Seismoelectric effects of electrokinetic origin

• Main Author: Butler, Karl Edward
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6270

Seismoelectric effects are electromagnetic signals that arise when seismic waves stress earth materials. Their measurement is challenging because they are typically much weaker than the ambient electromagnetic noise. For this study, specialized instrumentation, field methods, and data processing techniques were employed to eliminate acquisition artifacts, and optimize signal-to-noise ratios. Experimental data were acquired which demonstrate clearly that seismoelectric effects can be measured in the field, and used to map shallow boundaries in porous sediments. Two types of seismoelectric signals were observed during field experiments at Ffaney, BC. The primary response was generated as compressional waves impinged upon a boundary between road fill and impermeable glacial till. Sledgehammer and blasting cap seismic sources positioned up to 7 m away from the boundary induced seismoelectric conversions with amplitudes of up to 1 mV/m, which were measured at the surface with grounded dipole receivers. This response arrived simultaneously at widely separated receivers, and was the dominant signal observed at near offsets. Recordings taken by receivers farther from the shotpoint were dominated by a second type of seismoelectric arrival which originated in the immediate vicinity of each dipole. Attempts to use seismoelectric conversions to map boundaries at other sites were not successful, but signals like the secondary arrivals at Ffaney were observed in all cases. Electrokinetic effects, arising from motion between the pore liquid and solid frame, are considered the most likely mechanism for the seismoelectric responses presented here. A simple conceptual model for seismically-induced electrokinetic effects can account for the two types of arrivals observed in field data. The model predicts that charge separations induced by a compressional wave in porous media produce electric fields that are observed (i) as the seismic wave passes by a receiver, and (ii) when the distribution of charge associated with the seismic wave is altered by a boundary or other inhomogeneity. In principle the boundary may separate regions with differing elastic or electrical/electrokinetic properties, permeabilities, or pore fluids since all of these factors influence charge transport. The experimental results from Haney, BC, support this model and other more elaborate theories for seismoelectric effects of electrokinetic origin.