JPEG formatted images of EdgeTech SB-512i and SB-424 chirp seismic-reflection profiles collected by the U.S. Geological Survey offshore of Massachusetts between Duxbury and Hull (DH_SeismicProfiles)

Field activity 06012: The SB-0512i was mounted on a catamaran sled and towed at the sea surface approximately 30 - 40 m astern of the R/V Megan T. Miller. Position data were provided by a Global Positioning System (GPS) navigation receiver mounted on the catamaran, and data were transmitted to the acquisition computer on the vessel via a 2.4 GHz radio link. During some portions of the survey, technical difficulties caused the GPS system mounted on the catamaran to malfunction. In these instances Real Time Kinematic (RTK) GPS navigation was obtained from a receiver mounted above the interferometric sonar head (This applies to files l25f2000 through l84f2000 acquired during Julian days 218 through 222). Layback positions, which account for the linear distance between the shipboard RTK-GPS receiver and the towed catamaran vehicle, were calculated trigonometrically during post-processing (see processing step 3). While the vehicle mounted GPS antenna was operational, positional accuracy is assumed to be ± 10 m. While the vehicle mounted GPS was not operational, positional accuracy is assumed to be ± 20 m; increased uncertainty arises because layback calculations do not account for fish motion behind the vessel, which is caused by sea state and vessel speed induced changes in the angle and scope of the tow cable. Field activity 07001: The SB-424 was mounted on a rigid pole, approximately 1 m 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 SB-424 transducer 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 is assumed to be ± 10 m.