Investigation of seismoelectric effects in soft sediments

Abstract

Field trials of the seismoelectric geophysical method were conducted at two sites along the Saint John River in Fredericton, New Brunswick, Canada. These tests were performed in order to evaluate the suitability of this particular geophysical method for imaging subsurface interfaces in soft sediments in an urban environment. To this end, this method was used to investigate the presence and thickness of the clay aquitard that overlies the Fredericton Aquifer. Seismic refraction and seismic reflection data was collected at the same site for comparison purposes and for use in further studies of the Fredericton Aquifer. The test surveys were conducted at two separate locations over three days during the fall of 2002. Measurements made along the Saint John River west of Wilmot Park (behind the Fredericton Sheraton) were used to verify the operation of the equipment and produce sample results for processing. The survey conducted at Morell Park was the primary focus of the study. Seismoelectric shot records were collected utilizing blank shotgun shell and sledgehammer seismic sources. Extensive processing was required to remove background noise and enhance the signal to noise ratio. In particular, powerline frequency harmonics dominated the raw data and required removal by a specific processing algorithm. The measured electrical signals in the ground in the raw shot records were approximately 0.06 V/m. After processing, the magnitude of the seismoelectric signals were calculated to be approximately 0.002 V/m. The results of the seismoelectric survey were compared with the results of the seismic reflection and refraction survey as well as the available borehole data in order to correlate the results. Bulk and interfacial seismic effects were identified in the processed records.