Inter-vessel Field Backscatter Calibration for Kongsberg EM2040 Systems

Seafloor acoustic backscatter plays a critical role in many current seafloor classification methods. At the Office of Coast Survey, we are particularly interested in backscatter from multibeam echosounder systems deployed for bathymetric surveys, because we both execute and contract for a major survey effort with these systems on an ongoing basis. Recognizing that backscatter can be a valuable product from these surveys, we have been actively trying to understand both the requirements and necessary specifications for backscatter deliverables. While absolute calibration of these systems has been demonstrated in specialized test environments, practical full-calibration methods for deployed multibeam systems have remained, to date, elusive. For vessels utilizing more than one acoustic system, or the use of multiple vessels in a project area, a potentially useful intermediate step is to both account for the relative difference between acquisition parameters (e.g. power, pulse length) of a given sensor and the relative differences between sensors. We demonstrate a practical approach to this problem using a patch of seafloor as the calibration standard and illustrate the method using eight different Kongsberg 2040 systems- each mounted on survey vessel.As others working in this field have demonstrated [5, 9], the selection of an appropriate seafloor target patch is critical for this type of approach. In particular, temporal stability, annual accessibility, bottom type, predictable and anticipated environmental factors (traffic, variant sound speed), are critical factors. In 2017, we installed eight Kongsberg EM2040 MBES systems on NOAA Ship Rainier and Fairweather survey vessels and conducted inaugural test and acceptance trials in Puget Sound. We collected data for each azimuth, pulse length, and frequency for each system and determined relative calibration offsets for each. This direct comparison between identical MBES sonars mounted on near-identical vessels revealed anticipated differences between pulse lengths and frequencies in one test case, but unanticipated inconsistencies in the other. We attribute these inconsistencies to environmental factors, such as sound velocity irregularities from freshwater and vessel fathometer interference. In 2018, we selected a new calibration site in Port Madison, incorporating best practices identified by other workers in selecting the bottom type and paying greater heed to oceanographic stability. We acquired reference data sets across all eight systems over a two-week period, minimizing the possible temporal and environmental effects. Using commercial software (QPS FMGT), we created mosaics of each survey line. A cross-correlation analysis of the mosaic histograms gave distinct decibel differences for each vessel, frequency, and pulse length. We then use these offsets values to normalize between common acquisition modes in postprocessing, allowing for a more visually consistent mosaic and consistent backscatter data across these multiple platforms. We are now using the derived inter-system calibration values operationally. Our experiences in selecting an appropriate intersystem calibration area, calculating the relative offsets between operational parameters and vessels should be of interest to those operating multibeam systems, particularly multiple system, for seafloor mapping work.