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The increased use of backscatter measurements in time series for environmental monitoring necessitates the comparability of individual results. With the current lack of pre-calibrated multibeam echosounder systems for absolute backscatter measurement, a pragmatic solution is the use of natural reference areas for ensuring regular assessment of the backscatter measurement repeatability. This method mainly relies on the assumption of a sufficiently stable reference area regarding its backscatter signature. The aptitude of a natural area to provide a stable and uniform backscatter response must be carefully considered and demonstrated by a sufficiently long time-series of measurements. Furthermore, this approach requires a strict control of the acquisition and processing parameters. If all these conditions are met, stability check and relative calibration of a system are possible by comparison with the averaged backscatter values for the area. Based on a common multibeam echosounder and sampling campaign completed by available bathymetric and backscatter time series, the suitability as a backscatter reference area of three different candidates was evaluated. Two among them, Carré Renard and Kwinte, prove to be excellent choices, while the third one, Western Solent, lacks sufficient data over time, but remains a valuable candidate. The case studies and the available backscatter data on these areas prove the applicability of this method. The expansion of the number of commonly used reference areas and the growth of the number of multibeam echosounder controlled thereon could greatly contribute to the further development of quantitative applications based on multibeam echosounder backscatter measurements.

A new seafloor mapping system was installed on E/V Nautilus in early 2013, revolutionizing the way the ship explores the ocean. Not only can the Kongsberg EM 302 multibeam echosounder (Figure 1) create maps of the seafloor, it can also provide information on bottom type from backscatter data, similar to the side-scan sonar Diana. Additionally, the multibeam can be used to locate plumes in the water column created by hydrothermal vents or gas or oil seeps. Together with the new Knudsen 3260 subbottom profiler, these acoustic instruments will provide improved maps of the seafloor to help scientists choose dive locations and to improve situational awareness for navigators and ROV pilots. Data are archived at NOAA's National Geophysical Data Center, allowing anyone to obtain raw or gridded seafloor bathymetric data collected during Nautilus's exploratory cruises.