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

Data Processing

Navigation

Base station data were postprocessed through the National Geodetic Survey (NGS) On-Line Positioning User Service (OPUS), and time-weighted average positions were calculated. The time-weighted average positions were compared with the NPS network control numbers for each base station; the time-weighted elevations for each base station were within 2 standard deviations of the NPS control elevations. The NPS control coordinates were used for subsequent post-processing.

The base station coordinates were imported into GrafNav, version 8.4 (NovAtel Waypoint Product Group), and the data from the rover GPS (the GPS unit used to collect data at each sample site) were postprocessed to the concurrent GPS session data at the nearest base station. The position data were then exported as a GGA string in ASCII format and applied to the GPR data, replacing the GPS data collected during acquisition. The processed navigation data can be downloaded from the Data Downloads page.

Ground Penetrating Radar

Sandmeier Scientific Software’s Reflexw Version 7.2.2 geophysical near surface processing and interpretation software was used to process the GPR data. GPR data were acquired in Radan’s DZT format and later imported into Reflexw, where they were converted into a DAT file. For archival purposes, a non-proprietary version of the raw data was created by exporting the DAT file from Reflexw and saving it in American Standard Code for Information Interchange (ASCII) format. The processed GPR profiles were used to produce the JPEG images available for download in this report. The data were processed in a consistent order: (1) static correction was applied, (2) the mean value was subtracted (dewowed), (3) header gain removed, (4) manual Automatic Gain Control (AGC) gain applied, and (5) postprocessed DGPS data were imported into the trace headers to produce the elevation corrected profile. Data were visually inspected after each step in the process listed above and before Joint Photographic Experts Group (JPEG) images were exported; all profiles were analyzed for errors in navigation or trace data. A velocity of 0.06 meters per nanosecond (m/ns), a commonly used velocity for saturated sand (Seminack and Buynevich, 2013), was assumed and applied to all elevation-corrected profiles to display depth. The raw GPR data and elevation-corrected profiles can be downloaded from the Data Downloads page.