• Streamer Resistivity Surveys In Delmarva Coastal Bays

  • Application of Continuous Resistivity Profiling to Aquifer Characterization

  • Continuous Resistivity profiling in shallow marine and fresh water environments

  • River sediment salt-load detection using a water-borne transient electromagnetic system

  • Instream NanoTEM: providing increased resolution to stream salinisation and floodplain processes alo

Streamer Resistivity Surveys In Delmarva Coastal Bays

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This paper reports on streaming resistivity ("DC resistivity") surveys conducted in Maryland and Virginia Atlantic coastal bays in the spring of 2001. Surveys in Assawoman, Isle of Wight, and Chincoteague Bays, MD and VA, were used to study profiles of electrical resistivity of submarine strata to delineate submarine freshwater discharge and submarine saltwater interfaces and salinity distributions in submarine groundwater. The studies follow similar resistivity surveys in Rehoboth and Indian River Bays in spring of 2000 (Krantz and others, 2000; Madsen and others, 2001; and Manheim and others, 2001).

The Delmarva Peninsula coastal studies are part of larger cooperative programs between the U.S. Geological Survey, regional federal and state organizations, and academic institutions. They address the problem of excess nutrient discharge into Delmarva coastal bays. Like the Delaware coastal bays, Maryland and Virginia coastal bays receive excess nutrients due to human activities. The excess nutrients enhance growth of phytoplankton and fouling macroalgae, which impedes boat operation, coats beaches, and lays down organic-rich mats. This organic matter fosters anoxic conditions in the bottom sediments. Growing stagnation alters the habitat for benthic organisms and reduces biological diversity. Recent studies suggest that excessive organic growth inhibits natural mechanisms (like denitrification) that help transform and remove nutrients from the bay systems.

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.

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.

River sediment salt-load detection using a water-borne transient electromagnetic system

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The salinisation of major river systems in Australia, and in other countries, is primarily determined by the upward movement of saline water from regional aquifers into the river. The migration has been accelerated due to irrigation schemes and farming practices that have changed regional hydraulic gradients driving saline-water in aquifers towards the major drainage points in the landscape. In this paper, we describe results from a transient electromagnetic (TEM) system that has been deployed to monitor the influx of saline water through sub-riverbed sediments. The deployment was a floating arrangement of a commercial fast sampling (high resolution) TEM system that is sensitive to shallow (b50 m depth) resistivity variations.

The technique has been extensively trialed around the River Murray town of Waikerie in South Australia. An initial series of surveys along a 40 km section of river showed a range of sub-riverbed resistivities between 1 and 20 ohm-m, with a top layer of about 10-15 ohm-m closely following the water depth. Regions of high-resistivity in the riverbed sediments correlated well with saline-aquifer borehole pumping locations, indicating a localized drawdown of fresher river water. Low-resistivity anomalies have been interpreted as regions of saline water influx into the river. The technique is now used for routine mapping in Australia, with over 800 km of the Murray surveyed, and has potential application to other major world river systems.

Instream NanoTEM: providing increased resolution to stream salinisation and floodplain processes along the river Murray, southeast Australia

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Increasing salinity in the River Murray is well documented and is of concern environmentally, economically and socially. The Murray Darling Basin Commission and the Mallee Catchment Management Authority engaged the authors to collect base-line in-stream NanoTEM data within the River Murray from Lock 1 to Mallee Cliffs (675 km). This is a new application of a high resolution fast sampling Transient Electro-Magnetic (TEM) system, towed behind a boat, taking soundings every seven to ten metres along the river. The observed NanoTEM response was interpreted against the current understanding of the regional hydrogeology and groundwater processes in and around the river. This paper summarises some of the results from this investigation. The observed response correlates strongly with previously mapped major changes in underlying lithostratigraphy along the Murray River, and with gaining and losing reaches of the river. The extensive length of the survey provides an insight into potential interactions between the river, floodplain and groundwater, but does not replace the need for focussed ground-truthing programs to examine specific correlations. This rapid, portable technique should be applicable outside the Murray-Darling Basin as well as at additional locations within the Basin.

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.