Geophysical Data - Acquisition
The study area covers nearly 700 km2 of the lower shoreface and inner-continental shelf of Long Bay, from just seaward of the breaking waves (> 1 km) to ~ 10 km offshore (Figure 2). High-resolution sidescan-sonar, seismic reflection and interferometric sonar bathymetry were acquired during four USGS cruises: October-November 1999 aboard the R/V Atlantic Surveyor (USGS cruise ATSV99044, Hill and others, 2000a; Hill and others, 2000b; Roberts and others, 2002), March 2000 aboard the R/V Megan Miller (USGS cruise MGNM00014, Dadisman et al, 2001a; Dadisman and others, 2001b; Denny and Schwab, In Prep.), and May 2001 and 2002 aboard the R/V Coastal II (USGS cruises COAS01008 and COAS02011). Data from a fifth offshore cruise, conducted in June 2002 aboard the R/V Atlantic Surveyor (USGS cruise ATSV02014), extend southward between North Inlet and the mouth of Winyah Bay. Analysis of these data will be incorporated into this mapping in a subsequent report.
Acquisition systems for the 1999 and 2000 offshore cruises (ATSV99044 and MGNM00014) included a Datasonics SIS-1000 sidescan-sonar (100-120 kHz swept FM) and CHIRP sub-bottom profiler (2-7 kHz swept FM), a Huntec 300-3000 Hz boomer, and an SEA Ltd. Submetrix 2000 Series interferometric sonar system (234 kHz). Boomer and CHIRP seismic data were acquired digitally during both offshore cruises. During the ATSV99044 cruise, CHIRP data were acquired at a 0.27-s fire interval, a 274-ms sweep length, and a 0.263-ms sampling interval, and boomer data were acquired at a 0.5-s sampling rate, a 180-ms sweep length, and a 0.083-ms sampling interval. During the MGNM00014 cruise, CHIRP data were acquired at a 0.27-s fire interval, a 263-ms sweep length, and a 0.274-ms sampling interval, and boomer data were acquired at a 0.5-s sampling rate, a 250-ms sweep, and a 0.2-ms sampling interval. CHIRP data acquired with the SIS-1000 system were logged in the QMIPS format using Triton Elics International (TEI) ISIS acquisition software. CHIRP data were extracted and converted to single-channel SEG-Y standard format (Barry and others, 1975) using a USGS C program (QMIPSTOSEGY). Boomer data were acquired using TEI Delph Seismic acquisition software and recorded in SEG-Y standard format. Shore parallel lines were spaced at ~ 300 m, and shore perpendicular tie lines were spaced at ~ 2 km to provide adequate cross-shelf control within the seismic reflection data set.
The 2001 and 2002 cruises (COAS01008 and COAS02011) aboard the R/V Coastal II focused upon nearshore portions of the study area, from the inshore extent of the 1999 and 2000 surveys to approximately the seaward edge of the nearshore sand bar (typically < 200 m from the coast). Acquisition equipment used during these cruises included an Edgetech DF-1000 sidescan-sonar (dual frequency 100/500 kHz), an Edgetech SB-512 CHIRP sub-bottom profiler (500 Hz - 12 kHz swept FM), and an SEA Ltd. Submetrix 2000 Series interferometric sonar system. During both nearshore cruises, Edgetech CHIRP data were acquired digitally at a 0.25-s fire rate, a 10-ms pulse length, and a 1 to 5.5 kHz sweep (swept frequency). CHIRP data acquired with the SB-512 system were logged in the Edgetech raw seismic format using an Edgetech X-star acquisition system. These data were converted to single-channel SEG-Y standard format utilizing the SIOSEIS seismic processing package (copyright University of California, written and maintained by Paul Henkart at Scripps Institute of Oceanography). Variable line spacing was used during nearshore cruises to maximize seafloor coverage while navigating shallow waters.
Sidescan-sonar data were acquired digitally at a 0.125 second ping rate, yielding a 400 m swath, during all cruises. Sidescan-sonar data from both the SIS-1000 and DF-1000 systems were logged at a 2K sample rate, to QMIPS format, using the TEI ISIS acquisition software.
The SEA Ltd. Submetrix 2000 Series interferometric sonar was deployed on a side-mount and mounted below a Seatronics TSS DMS2-05 motion reference unit (MRU) during all cruises. The MRU calculates heave, pitch, roll and yaw of the survey vessel. Bathymetric data were acquired at a 0.133 second ping rate and logged at a 2K sample rate using the SEA Ltd. RTS2000 acquisition software. Bathymetric swath width varied as a function of depth, but averaged roughly 10 times water depth within the depth range between 6 to 14 meters.
Coastal Oceanographics HYPACK MAX hydrographic surveying software was used to acquire Differential Global Positioning System (DGPS) data and send a navigation string to each acquisition system. Precise measurements were made prior to survey operations to record the offsets between the navigation antenna and the MRU (the MRU is treated as the reference location for all systems).
During the ATSV99044 and MGNM00014 cruises, slant range distance to the SIS-1000 towfish was recorded using an acoustic ranging system. Using slant range measurements and ship navigation, accurate towfish positions (± 5 m horizontal) were calculated in real-time within HYPACK and incorporated into the navigation string sent to the ISIS system. This calculation assumes that the towfish follows directly behind the vessel, which is relatively accurate when running in a straight line with a small amount of tow cable deployed. However, because the towfish position error increases in turns by tens of meters, data collected during turns were discarded.
During the COAS01008 cruise, the position of the SB-512 tow vehicle was adjusted for layback during post-processing. The instrument was towed at the sea surface a constant 60 m behind the ship. Ship navigation recorded to the file headers were corrected for this 60 m layback during post-processing, providing accurate positioning (± 5 m horizontal) of the tow vehicle. During the COAS02011 cruise, a DGPS antenna and receiver were mounted on the SB-512 tow vehicle. A radio modem system relayed tow vehicle position (± 2 - 3 m horizontal) to the acquisition system aboard the survey vessel and they were recorded to the file headers.