Archive of Ground Penetrating Radar Data Collected During USGS Field Activity 13BIM01: Dauphin Island, Alabama, April 2013

Data Processing

Navigation

The coordinate values for the GNSS base station were checked using the time-weighted average of values obtained from the NGS On-Line Positioning User Service (OPUS). The average of the coordinates from all the occupations collected at the base during the survey did not differ significantly from the NGS published coordinates; therefore, the published coordinates were used for post-processing.  The kinematic GNSS data from the GPR were post-processed using the concurrent GNSS data from the base station. The raw base station and rover data were imported into NovAtel GrafNav version 8.4 software and were post-processed by differential correction.  During processing, steps were taken to ensure that the trajectories between the base and rover were clean, resulting in fixed positions. By analyzing the graphs, trajectory maps, and processing logs that GrafNav produces for each GNSS session, data from satellites flagged by the program as having poor health or satellite time segments that had cycle slips could be excluded, or the satellite elevation mask angle could be adjusted to improve the position solutions. The final, differentially corrected, precise DGPS positions were computed at 1-s intervals for each roving GNSS session and exported in American Standard Code for Information Interchange (ASCII) text format (as a GGA string), which replaced the uncorrected real-time rover positions recorded during acquisition. The GPS data were acquired, processed, and exported in the World Geodetic System of 1984 (WGS84) (G1150) geodetic datum. During acquisition of lines 66–116, the signal between the GPS rover unit and multiple satellites was interrupted or lost, most likely due to dense tree canopy within the survey area. This lack of communication with the satellites caused either incorrect values to be recorded or missing/incomplete GPS data to be collected. For lines missing post-processed GPS and (or) elevation data, elevation and location data were extracted from a topographic light detection and ranging (lidar) survey, which was contracted by the USGS in July 2013 (Guy and Plant, 2014) and used instead. In cases where the DGPS data were incomplete or corrupted and lidar data were unavailable, non-elevation corrected profiles have been provided.  Lines with post-processed GPS and elevation data issues are noted in the associated metadata files.

Ground Penetrating Radar

The GPR data were processed using Sandmeier Scientific Software’s Reflexw version 7.2.2 geophysical near surface processing and interpretation package. Each GPR data file was imported into Reflexw, where it was converted from Radan’s DZT (.dzt) format to a .dat file. Processing steps used during post- processing included applying static correction, subtracting mean (dewow), removing header gain, and applying manual Automatic Gain Control (AGC) gain. After these initial processing steps, the position (either post-processed DGPS or lidar-derived coordinates) and elevation data were integrated into the trace headers. Lastly, all profiles were inspected for data quality to ensure no navigation or trace data gaps were present before being output in ASCII format for the trace data and Joint Photographic Experts Group (JPEG) format for the profile images. If any issues were discovered during the quality assurance/quality control (QA/QC) process, the information was noted for each affected line and recorded in the metadata included in this report. A velocity of 0.06 meters per nanosecond was used to display depths on the elevation-corrected profiles.