|
Streamer-Resistivity Surveys in Delaware Inland Bays Delineate Submarine Fresh Water
 |
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: 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.
 |
|
 |
July 2000
in this issue:
Snake Pond
Delmarva Inland Bays
Barataria Barrier-Shoreline
OBS Recovery
Lake Mead
Joint USGS/BRD/WRD Studies
SeaMap Program
Coastal Erosion
Woods Hole Field Center
cover story:
Marine Ecology Workshop
Southern California Academy of Sciences
Penrose Conference
Visitors
July Publications List
|
U. S. Department of the Interior | U.S. Geological Survey
previous story