The Reson 8125 multibeam sonar was the primary survey tool for investigating the high-priority targets selected by the NHC. The Reson 8125 is the first of a new generation of dynamically focused multibeam sonars. Operating at 455 kHz, the system forms 240, 0.5° x 1.0° beams over a 120° swath. The very narrow beams are achieved by the use of a relatively long array (0.5 m), which, in a standard multibeam sonar, would preclude working at short ranges from the transducer face (in the near field). The 8125, however, uses dynamically focused beam-forming that allows for operation of the sonar in the near field. Thus very high frequencies and short pulse lengths can be used (providing excellent vertical resolution) while maintaining very narrow beam widths (providing excellent lateral resolution). In addition, the focused beam-forming process reduces the energy levels associated with side lobes, making for an inherently cleaner return and, particularly important for archaeological surveys, the ability to coherently track features in the complex environment of wreckage.
The 8125 was deployed on a custom-built pole mounted to the starboard side of the M/V Genesis, an 11.3-meter catamaran chartered by the NHC from Tech Marine Service, out of Great Yarmouth, UK (Figs. 4a and b). The Genesis is very stable and has the ability to transit at speeds of 20 - 30 knots (surveys were conducted at 3 - 6 knots), making it an extremely efficient platform for operations. A TSS DMS2-05 dynamic motion sensor monitored vessel motion, and a TSS Meridian gyrocompass measured heading. A Trimble AG132 differential GPS receiver using Fugro SeaSTAR corrections and local RTCM, determined vessel position with sub-meter accuracy. The Reson SVP-C sensor continuously monitored sound speed at the transducer head and a Reson SVP-14 sound speed profiler established velocity profiles. Offsets between all of the sensors were carefully measured and standard hydrographic patch test and calibration procedures followed to minimize any integration or alignment errors.
Sound speed profiles were taken before the start of every new survey, at intervals of no longer than three hours, and whenever real-time quality control (QC) indicated a potential problem. For the most part, the water-column structure remained remarkably constant through the seven days of survey work. The tidal range off Normandy is large (on the order of 6 m) and tidal corrections are critical. The Service Hydrographique et Oceanographique de la Marine (SHOM) of France, kindly provided predicted tide models for each of our work areas.
Since tides controlled entry into port, accurate weather data provided a critical link between sea state, personnel safety and the ability to collect quality data. The U.S. Naval European Meteorology and Oceanography Center (NEMOC) in Rota, Spain, provided 48-hour regional weather forecasts and emergency fax broadcasts. The harbormaster at the port authority of Grandcamp-Maisy posted a local 24-hour forecast and alerted the Genesis crew of any hazardous conditions reported by the fishing fleet. A Reson 6042 data acquisition system was used to digitally acquire data, integrate data from the ancillary sensors, and to store raw data files. Data from the 6042 were exported into a Triton Extended Format (XTF) for further processing and cleaning with CARIS HIPS, which produced cleaned and gridded data. Interactive Visualization Systems' Fledermaus software was used to produce interactive 3-D visualizations of the targets and their surrounding environment. Depending on the complexity of the data set collected, the turn-around time from data acquisition to interactive 3-D viewing ranged from minutes to several hours.