Design and field testing of a non-linear single-beam echosounder for multi-frequency seabed characterization

Seabed mapping and characterization are best performed using several frequencies and several angles of incidence. This is often an issue because of the need to employ different sonars, with distinct frequencies but co-located as much as possible to image the same patch of seafloor. This article presents the design, calibration and field testing of a multiple-frequency single-beam echosounder (SBES), mounted on a mechanical pan-and-tilt head. It uses very high transmitting levels to produce non-linear effects and generate harmonics of a 100 kHz fundamental frequency. PZT transducers are used to transmit high acoustic powers and PDVF transducers enable the reception of scattering levels over a very broad frequency band (for the different harmonics). Tank experiments are used to verify effective harmonic generation. The shock distance (at which harmonics are at their maximum level) is measured as 2 m from the transmitter and recommended as the minimum far-field range. Non-linear transmission losses (distinct from linear losses) are calibrated using a full metal sphere 38.1 mm in diameter and of known frequency response, up to ranges commensurate with the depths expected in the field (30 m). The −3 dB beamwidth varies from at 100 kHz to at 300 kHz. Harmonics are used to resolve phase ambiguities in detecting seabed depths. Backscattering strengths are matched to the Generic Seafloor Acoustic Backscatter (GSAB) model to derive the best-fitting parameters. Field validation took place in the Bay of Brest (France) in May 2016, over three different types of seafloor (namely: sandy mud; gravel; gravelly coarse sand with maerl). Additional in situ calibration was used. The echosounder was pointed at angles from (nadir) to by steps. One of the areas surveyed (“Carré Renard”), commonly used for instrument calibration and comparison with other measurements, showed differences 1 dB at 200 kHz. Videos and photographs of the seafloor were used to ground truth interpretations of the curves. The results show that these curves measured with the echosounder are relevant for seabed classification and characterization. The different shapes and levels of BS when compared to ground truth are coherent with the Jackson model. The main limit of this prototype of echosounder is the signal to noise ratio, in particular for high frequency harmonics ( kHz). The in situ calibration is unavoidable because of the non-linear parameter variations with water characteristics (temperature, salinity…). Calibrated curves from 100 kHz to 300 kHz can be directly compared to other measurements, for example to calibrate other instruments.