Step 1: Seismoelectric waves
The Figure shows an example of seismoelectric layout. In this example, the seismic source consists of a sledgehammer hitting a plate. The receivers are a string of dipoles connected to digitizers.
Seismoelectric conversions mainly give rise to two types of waves:
When the subsurface contains fluid, the seismic wave gives rise to an accompanying electromagnetic wave that travels within the seismic disturbance. As it travels at the same velocity as the seismic wave that induces it, it is dubbed a coseismic wave; an example of coseismic raypath is depicted in black in this figure.
When the incident seismic wave encounters an interface between two media exhibiting different mechanical or electrical properties (layers 1 and 2), it may give rise to an interface response (IR) that can be modeled by a dipole oscillating at the first Fresnel zone: the IR is represented in red
Step 2: Seismoelectric wave and Seismic waves
Figure displays two synthetic records side by side:
A seismic recording on the left-hand side: the inline horizontal displacement.
A seismoelectric recording on the right-hand side: the horizontal electric field.
The model is a simple two-layer tabular model consisting of a sand layer on top of a sandstone half-space. Some of the seismic arrivals show clear seismoelectric counterparts: these are coseismic waves.
Step 3: Interface response
When the incident seismic wave encounters an interface between two media exhibiting different mechanical or electrical properties, it may give rise to an interface response (or IR), travelling with the velocity of an electromagnetic wave. As its velocity is much higher than seismic velocities, the IR looks like it seen almost instantaneously by all receivers; it therefore appears like a "horizontal" event in the seismoelectric recording.
The coseismic signals provide information about the medium in the vicinity of the receivers, that is, in the very near surface. The interface response, on the other hand, provides information about the subsurface at depth.
Unfortunately, the interface response may be hard to recover, because its amplitude is generally much weaker than that of the coseismic response. Therefore, the weak interface response is often "masked" by the stronger coseismic signals, as seen in this Figure.
Figure shows a synthetic seismoelectric recording modeled for a two-layers tabular medium consisting of a sand layer on top of a sandstone half-space. The horizontal event seen at about t=0.02s corresponds to the interface response.
Step 4: Interface response identification
Figure (bottom left) shows an example of interface response IR It is a synthetic recording. In this example, the amplitude of the IR has been artificially boosted; otherwise the IR would be concealed by the stronger coseismic arrivals, as seen in the top left part of the figure. In this example the IR can be approximated as a dipole oscillating at the first Fresnel zone at the interface: this means that its amplitude is not maximal right below the shot point, but instead reaches its maximum at a certain offset away from the source. The amplitude variations of the IR (in absolute value) with respect to the offset are displayed in Figure (bottom right). It can also be noted that the IR exhibits opposite polarities on either side of the shot point: this characteristic may be used to discriminate the IR from other arrivals.
