Backscatter calibration of high-frequency multibeam echosounder using a reference single-beam system, on natural seafloor

Abstract

The calibration of multibeam echosounders for backscatter measurements can be conducted efficiently and accurately using data from surveys over a reference natural area, implying appropriate measurements of the local absolute values of backscatter. Such a shallow area (20-m mean depth) has been defined and qualified in the Bay of Brest (France), and chosen as a reference area for multibeam systems operating at 200 and 300 kHz. The absolute reflectivity over the area was measured using a calibrated single-beam fishery echosounder (Simrad EK60) tilted at incidence angles varying between 0° and 60° with a step of 3°. This reference backscatter level is then compared to the average backscatter values obtained by a multibeam echosounder (here a Kongsberg EM 2040-D) at a close frequency and measured as a function of angle; the difference gives the angular bias applicable to the multibeam system for recorded level calibration. The method is validated by checking the single-and multibeam data obtained on other areas with sediment types different from the reference area.

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“…The sounder measures depth, which relates to the signal's two-way travel time. It is also possible to extract information of the seabed type from the analysis of the echo [25]. Although the main use of SBES is to produce bathymetry maps, its capability to detect echoes from targets in the water column has turned it into a valuable tool for fishing.…”

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“…There exists a wide range of transducers that deliver results of interest for marine scientists, fisheries, and conservation managers, including echosounder and sonar systems. During the last few decades, the main advance is the advent of multibeam (e.g., Guillard et al, 2011) and wideband (Demer et al, 2017; acoustic systems, although calibration and data treatment are still challenging with existing protocols (e.g., Perrot et al, 2014Perrot et al, , 2018Demer et al, 2015;Korneliussen et al, 2016;Eleftherakis et al, 2018).…”

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“…This starts with standardizing operational procedures, in terms of acquisition and processing, ideally inspired from community-driven experiences (e.g., the GeoHab backscatter working group [11]). Accuracy of an MBES system is largely dependent on the calibration process, requiring manufacturer-based operations (i.e., providing users with measurements/calibration tests results) and/or dedicated facilities and instrumentation to carry out in situ calibration (otherwise unfeasible for hull-mounted systems-see [21,29] for detailed considerations regarding calibration). Data repeatability refers to controlling the spatio-temporal consistency of the acoustic data in terms of instrumental and environment-caused drifts.…”

“…The AR forms a shape ("the AR curve") which reflects the dominant acoustic phenomena occurring along the angular domains: high-intensity specular reflection around the nadir and lower-level scattering at oblique angles, strongly decreasing at shallow grazing angles. Where absolute calibration of the BS is achievable, the BS AR is to be considered as an objective measurement for which different methods exist ( [21] and references therein). The mosaic backscatter is a further derivative of the backscatter data, where BS levels are presented, usually in a georeferenced frame, in the form of a grayscale image with the angular dependence removed via statistical compensation.…”

“…Inversion of the AR into sedimentologically relevant information is a principle known for long which is currently hindered by a lack of high-frequency geoacoustic models dedicated to solving the "inversion problem" and should be perceived as an advancing application within the realm of acoustical oceanography. However, it remains promising considering the rapid advances in MBES system absolute [21,27,28] and relative calibration [29] and in stability and repeatability controls [30], together promoting the comparability of data in space and time. This would also allow compiling acoustic inventories that are calibrated against substrate types (and of associated features and combinations) to be used more globally.…”