More information about this field activity and the data collected can be found at the Field Activity Web Page (<http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2011-005-FA>)
Boomer seismic reflection data collected during USGS Woods Hole Coastal and Marine Science Center field activity 1996-040-FA were also used to aid in the interpretation of stratigraphic units and unconformities (Lines 7, 13, 19, 22, 24, 25, 35, 36 and 79). Distance between lines ranged from 500 meters to 4 kilometers. Nominal resolution of the boomer system is approximately 1 meter. (<http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=1996-040-FA>)
Sparker seismic reflection data collected during USGS Woods Hole Coastal and Marine Science Center field activity 1997-011-FA were also used to aid in the interpretation of stratigraphic units and unconformities (Lines l42f1, l45f1, l57f1, l60f1, l63f1, l66f1, l72f1, l74f1 - l81f1). Distance between lines ranged from 1 to 4 kilometers. Nominal resolution of the sparker system is approximately 1 meter. (<http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=1997-011-FA>)
Large areas within the survey area have few to no direct picks of the coastal plain unconformity because the surface was not imaged with the seismic systems (chirp (2011), boomer (1996) and sparker (1997)). These areas were interpolated in order to produce a continuous surface. Thus, overall vertical accuracy is assumed to be on the order 10s of meters.
Boomer seismic reflection data collected during USGS Woods Hole Coastal and Marine Science Center field activity 1996-040-FA were also used to aid in the interpretation of stratigraphic units and unconformities (Lines 7, 13, 19, 22, 24, 25, 35, 36 and 79). (<http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=1996-040-FA>)
Sparker seismic reflection data collected during USGS Woods Hole Coastal and Marine Science Center field activity 1997-011-FA were also used to aid in the interpretation of stratigraphic units and unconformities (Lines l42f1, l45f1, l57f1, l60f1, l63f1, l66f1, l72f1, l74f1 - l81f1). (<http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=1997-011-FA>)
Survey: Survey lines were run at an average speed of 5 knots. Lines 14 through 27 were run at a 75-m line spacing to achieve full coverage of the seafloor with sonar systems in a priority area of interest in the nearshore, in water depths less than 15 meters. Lines 28 through 92 were run at a 150-m line spacing, with the exception of tie lines (lines 52 through 60, 65 through 67, 89 through 92), which were run at approximately a 2-km line spacing.
Seismic Data: 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 M/V Scarlett Isabella. Chesapeake Technologies' SonarWiz (v.5.03.0016) seismic acquisition software was used to control the Geo-Star topside unit, digitally log trace data in the SEG-Y Rev. 1 format (IEEE floating point), and record DGPS navigation coordinates to the SEG-Y trace headers (in arc seconds of Latitude and Longitude, multiplied by a scalar of 100). 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 200 ms (4340 samples/trace and .000046-s sample interval).
Wayne E. Baldwin performed this and all subsequent process steps.
During the initial loop through the script: 1) Easting and northing coordinates (UTM Zone 18, 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 DGPS receiver (51.5m lines l14f1- l74f1; 43.2m lines l75f1 - ll92f1), 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 first 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.