We study seismoelectric waves due to the electrokinetic effect in vertical transversely isotropic (VTI) media. An analytical method is presented for solving 3D seismoelectric waves generated by a point source and numerical simulations are conducted to investigate the characteristics of the seismoelectric waves. The results indicate that three types of seismoelectric signals can be observed, namely, a direct electromagnetic (EM) wave radiated by a seismic source, coseismic electric/magnetic fields accompanying seismic waves (including qP, qSV, and SH waves), and an EM wave converted at an interface from a seismic wave (i.e., the interface seismoelectric response). The seismoelectric signals in a VTI medium have some specific properties different from those in an isotropic medium. For example, a qP wave has a coseismic magnetic response, whereas in an isotropic medium a P wave cannot generate any coseismic magnetic field. All the qP, qSV, and SH waves have a direction dependent or anisotropic ability to generate coseismic electric and magnetic fields. The qP wave has the strongest ability to generate the electric field, 1–3 orders stronger than the qSV and SH waves. The qSV and SH waves have a comparable ability to generate the magnetic field, one order stronger than the qP wave. The results also indicate that the anisotropy in velocity, permeability, and conductivity has great impacts on the interface response. Simulations of an earthquake source demonstrate that the coseismic electric fields are dominated by the qP, qSV, and Rayleigh waves, whereas the coseismic magnetic fields are dominated by the SH and Love waves.

Earthquakes are frequently accompanied by electromagnetic (EM) anomalies. These anomalies are thought to be caused by earthquakes but the generation mechanism is still unclear. The piezoelectric effect has been proposed as a possible mechanism but the EM responses to earthquakes due to such an effect has not been well understood. In this article, we study the EM signals generated by an earthquake source due to the piezoelectric effect. We develop a semi-analytical method to solve the seismic and EM fields in a 3D layered model and conduct numerical simulations to investigate the characteristics of the EM fields. The results show that the earthquake can generate two kinds of EM signals. One is the early-EM signal which arrives earlier than the seismic wave. The other is the co-seismic EM signal accompanying the seismic wave. For an earthquake the co-seismic electric field can reach ~10 μV/m and the magnetic field can reach ~10-4 nT. We also study the sensitivity of the co-seismic EM fields to the rock conductivity. The results show that the co-seismic EM fields are mainly affected by the conductivity of the shallow layer, and they are also affected by the conductivity of the deep layer when the top layer is thin.

Summary In this study, we present a semi-analytical method to simulate the propagation of seismic waves in horizontally stratified double-porosity media. We solve the governing equations in the frequency-wavenumber domain and then compute the time-space domain solutions by Hankel transform and fast Fourier transform. We conduct numerical simulations to study the properties of the seismic waves in the double-porosity media. The results show the existence of three P waves, one fast P wave and two slow P waves. The two slow P waves are highly attenuated and can be observed by assuming a low fluid viscosity. By comparing the fast P wave in a single-porosity medium and that in a double-porosity medium, we find that the double-porosity model predicts much higher attenuation of the fast P wave due to the local fluid flow between the background medium and the inclusions. We also study the parameters affecting the attenuation, such as the radius of the spherical inclusions, the viscosity of the pore fluid, the permeability of the background medium and the porosity of the inclusions. Finally, we present a cross-well survey model to investigate the seismic responses in the double-porosity media and compare them with the responses in the single-porosity media. We find that both the amplitude and velocity of the fast P wave decrease with the increase of the volume ratio of the inclusions.

Summary We propose a frequency-domain finite-element (FDFE) method to simulate the 2-D SHTE mode seismoelectric and electroseismic waves. By neglecting the secondary weak wave field feedbacks, the SH and TE waves are solved, separately. In a finite plane region, propagations of both SH and TE waves can be described as the Helmholtz equation with boundary conditions, which is proved to be equivalent to the extremum of functional by conducting calculus of variation. The computation region is partitioned into structured rectangular elements with the bilinear interpolation. The proposed FDFE algorithm solves the wavefield in frequency domain and avoids adopting the quasi-static approximation. One advantage of the proposed algorithm is its ability to accurately simulate the seismoelectric and electroseismic responses generated from the free surface. We verify the proposed algorithm based on a layered model beneath a free surface by comparing the waveforms calculated using the FDFE algorithm with those calculated using analytically-based methods. The proposed algorithm is applied in feasibility studies of interface seismoelectric and electroseismic responses in exploring the hydrocarbon reservoir and monitoring the time-lapse pollutant within a sand channel.