• Continuous Resistivity Profiling in Shallow Marine and Fresh Water Environments

  • Application of Continuous Resistivity Profiling to Aquifer Characterization

Continuous Resistivity Profiling in Shallow Marine and Fresh Water Environments

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In this paper, we describe an instrument system for performing continuous resistivity profiling in shallow freshwater and marine environments. Using a streamer cable containing 9 electrodes, the system continuously samples the dipole-dipole resistivity at n-spacings 1 through 6. The system can be installed aboard a variety of small inboard or outboard powered vessels in a few hours. Hand-held or marine GPS units provide location information that is recorded by a laptop computer. With this system, up to 40 line-km/day of dipole-dipole data have been collected. The resistivity data are merged with the GPS positions as a post-processing step. The final step in the post-processing is the inversion of overlapping segments of each profile using a 2-D smooth model. The inversions provide high resolution images of the geoelectric cross-section. The depth of investigation ranges from 20-30 m, with a 10 m dipole spacing. Over the last 4-years, we have performed surveys on the Ohio River, near Louisville, KY, on tidal estuaries and bays along the Atlantic coast in Delaware, Maryland, Virginia, and North Carolina, and in Tampa Bay, Florida. Data from these surveys will be used to illustrate the final deliverable from a survey.

Application of Continuous Resistivity Profiling to Aquifer Characterization

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This paper presents the results of a continuous dipole-dipole resistivity survey conducted along a section of the Ohio River near Louisville, KY in the summer of 1997. Louisville, and no doubt many other municipalities along major rivers such as the Ohio River, draw their municipal water from the alluvium beneath the river using large vertical caissons from which horizontal perforated casings are pushed into the river. The high capacity of the pumping sites (200,000 gal/min) requires direct and rapid recharge of the drainage area of the intake site. Recharge rate and hence pumping capacity can be seriously compromised by the presence of clay on the river bottom directly over the intake drainage area retarding the recharge of the alluvium beneath. The objective of the resistivity survey was to characterize the nature of the river bottom for the purpose of siting new intakes for the municipal water supply of the city of Louisville. The paper describes a resistivity system assembled from commercially available ground resistivity instrumentation. Navigation information was coupled into the system using an integrated L-band differential GPS receiver. The equipment was installed and tested on a small pontoon barge powered by an outboard motor in less than a day. Using a streamer containing 9 electrodes spaced at 10-m intervals, 35 line-km of continuous dipole-dipole resistivity (1 = n = 6) data were acquired at approximate intervals of 5 m. The data were acquired in approximately 10 hours (3-5 km/hr) over a period of two 2 days. The resulting resistivity maps and pseudo-section profiles effectively delineate areas where clay is known to be present in the river bottom, detect the presence of culture (e.g., pipes and casing in the river bottom), and provide the basis for siting new water intake installations. The survey demonstrates that resistivity profiling provides a rapid and economical means for the characterization of sediments beneath shallow fresh water.

Application of Continuous Resistivity Profiling to Aquifer Characterization

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This paper presents the results of a continuous dipole-dipole resistivity survey conducted along a section of the Ohio River near Louisville, KY in the summer of 1997. Louisville, and no doubt many other municipalities along major rivers such as the Ohio River, draw their municipal water from the alluvium beneath the river using large vertical caissons from which horizontal perforated casings are pushed into the river. The high capacity of the pumping sites (200,000 gal/min) requires direct and rapid recharge of the drainage area of the intake site. Recharge rate and hence pumping capacity can be seriously compromised by the presence of clay on the river bottom directly over the intake drainage area retarding the recharge of the alluvium beneath. The objective of the resistivity survey was to characterize the nature of the river bottom for the purpose of siting new intakes for the municipal water supply of the city of Louisville. The paper describes a resistivity system assembled from commercially available ground resistivity instrumentation. Navigation information was coupled into the system using an integrated L-band differential GPS receiver. The equipment was installed and tested on a small pontoon barge powered by an outboard motor in less than a day. Using a streamer containing 9 electrodes spaced at 10-m intervals, 35 line-km of continuous dipole-dipole resistivity (1 = n = 6) data were acquired at approximate intervals of 5 m. The data were acquired in approximately 10 hours (3-5 km/hr) over a period of two 2 days. The resulting resistivity maps and pseudo-section profiles effectively delineate areas where clay is known to be present in the river bottom, detect the presence of culture (e.g., pipes and casing in the river bottom), and provide the basis for siting new water intake installations. The survey demonstrates that resistivity profiling provides a rapid and economical means for the characterization of sediments beneath shallow fresh water.