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Fieldwork

Streamer-Resistivity Surveys in Delaware Inland Bays Delineate Submarine Fresh Water


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Towing the array in Assawoman Canal
Towing the array: Streaming in Assawoman Canal (note distant surface floats belonging to the array, arrow).
The first field deployment of a multichannel streamer-resistivity array in salty estuarine waters showed results well beyond expectations. A 140-m-long dipole-dipole system towed behind a small estuarine boat was able to measure resistivities continuously to depths between 10-20 m, where fresh water was present directly under saltier bay waters, and up to 40 m where saltier strata were encountered. The estuarine boat was provided by the USGS Water Resources Division (WRD) Geophysical Research Center, Storrs, CT. The results dramatically confirm previous indications of offshore freshwater discharge obtained by Delaware Geological Survey remote sensing overflights. This work was part of the cooperative USGS (CMGP & WRD)-CISNET (Coastal Intensive Sites Nutrient Evaluation Program) consortium studies of the role of ground water in delivering excess nutrients to Delmarva coastal bays. Delaware Sea Grant provided funding to the University of Delaware to extend the field surveys. One hundred kilometers of survey tracks were obtained. Research participants included Frank Manheim (Chief Scientist-Reston), John Bratton (WHFC), David Krantz (WRD-Dover, DE), Pete Haeni and Eric White (WRD-Storrs), John Madsen (Sea Grant Principal Investigator), Jim Black, (University of Delaware, Newark), and Skip Snyder (Chief of Research for the Zonge Co. of Tucson, AZ, which developed the streamer array and post-processing system).

The crew Skip Snyder with receiving apparatus
The crew: some of the operating crew (Pete Haeni, David Krantz, Skip Snyder, and Eric White)    Receiving gear: Skip Snyder again with receiving apparatus, and GPS-navigation and bathymetric-sounding equipment.

Fig. 1. (below) An inversion model (apparent resistivities converted to a layered depth image) for a section of a profile from Assawoman Canal to White's Creek, Indian River Bay, MA. Throughout this segment, ambient waters were salty. The figure shows shallowing high-resistivity layers, denoting fresh water within the subsea sediments upon entering the canal area. Short cores and interstitial water analysis confirmed the presence of fresh water at shallow levels in the subsea strata.
Resistivity plot

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