Delaware River near Wilmington Floating Electromagnetic Surveys from August 2020
Dates
Publication Date
2021-01-22
Start Date
2020-08-26
End Date
2020-08-27
Citation
Rey, D.M., Briggs, M.A., Terry, N.C., and White, E.A., 2021, Delaware River near Wilmington Floating Electromagnetic Surveys: U.S. Geological Survey data release, https://doi.org/10.5066/P9BL9T50.
Summary
Note: this data release has been depecrated. Find the updated version here: https://doi.org/10.5066/P9X8RUBW. Electromagnetic (EM) geophysical methods provide information about the bulk electrical conductivity of the subsurface. EM data has been widely used to investigate aquifers and geologic structures. In the following study, the United States Geological Survey conducted a boat-towed, waterborne transient electromagnetic (FloaTEM) survey to examine conductivity within the subsurface of the Delaware River channel. These conductive zones determine the location of the groundwater freshwater/saltwater interface within the Delaware River, downstream from Wilmington, DE. The FloaTEM system transmits a primary electrical current through [...]
Electromagnetic (EM) geophysical methods provide information about the bulk electrical conductivity of the subsurface. EM data has been widely used to investigate aquifers and geologic structures. In the following study, the United States Geological Survey conducted a boat-towed, waterborne transient electromagnetic (FloaTEM) survey to examine conductivity within the subsurface of the Delaware River channel. These conductive zones determine the location of the groundwater freshwater/saltwater interface within the Delaware River, downstream from Wilmington, DE. The FloaTEM system transmits a primary electrical current through a transmitter loop (Tx) wire. This creates a static primary magnetic field. Then, the current in the TX loop is subsequently turned off, resulting in secondary electrical currents being induced in the earth. These induced electrical currents decay with time, and this rate of decay in the secondary electrical field is a function of the bulk conductivity of the subsurface material. As the secondary electrical field decays, a secondary magnetic field is induced and measured at a receiver (Rx) loop towed behind the Tx loop. The Rx loop measures the decay of the secondary magnetic field as a function of time (dB/dt). Measured dB/dt decay curves can be inverted to recover the depth-dependent resistivity structure of the earth. FloaTEM surveys were conducted downstream from Wilmington, DE on 8/26/2020 and 8/27/2020. Data on 2/26/2020 were collected around the Augustine Wildlife Area boat ramp, and data on 8/27/2020 were collected near the Collins Landing boat ramp.This data release includes the averaged, culled and inverted TEM data showing resistivity (in ohm-meters) with depth for each of the survey sites.
(1) Files with *AVERAGED.csv are comma-delimited ASCII files that contain the least processed data where transients were averaged together and most coupled data were removed. Data were subsequenty manjually inspected for quality. Data locations are provided as UTM Zone 15 N projection and datum of WGS-84.
(2) Files with *INVERTED.csv are comma-delimited ASCII files containing the inversion model results. Model locations are provided as UTM Zone 15 N projection and datum of WGS-84.
(3) Files with *depth_conductivity.csv are comma-delimited ASCII files containing the water-depth and conductivity data as measured from the bottom of the transducer. Water-depth and conductivity measurement locations are provided as UTM Zone 15 N, WGS-84.
Click on title to download individual files attached to this item.
DE_FTEM_final.xml Original FGDC Metadata
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11.07 KB
application/fgdc+xml
Purpose
Saltwater is more electrically conductive than freshwater, making EM methods a useful tool for mapping the spatial extent of freshwater/saltwater interfaces. Little information is available about the spatial extent of the freshwater / saltwater groundwater interface near the mouth of the Delaware River. Consequently, FloaTEM surveys were conducted to characterize this interface.
