Multibeam echosounders make it possible to determine the seafloor topography, and provide a measurement of the reflectivity level, usable to determine the seabed nature (sand, rock, seaweeds) or to monitor its changes. It implies accurate methodological constraints in the operation f multibeam echosounders, specifically addressed in the framework of the Ifremer project R403-06 (R&D in underwater acoustics).

The main objective of the series of cruises REM2040 (conducted since 2014) is to explore the capabilities of (1) calibration of the intensity measurements done with multibeam echosounders and (2) exploitation of the absolute reflectivity level as a function of the seabed nature and the environment parameters. For calibration, the solution selected here is a cross-comparison with a calibrated single-beam echosounder, and the survey of reference areas. The "instrumental" approaches (in test tank, or using reference targets) are useful for defining reference performances but are practically difficult to operate as a routine.

This application file for a new cruise (5 days in 2018) is aimed at continuing on the long term and completing the works achieved during the previous cruises REM2040 : without fundamental evolutions in the methodology, we aim at extending the time series of results, widening the frequency range  previously used (200 and 300 kHz) toward lower values (120 and 70 kHz) and to log data in missing configurations. The year period we ask for (May or June) accounts for the schedule constraints of the Ifremer-NSE teams. The instrumentation will be similar to the previous cruises: multibeam echosounder EM2040, single-beam echosounder EK-80 deployed from a side-mounted pole on RV Thalia ; underwater video system and sediment grab ; seawater measurements. This new cruise will make it possible:

• To continue the monitoring of the time stability of the reflectivity level in the reference areas for multibeam echosounders calibration ; in particular completing the time series of measurements taken on the reference areas of SHOM (Carré Renard, Pierres Noires, Rascas);

• To complete the multi-parameter analysis of data acquired on the various areas already identified, for which a number of missing configurations have been identified. We will specially insist on :
  • Operation of the single-beam echosounder Simrad EK-80 for analysing the incidence angle dependence (Pan & Tilt device)  at several frequencies ;
  • The frequency dependence, which is a fundamental element in understanding and exploiting the seafloor reflectivity, while still insufficiently mastered today ;  
  • The survey of new sedimentary areas or measurement configurations, in order to complete the database currently available.