More information about the individual USGS surveys conducted as part of the northern Cape Cod Bay project can be found on the Woods Hole Coastal and Marine Science Center Field Activity webpage:
06012: <http://quashnet.er.usgs.gov/data/2006/06012/> 07001: <http://quashnet.er.usgs.gov/data/2007/07001/> 07002: <http://quashnet.er.usgs.gov/data/2007/07002/> 07003: <http://quashnet.er.usgs.gov/data/2007/07003/> 08002: <http://quashnet.er.usgs.gov/data/2008/08002/>
Field activities 07001 and 08002: The SB-424 towfish (07001) and Chirp 3200 transducers (08002) were mounted on a rigid pole, approximately 1 and 0.5 m, respectively, below the sea surface on the starboard side of the R/V Rafael. Position data were provided by a RTK-GPS navigation receiver mounted directly above the interferometric sonar head. While horizontal offsets between the seismic-reflection sources and the RTK-GPS navigation receiver were not corrected in the sonar setup, this distance was less than 3 m, and the resulting positional accuracy during each field activity is assumed to be ± 10 m.
Seismic-reflection data acquisition 07001: Chirp seismic data were collected using an EdgeTech Geo-Star FSSB sub-bottom profiling system and an SB-424 towfish (4-24 kHz), which was mounted on a rigid pole on the starboard side of the R/V Rafael of Woods Hole, MA. EdgeTech J-Star and Triton Imaging Inc. SB-Logger seismic acquisition software was used to control the Geo-Star topside unit, digitally log trace data in EdgeTech JSF and SEG-Y Rev. 1 formats, respectively, and record GPS navigation coordinates to the JSF or SEG-Y trace headers. Data were acquired using a 0.25-s shot rate, a 10-ms pulse length, and a 4 to 16 kHz frequency sweep. Recorded trace lengths were approximately 250 ms. Northwest-southeast oriented tracklines were spaced between 75 and 100 m apart.
Seismic-reflection data acquisition 07002: Chirp seismic data were collected using an EdgeTech Geo-Star FSSB sub-bottom profiling system and an SB-0512i towfish (0.5-12 kHz), which was mounted on a catamaran and towed astern of the R/V Megan T. Miller of Port Jefferson, NY. EdgeTech J-Star seismic acquisition software was used to control the Geo-Star topside unit, digitally log trace data in the EdgeTech JSF format, and record GPS navigation coordinates to the JSF trace headers. Data were acquired using a 0.25-s shot rate, a 5-ms pulse length, and a 0.5 to 8 kHz frequency sweep. Recorded trace lengths were approximately 250 ms. Northwest-southeast oriented tracklines were spaced between 100 m and 1 km apart, while northeast-southwest oriented tracklines were spaced between 1 and 3.5 km apart.
Seismic-reflection data acquisition 08002: Chirp seismic data were collected using a dual frequency (3.5 and 200 kHz) Knudsen Engineering Limited Chirp 3200 system with transducers mounted on a rigid pole on the starboard side of the R/V Rafael of Woods Hole, MA. Knudsen SounderSuite seismic acquisition software was used to control the Chirp 3200 system, digitally log trace data in the SEG-Y Rev. 1 format, and record GPS navigation coordinates to the SEG-Y trace headers. Data were acquired using a 0.25 or 0.5-s shot rate, a 1 or 2-ms pulse length, and a peak frequency of 3.5 kHz. Recorded trace lengths were approximately 67 and 135 ms. Northwest-southeast oriented tracklines were spaced between 60 and 150 m apart.
During the initial loop through the script: 1) Easting and northing coordinates (UTM Zone 19, WGS84) for the first five traces of input navigation were read and easting and northing differentials between the consecutive positions were calculated; 2) The signs (+/-) of the differential values were compared to a look-up table to determine the appropriate conversion of the arc tangent (atan2(dy,dx)) angle between consecutive positions to a polar azimuth; 3) The average of the polar azimuths was calculated; 4) The sine and cosine of the average azimuth was calculated and multiplied by the linear distance between the catamaran and the shipboard RTK-GPS receiver, providing absolute values for easting and northing offsets, respectively; 5) A look-up table was used to determine the quadrant of the average azimuth and appropriately add or subtract the calculated offsets to the easting and northing coordinates of the first three input traces, producing final layback positions for those traces; 6) Layback and original easting and northing coordinates for the three adjusted traces were printed to a new layback navigation file that also retained additional attributes input records; and 7) Easting and northing coordinates of the fourth and fifth traces, the three azimuths computed between traces two, three, four, and five, and the average azimuth were held as input for calculations conducted in the subsequent loop.
During subsequent loops through the script: 1) Easting and northing coordinates for three additional traces from input navigation were read, and easting and northing differentials were calculated between the consecutive positions, including the last trace position held from the previous loop; 2) Three new polar azimuths were calculated using the differential values, then a new average azimuth was calculated from the three that were held, the new three, and the average held from the previous loop (the previously calculated average was factored into the new average to smooth "kinks" along the layback navigation that can result from significantly different average azimuths calculated from one loop to the next); 3) New layback offset values were computed, and applied to the easting and northing coordinates of the last two traces input during the previous loop, and the fist trace input during the present loop. 4) Layback and original easting and northing coordinates for the three adjusted traces were appended to the layback navigation file started in the previous loop; and 5) Easting and northing coordinates of the second and third traces, the three new azimuths, and the average azimuth from the present loop were held as input for calculations conducted in the subsequent loop.
Near the end of the input navigation file: 1) If less than three traces were present during a new loop, the layback offsets calculated during the previous loop were applied to remaining trace coordinates; 2) Layback and original easting and northing coordinates for the remaining adjusted traces were appended to the layback navigation file; and 3) The script reached its end, closed, and saved the layback navigation file.
In this fashion, the script approximated a moving window, in which the average of six trace to trace azimuths was used to calculate layback offsets for three central trace positions. Exceptions were at the start of a file, where the first three input trace positions were adjusted using offsets calculated from the average of only four azimuths, and possibly at the end of a file, where remaining traces may have been adjusted using the offsets calculated during the previous loop.