh waves from seismoelectric signals, namely, electromagnetic (EM) signals originating from seismoelectric conversion. Adopting a vertical force source acting on the free surface, we simulate seismic and EM signals in the layered half-space porous media. The dispersion spectrograms of seismic and EM signals are obtained by the frequency‐Bessel transform method. Through modeling results, we find that the EM signals mainly contributed by evanescent seismoelectric waves also contain the Rayleigh wave dispersion information and can provide the dispersion information of higher modes that seismic waves cannot. We analyze the reason generating this phenomenon by investigating the dispersion characteristics of seismic and EM signals at different depths. We propose to use the radial electric component in practice to improve the Rayleigh wave dispersion spectrogram, because it has much higher detectability than other EM components. We also discuss the factors affecting the strength of EM signals. Our study suggests the Rayleigh wave dispersion information with abundant higher modes and good frequency-range coverage can be obtained by using seismic and seismoelectric signals together.