Data Acquisition

Navigation

Navigation data acquisition methods were adapted from previous reports (Forde and others, 2016). Latitude, longitude, and elevation data for each GPR profile, and a few topographic transects not associated with GPR data collection, were constrained by postprocessed differential correction using a Continuous Operating Reference Stations- (CORS) to-rover setup. Field setup consisted of a roving GPS receiver collecting location data (horizontal and vertical) at a frequency of one geographic position per second. The rover unit consisted of an Ashtech Z-Xtreme receiver and a geodetic antenna that were mounted to a backpack and connected to the GPR’s Digital Control Unit (DCU). The height of the GPR antenna was measured and recorded whenever the system was assembled. Raw GPS data positions were transmitted to the GPR as a National Marine Electronics Association (NMEA) Global Positioning System fix data (GGA) string at a 9600 baud rate and a temporal resolution of 1 second.

Ground-Penetrating Radar

GPR data acquisition methods were adapted from previous reports (Forde and others, 2016). A total of 41 GPR transects (fig. 3, 4, 5), representing a linear distance of approximately 5.9 km, were acquired during field activity 2015-611-FA. The GPR data were collected with a Geophysical Survey Systems, Inc. (GSSI) TerraSIRch SIR System-3000. This unit acquires single-channel GPR data and includes a DCU model DC-3000, a 400 Megahertz (MHz) antenna, and associated cables and harness. For calculation of the distance covered by a profile, a 16-inch diameter survey wheel (GSSI, model 620) was attached to the back of the antennas. To calibrate the wheel, an accurate wheel-to-distance revolution ratio was acquired using the following procedure: (1) measurement of a 10-meter (m) long calibration transect on the ground surface followed by entering the calibration distance into the digital control unit, (2) positioning the antenna at the start of the calibration line, and (3) pulling the wheel the measured 10 m to ensure the wheel covered the entire distance. The steps were repeated until reproducible values were acquired and then the values were averaged. The averaged value was entered into the DCU and used as the calibration standard. The GSSI was used with an external GPS Acumen data logger, GPS hardware, harness, and tow strap. Acquisition settings were adjusted when changes to the subsurface geology necessitated adjustment. Settings for each subsurface profile can be found on the Data Downloads page within file 2015-611-FA_GPR_profile_parameters. 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 the Radan file format Data Zero Time (DZT). GPR data were collected along both shore parallel and shore perpendicular lines (fig. 4, 5) based primarily on the location of coring transects.