Phase 1B Underwater Archaeological Report for the Onondaga ...

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FINAL Phase IB Underwater Archaeological Resources Report for Onondaga Lake Superfund Site Sonar data were next inspected to assess the presence of objects on the bottom near each anomaly. When a bottom feature was observed, the position and dimensions of the feature were recorded and a high-resolution image of the feature was captured. These images and measurements were used to compile a Contact database and report for each anomaly. The databases were exported in SHP format for use with ArcGIS software. Sonar resolution is defined as the ability of the sonar system to discriminate between two adjacent objects of a particular size and separation. This resolution decreases with increasing range from the sensor due to signal spreading. The theoretical resolution of the side scan sonar data is determined by swath width (range setting), frequency, beam width (0.3-degrees for this system), ping rate, ping duration, and vessel speed. Data collected using a 1,250-kHz signal and 82 feet (25m) range has a minimum resolution of approximately 0.3937in (1cm) across-track and 2.76in (7cm) along-track depending on the distance from the towfish. At a typical range of 30.5 feet (10 meters) (mid-swath) sonar resolution is estimated as 0.18 inches 2 (3cm 2 ). Full resolution geo-referenced mosaics were provided digitally to facilitate additional characterization of anomalies. The resolution of the processed and georeferenced sonar mosaic was set to 1.2 inches (3.048cm) per pixel. These digital side scan mosaics were imported to ArcGIS software along with the locations of the digitized Contacts. Scaled maps of each anomaly were created in GIS. Scanning Sonar A high-resolution Kongsberg MS1000 675-kHz sector scanning sonar and a Hemisphere VS-100 DPGS and digital compass were used to collect scanning sonar data from each anomaly (Figure 27). The sonar and DGPS were interfaced to a laptop computer running Kongsberg data acquisition software via USB and serial connections, respectively. HYPACK software was used to navigate R/V Lophius to each anomaly. The sonar system was mounted inside a stainless steel cage with the head oriented downward. This mounting arrangement allowed the system to be deployed by lowering the cage to the lakebed or by securing the cage in a fixed orientation to the bow of R/V Lophius (Figure 28). The latter deployment method was preferred for shallow anomalies because it minimized the effects of aquatic vegetation on data quality, optimized beam geometry, and allowed accurate georeferencing of data through interfacing with the Hemisphere digital compass. Several digital scans were made at each anomaly. Preliminary wide-area scans were made at ranges up to 98.4 feet (30m) to confirm the location of the anomaly and to allow precise repositioning of the vessel for close range scans. High resolution scans were conducted using ranges between 16.4 and 49.2 feet (5 and 15m), with resulting data (image) resolutions of approximately 0.3937in (1cm). 39
FINAL Phase IB Underwater Archaeological Resources Report for Onondaga Lake Superfund Site Figure 27. Kongsberg MS1000 sector scanning sonar is stainless steel cage (courtesy CRE). Figure 28. Deployment of scanning sonar using the bow of the boat to secure the cage (LCMM Collection). 40
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