Sea Technology

JUN 2018

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16 ST | June 2018 survey lines were needed to map 10.6 hectares with an average depth of 0.8 m. The average height above the bottom at nadir was 0.73 m and ranged from 0.10 to 4.9 m. The sonar performed well at these shallow depths. Typical swath widths of side scan imagery were 40 and 24 m for the 550 and 1,600 kHz, respectively, with approximately 90 percent of that swath width being us- able. At 0.10 m above the bottom, approximately 90 to 95 percent of the 24-m swath from the 1,600-kHz frequency was usable. The changes in salinity in the Her- ring River estuary mentioned above did not render the data unusable or noticeably degraded. Some survey lines were collected from the head of the river to the mouth, and changes in salinity were not detectable based on visual inspection of the side scan im- agery or bathymetric data. With regard to bathymetric data, our experience with this instrument suggests a consistent usable swath width to depth ratio between 6:1 and 8:1, in 0 to 10 m of water. While the data are noisier than those from multi- beam echosounders, and hand clean- ing of PMSS data is more time-con- suming, our operators have found that 6 hours' worth of survey data will re- quire approximately 8 to 10 hr. of pro- cessing, including filtering and hand cleaning. This is a generalization, and the time needed can be much longer given equipment problems, environ- mental conditions and other expected survey complications. Howev- er, it should be noted that the PMSS technology can map the seafloor at a higher rate than the MBES, in most cases two to three times faster. Therefore, the ratio of area mapped versus pro- cessing times is comparable to MBES, and the survey times are significantly reduced and have the added benefit of co-regis- tered side scan imagery. The processing times for the imagery are consistent with traditional side scan instruments. Recent and ongoing projects funded by NOAA and the Na- tional Fish and Wildlife Foun- dation (NFWF) to locate, iden- tify and retrieve derelict fishing gear using vessel-based acous- tic methods have demonstrated the utility of PMSS. The co-lo- cation of these data sets allow for detailed 2D and 3D spatial analyses of these areas and are often complementary. For ex- ample, the side scan imagery is of high enough resolution to clearly identify lobster pots, the connecting line (2.54-cm diameter), slight variations in bottom grain sizes and small oscillatory ripples (30- to 40-cm spacing). However, the bathymetry shows that the areas of finer grain sizes are up to 50 cm higher than the surrounding area; this is at first counterintuitive. (Top) Upper: single line of bathymetry overlain on 550-kHz side scan imagery at approximately 5-m water depth. Lower left: inset of bathymetric high 50 cm. Note small ripples (30- to 40-cm spacing) can be seen in bathymetry. Lower right: inset of pots and different grain bottom sizes. SS-1, coarse sand (D 50 = 0.31 mm). SS-2, fine sand (D 50 = 0.16 mm). Underwater pho- to of lobster pot, 2.54-cm line at site SS-1. PVC pipe for scale (30-cm length). (Right) Nearshore resource assessments. Upper: side scan mosaic (1,600 kHz); white trans- parent polygons denote extent of eelgrass. Middle: swath bathymetry (10-cm grid cell); gray transparent polygons denote extent of eelgrass. Lower: inset from middle panel; dashed polygon delineates areas of prop scars, and black star denotes areas of eel- grass visible in bathymetry.

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