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Numerical simulation and analysis of seismoelectromagnetic wave fields excited by a point source in layered porous media

To understand the characteristics of the electromagnetic(EM)fields during an earthquake,we studied the EM fields induced by the seismic waves in a porous medium due to the electrokinetie effect.Using the Pride's equations for the coupled poroelastic and electromagnetic fields,we derived the displacement-stress-EM-discontinuity vector for a double couple source, and then calculated the full waveforms generated by such a double couple source.As a comparison,waveforms generated by an explosion source are also simulated.The result shows that a P wave can induce an accompanying electric field,of which each component has a similar but opposite-phased waveform with the corresponding component of the solid displacement; an Swave can induce accompanying a magnetic field,which has similar waveform with the solid displacement.The double couple source also generates a kind of radiating EM wave,which propagates independently.Such EM wave is at least one order of magnitude faster than the p- wave,thus arrives at all receivers almost at the same time.However,its amplitude is much weaker than that of the accompanying electric and magnetic fields,and decreases rapidly when offset increases.It is also shown that the amplitude of the accompanying electric field induced by the P-wave not only depends on the amplitude of the P-wave and the electrokinetic coupling coefficient,but also depends on the fluid-solid dynamic compatibility,which is determined by the pore structure.There exists a special kind of porous medium,in which the P-wave cannot induce any accompanying electric field.