Data Series 1078
| Data ProcessingNavigationBase station data were post-processed through the National Geodetic Survey (NGS) On-Line Positioning User Service (OPUS). The time-weighted position calculated from all base station occupations did not differ significantly from the NPS control coordinates; therefore, the control coordinates were used for post-processing. The base station coordinates were imported into GrafNav, versions 8.5 and 8.7 (NovAtel Waypoint Product Group), and the data from the rover GPS were post-processed to the concurrent base station session data. The final, differentially corrected, DGPS positions were computed at 1-s intervals for each rover GPS session and exported in American Standard Code for Information Interchange (ASCII) text format as a NMEA GGA string, which replaced the uncorrected real-time rover positions recorded during acquisition. The GPS data were acquired and processed in the World Geodetic System of 1984 (WGS84) (G1150) geodetic datum. Along some GPR profiles that were collected within communities with closely spaced buildings or in coastal forests, the satellite signals were obscured and post-processed GPS positions could not be computed for every epoch. Gaps greater than about 15 m in length were filled by interpolating elevations from National Oceanic and Atmospheric Administration (NOAA) Light Detection and Ranging (lidar) data collected in 2014 (NOAA, 2015). The final navigation files provided in Forde and others (2018) were exported from GrafNav to the North American Datum of 1983 (NAD83) (2011) and North American Vertical Datum of 1988 (NAVD88) and were derived using the GEIOD12A geodetic model. Ground Penetrating RadarReflexw Version 7.2.2 (Sandmeier Scientific Software) 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 ASCII 4-column format. The data were processed in a consistent order: (1) static correction was applied to account for the time delay of the first arrival; (2) the mean value was subtracted (dewowed); (3) header gain applied during acquisition was removed; (4) manual Automatic Gain Control (AGC) gain was applied; and (5) post-processed DGPS data were imported into the trace headers. Data were visually inspected after each step listed above and before elevation-corrected profiles were exported; all profiles were analyzed for errors or data gaps in the navigation and trace data to ensure data quality was maintained throughout. Hyperbola analyses were performed on selected profiles to estimate the radar-wave velocities through the sediment. Radar-wave velocities are inversely related to the dielectric permittivity of the material through which the electromagnetic (EM) wave passes:
where v is the velocity in meters per nanosecond (m/ns), C is the speed of light (0.2998 m/ns), and εr is the dielectric constant of the material (Buynevich and Fitzgerald, 2017). Radar-wave velocities of some sediments commonly found in the study area are listed in table 1. Calculated velocities (N=30; table 2) ranged from 0.06 to 0.2 m/ns and averaged 0.094 m/ns (εr = 10). The processed profile data were re-imported into RADAN 7 and the surface normalization processing algorithm was applied, adjusting the profile to both the measured terrain and the site-specific, averaged radar-wave velocity (table 2). Finally, elevation- and velocity-corrected profiles were exported as Joint Photographic Experts Group (JPEG) images. At sites 1 and 2, the calculated velocities were highly variable, and the profiles were processed using an average velocity of 0.10 m/ns (εr = 9). At site 3, lines 1–11 were processed using an average velocity of 0.08 m/ns (εr = 14) and lines 12–14 were processed using an average velocity of 0.06 m/ns (εr = 25). The raw GPR data; processing parameters, including raw velocities estimated from hyperbola analyses; and elevation-corrected profiles can be downloaded from the Data Downloads page or from the associated data release (Forde and others, 2018). In addition to dielectric permittivity, electric conductivity and (or) magnetic permeability affect the behavior of the EM wave through sediments. Highly conductive materials (for example, saltwater) and highly magnetic materials both attenuate the EM signal, limiting or precluding penetration (Buynevich and Fitzgerald, 2017). GPR data collected from beach environments were highly attenuated, and in most cases no reflectors were distinguishable in these profiles. This attenuation is likely due to the presence of saline pore water and (or) localized concentrations of garnet and magnetite in the beach sediments.
Table 1. Radar-wave velocities (in meters per nanosecond [m/ns]) and corresponding dielectric constants (εr) of some common sediments in the study area (modified from Reynolds, 1997; ASTM, 2011; Robinson and others, 2013). [Radar-wave velocities through air and water are given for comparison.]
Table 2. Summary of radar-wave velocities (in meters per nanosecond [m/ns]) estimated from hyperbola analyses of selected ground-penetrating radar (GPR) profiles collected during this study, and corresponding dielectric constants (εr).
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U.S. Department of the Interior |
U.S. Geological Survey
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Page Last Modified: Wednesday, 21-Feb-2018 08:36:24 EST
