Raw and Processed Ground-Penetrating Radar and Postprocessed Differential Global Positioning System Data Collected from Assateague Island, Maryland, October 2014

Data Acquisition

Navigation

Latitude, longitude, and elevation data for each GPR profile were constrained by postprocessed differential corrections using a base station to rover setup. Field setup consisted of a rover and base stations collecting GPS data at a frequency of one capture per second. Thales choke-ring antennas, tripods, and Ashtech Z-Xtreme receivers were set up on National Park Service (NPS) control sites located within 6 km of the study areas. The rover unit used a similar configuration: an Ashtech Z-Xtreme receiver and geodetic antennas mounted on a backpack and connected to the GPR’s Digital Control Unit. The rover antennas height above the ground and the horizontal distance between the rover antennas and GPR antennas/unit were measured and recorded daily. Raw GPS data positions were output to the GPR Digital Control Unit as a National Marine Electronics Association (NMEA) GGA string at a 9600 baud rate and a temporal resolution of one capture per second.

Ground-Penetrating Radar

A total of 44 GPR lines (figs. 3, 4), representing a linear distance of 10.5 km, were acquired during field activity 2014-322-FA. The GPR data were collected with a Geophysical Survey Systems, Inc. (GSSI) TerraSIRch SIR System-3000, which acquires single-channel GPR data and includes a Digital Control Unit (model DC-3000), 200 Megahertz (MHz) and 400 MHz antennas, and the associated cables and harness. For calculating the distance covered by a profile, a 16-inch survey wheel (model 620) was attached to the back of the antennas in use. To calibrate the wheel, an accurate revolution of wheel-to-distance ratio was acquired using a specific procedure: (1) measurement of a 10-meter (m) long calibration line on the ground surface and entering the calibration distance into the Digital Control Unit, (2) positioning the antennas at the start of the calibration line, (3) pulling the wheel the measured 10 m and ensuring that the wheel covered the entire distance. These steps were repeated until a set of reproducible values were created and averaged. The resultant average was entered into the digital control unit and used for calibration. The GSSI was used with an external GPS Acumen data logger, GPS hardware, harness, and tow strap. Acquisition settings were adjusted when necessitated by changes to the subsurface geology or when the antennas (200 Mhz or 400 Mhz) were switched; settings for each subsurface profile can be found at “GPR profile parameters” on the Data Downloads page. An Infinite Impulse Response (IIR) filter was used (Lowpass = 600 MHz and Highpass = 50 MHz) to increase the signal-to-noise ratio of the recorded GPR data. Data were acquired using RADAN 7 proprietary software (in TerraSIRch mode) and saved in Data Zero Time (DZT) format.