During October-November 2017 twelve time domain electromagnetic (TDEM) surveys were collected in Nassau and Queens County. The data were collected to estimate the extent of saltwater intrusion in support of the Long Island Groundwater Sustainability Project in cooperation with the New York State Department of Environmental Conservation.
In the TDEM method, an electrical current is run through a wire in a transmitter loop, which in turn produces a magnetic field. The strength of the magnetic field is proportional to the strength of the electrical current. The current is rapidly shut off and due to Faraday’s Law, a secondary current is induced. The strength of the secondary current is proportional to the rate of change (dB/dt) of the magnetic field. These induced currents travel into the subsurface and produce secondary magnetic fields that induce voltages in the central receiver coils. Conductive bodies in the subsurface create higher induced voltages in the receiver coils (Fitterman, 1986). The magnitude and decay of the voltages measured in the receiver coil are used to model a 1D sounding of resistivity with depth.
TDEM data were collected using the ABEM WalkTEM acquisition unit. Depending on the size of the site, one of the following transmitter loop geometries were used: 1) 40 by 40 meter square, or 2)100 by 100 meter square. A central induction array was used where two receiver loops were set up within the transmitter loop: 1)A 0.5 by 0.5 meter square, and 2) a 10 by 10 meter square. Due to the quantity of loops in each receiver coil, the effective areas were 35 square meters and 1400 square meters, respectively. Two to three surveys were conducted at each site using various time-gate scripts in the WalkTEM unit and the data were filtered, stacked 10 times, and averaged.
This data release includes XML metadata, a site location map, the raw and processed TDEM data, and the inverted soundings showing resistivity (in ohm-m) with depth at the 12 survey sites in Nassau and Queens County.
REFERENCES CITED: Fitterman, D.V., and Stewart, M.T., 1986, Transient electromagnetic sounding for groundwater: Geophysics, v. 51, p. 995-1005.