The ANR-Astrid S2MF project (multi-frequency sonar and sounders for imaging and accurate measurement of backscattering in seafloor sediment layers), launched in 2015, aims to develop and validate multi-frequency sonar technology, using the non-linear generation of harmonics in the signal's nominal frequency - by voluntarily emitting at very high levels. The idea is to be able to obtain seabed reflectivity data at several frequencies covering a fairly broad range (typically from 100 to 400 kHz) both simultaneously and with the same instantaneous footprints, making it possible to better distinguish types of sediment. Partners of the project are the UPMC Institut d'Alembert (lead partner), Ifremer and ENSTA-Bretagne. Two S2MF prototypes have been developed over the course of the project: a vertical sounder with a small disc antenna; a side-scan sonar with a more cumbersome linear antenna. Both systems underwent trials in 2015-2016 in the test tank (Ifremer, Brest Centre) and from a launch in the sheltered bay (Canopée, ENSTA-Br). These operations are designed to progressively assess their performances under increasingly complex conditions and are complemented by a "life-size" acquisition cruise aboard Thalia, on already known geological sites where calibrated sonar acquisitions have already been made at similar frequencies (between 100 and 400 kHz). To this end, previous REM2040 missions aboard Thalia provided repeated measurements on targeted zones (particularly on Renard bank in the Bay of Brest) and enabled some twenty reference zones to be selected (see map below) and then characterised via sampling (Shipeck grabs) and video observations.