Ground Penetrating Radar Profiles collected in Charleston, SC, in June 2015 for imaging shallow faults

This Data Release contains a series of reconnaissance Ground-Penetrating Radar (GPR) profiles collected in 2015 to image potential shallow faulting related to the 1886 M~7 earthquake near Charleston, South Carolina, and any other faults in the area. The data include coordinates for the profiles as .kmz files, and processed GPR data in SEGY format, with both the unmigrated and migrated profiles. The raw data, totalling about six gbytes, are archived as .zip files containing the coordinate files (.cor) and the data in Mala format (.rd3). We also archive the merged and stacked profiles in which the single-channel traces (single source and receiver) were combined (stacked) into 1-m distance bins starting at the beginning of each profile (i.e. all traces from 0 to 1 m; all traces from 1 to 2 m; all traces from 2 to 3 m; etc.). The profile was then migrated and depth converted using a constant velocity that set the prominent reflector at the top of the Cooper Group (Cooper "marl") to the approximate depth determined from drill holes.

All profiles were collected with a Mala GPR system with an unshielded 25 mHz rugged terrain antenna attached to a ProEx control unit. The antenna was a 13 m long cable with the transmitter and receiver aligned end-to-end and separated by 6 m. The antenna was towed along the shoulders of roads behind either a person walking with the acquisition system on a backpack, or behind a slow-moving vehicle driving at about walking speed along the road shoulder with the acquisition system in the back of the vehicle. For the Ashley River profile, the streamer was towed in a waterproof sleeve behind a small boat.
 
The profiles and acquisition directions, shown on the map, are:

6/26/2015
Shipyard Lane (southeast to northwest) - profile segments 248, 253

6/27/2015
Railroad tracks near Rantowles (west to east) - profile segments 267, 273, 277, 280, 281
Sandy Lane near Rantowles (2 profiles; west to east and then east to west) - profile segments 259, 261

6/28/2015
Highway 17 near Rantowles (east to west) - profile segments  10, 11
Ashley River Road (just south of the Ashley River) - Part 1 (east part; collected southeast to northwest) - profile segments 17, 19 (renamed 12, 13)

6/29/2015
Ashley River (from a boat; collected northwest to southeast going downriver) - profile segments 24, 2 (in order from northwest to southeast)

6/30/2015
Highway 176 north of Summerville (southeast to northwest) - profile segments 1, 3, 4
Ashley River Road (just south of the Ashley River) - Part 2 (west part; collected northwest to southeast) - profile segments 2, 3, 9, 7, 6, 4 
      (these segments are listed in sequence from southeast to northwest; but each profile was collected from northwest to southeast [reverse order from above])

The .kmz file can be used to see the line locations on Google Earth, as shown in the image on this site. Each profile was collected as a series of line segments because we sometimes needed to stop to clear disk space or deal with logistics such as traffic. The segments need to be merged to make a continuous profile, which we have done in the set of merged files. On Google Earth (the .kmz file), each line segment is colored differently to show the individual segments, and clicking on the line will give the name of the segment. 
 
The final profiles are as much as 27 km in length after merging the segments. The profile segments were generally collected in a consistent direction, with the exception of Ashley River Road in which the western segments were collected in the opposite direction (W to E) as the eastern segments (E to W). Sandy Lane was collected twice, once in each direction (W to E [line 1], and then E to W [line 2]), but the latter is flipped in the processing so that the images are in the same direction.

Acquisition parameters varied between lines, but generally traces were collected on a timed increment with about 20 traces per second collected in Mala GPR format. All data were converted during processing to Society of Exploration Geophysicist’s SEGY format, and most are summed in 1-m bins to produce 1 trace per m of profile. For the profiles with GPS coordinates, we summed the traces into 1-m bins using the coordinates within the trace headers.  For profiles without GPS coordinates (we sometimes had trouble getting the GPS receiver integrated into the recording system), we picked the endpoints of the profile to compute a total length, and then divided that length by the total number of traces to determine the average number of traces per m. We then summed the appropriate number of traces to produce an average 1 m trace spacing. Some profile segments are thus linearly interpolated assuming a constant acquisition speed, but locations of obvious features such as bridges show that the interpolation was fairly accurate (i.e. the acquistion speed for a profile was fairly constant).

Processing consisted of removing the average trace value to remove the direct (horizontally-traveling) source pulse, trace stacking, bandpass filter, deconvolution, migration and depth conversion. For the migration and depth conversion, we used velocities that collapsed diffractions or, more commonly, set the prominent reflector marking top of Oligicene Cooper Group (‘Cooper Marl’) to match the average depth of the top of that unit described in nearby drillholes (Weems et al., 1987 and 2014). The velocities used for migration and depth conversion were consistent with GPR velocities in wet or dry sands (0.06 to 0.120 m/ns).  All data were processed using Colorado School of Mines’ Seismic Unix (SU) software, and the processing scripts with parameters are included in this data release. Note that in the processing scripts the time sample (dt) on the data is different than assumed by the seismic processing software (microsec versus millisec), so the filter parameters are not the true frequencies and the velocities are a factor of 1000 greater then the true velocities (e.g. 70 instead of 0.07).

Most of these GPR profiles are displayed and interpreted in the publication:
Pratt, T.L., Shah, A.K., Counts, R.C., Horton, J.W., Jr., and Chapman, M.C., Shallow Faulting and Folding in the Epicentral Area of the 1886 Charleston, South Carolina Earthquake, in review at the Bulletin of the Seismological Society of America, 2021.

The images contained in the zipped "profile_images" directory correspond with the figures in the above paper.

Acknowledgments:
We thank Scott Harris of the College of Charleston for helping us collect the marine profile along the Ashley River. We thank reviewers Eric Jones and William Stephenson, both of the USGS.
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The Seismic Unix software is open-source, and can be downloaded from: https://wiki.seismic-unix.org/doku.php.

References:
Weems, R.E., E.M. Lemon, Jr., G.S. Gohn, and B.B. Houser (1987). Detailed sections from auger holes and outcrops in the Clubhouse Crossroads, Johns Island, Osborn, and Ravenel quadrangles, South Carolina, U.S. Geol. Surv. Open-File Rept. 87–661, 159 p.

Weems, R.E., W.C. Lewis, and E.M. Lemon, Jr. (2014). Surficial geologic map of the Charleston, region, Berkeley, Charleston, Colleton, Dorchester, and Georgetown Counties, South Carolina, U.S. Geol. Surv. Open-File Rept. 2013-1030, scale 1:100,000, 1 sheet, https://doi.org/10.3133/ofr20131030