Introduction

A seismic wave traveling through a fluid-saturated porous material carries with it a charge separation created by the pressure-induced flow of pore fluid. The pore fluid carries a small (but not inconsequential) amount of electric charge relative to the adjacent grains due to the electric double layer () that exists at the grain-fluid boundary. Thus, an electric field (Figure ) is co-located with a compressional (P) wave propagating through such a material (). We refer to this field as the ``coseismic'' field.

 

esbasics
Figure 1 Electroseismic phenomena depend on the charge separation created by streaming currents that flow in response to the pressure gradient of a seismic wave. The electric double layer is responsible for streaming currents at the grain scale.

The second aspect of the electroseismic response occurs when the P-wave encounters an interface in material properties (elastic, chemical, flow-related, etc). The charge separation in the wave (Figure ) is disturbed, causing asymmetry and results in what can be approximated as an oscillating electric dipole at the first Fresnel zone (). Essentially, the entire region of the first Fresnel zone acts as a disk of vertical electric dipoles. Thus, the resulting electric field is that of a dipole, with opposite polarity on opposite sides of the source point and amplitudes diminishing as 1/z³ (where z is the depth to the interface). This field (Figure b), called the ``interface response,'' can be measured almost immediately at the Earth's surface since the travel-time of electromagnetic radiation is negligible compared with seismic travel-times (341#341).

 

2effects
Figure 2 Two types of electroseismic effects that can be measured with electrode dipoles at the Earth's surface: (a) the coseismic field of a P-wave at the surface (represented here by the charge accumulations ``+'' and ``-''), and (b) the interface response created when the P-wave hits an interface at depth.

Both effects can be measured in the field using a standard seismograph equipped with electrode dipoles instead of geophones (, , ). The coseismic field traveling with the seismic wave is not particularly interesting since it contains information only about the properties of the surface of the Earth. The interface response, on the other hand, can provide new information about the subsurface. In particular, the interface response is created even for very thin layers, such as a thin fracture zone in otherwise solid rock, or a thin impermeable layer in an aquifer or reservoir. () show that the interface response from a saturated permeable layer 0.6-m thick can be reliably observed. Numerical simulations show that the interface response from a 1-cm embedded impermeable layer is significantly greater than that from an interface between two layers (Stephane Garambois, personal communication 2001). Thus the electroseismic method promises to provide valuable information about important subsurface targets that can not be imaged using other geophysical methods, including information about the location of changes in flow properties. It is our goal to develop a protocol that can be used to reliably and repeatably acquire, process, and interpret electroseismic data.

Unfortunately, electroseismic data collected with a geometry similar to conventional surface seismic data is comprised of both the interface response from subsurface layers and unwanted coseismic energy recorded simultaneously. Coseismic energy, roughly 100 times the amplitude of the interface response, therefore represents a formidable form of coherent source-generated noise. The removal of this noise is essential to the utility of the electroseismic method, so the development of an effective data processing approach is an important step toward this goal. The use of transforms (e.g., f-k filtering) has proven ineffective on available data due to the overpowering amplitude of the coseismic noise, and the fact that the top of the coseismic energy hyperbola tends to be smeared across the record during the inverse transform. It is essential that all horizontal energy remaining in the record after processing be only from the the interface response. Each shot record eventually will be stacked to produce a single trace corresponding with the subsurface region beneath the shot point. Thus, smeared coseismic energy would be very detrimental to the final stack in much the way that inclusion of ground roll or refractions negatively impacts a stacked seismic reflection section. In addition, the smearing of energy represents a loss of amplitude information and the retention of relative amplitudes is desired.

We present a data processing strategy that separates the signal of interest from the stronger coherent noise. This strategy incorporates the coherent noise subtraction approach described by () while building on the use of PEF's described by (). An important feature of our approach is its preservation of the signal amplitude made possible by our use of iterative inversion. We also present other data processing options and discuss the work remaining to be completed before the electroseismic method can be considered a reliable tool.