Backscatter mosaics based on a multi-frequency multibeam echosounder survey in the continental shelf setting of the North Sea were compared. The uncalibrated backscatter data were recorded with frequencies of 200, 400 and 600 kHz. The results showed that the seafloor appears mostly featureless in acoustic backscatter mosaics derived from 600 kHz data. The same area surveyed with 200 kHz reveals numerous backscatter anomalies with diameters of 10-70 m deviating between −2 dB and +4 dB from the background sediment. Backscatter anomalies were further subdivided based on their frequency-specific texture and were attributed to bioturbation within the sediment and the presence of polychaetes on the seafloor. While low frequencies show the highest overall contrast between different seafloor types, a consideration of all frequencies permits an improved interpretation of subtle seafloor features.

The presented 3D seismic system operates three transducers (130 kHz) from a stationary lander and allows non-destructive imaging of small-scale objects within the top decimeters of silty sediments, covering a surface area of 0.2 m2. In laboratory experiments, samples such as shells, stones, and gummy worms of varied sizes (down to approx. 1 cm diameter) could be located in the 3D seismic cube to a depth of more than 20 cm and differentiated by a reflected amplitude intensity and spatial orientation. In addition, simulated bioturbation structures could be imaged. In a practical application, the system allows to determine the abundance of endobenthos and its dynamic in muddy deposits in-situ and thus identify the intensity of local bioturbation.

Quantitative acoustic marine habitat mapping needs to consider the impact ofmacrobenthic organisms on backscatter data. However, the sensitivity of hydroacoustic systems toepibenthic life is poorly constrained. This study explores the impact of a benthic community withsparse abundance on seafloor microroughness and acoustic backscatter at a sandy seafloor in theGerman North Sea. A multibeam echo sounder survey was ground-truthed by landermeasurements combining a laser line scanner with sub-mm resolution and broad-band acoustictransducers. Biotic and abiotic features and spatial roughness parameters were determined by thelaser line scanner. At the same locations, acoustic backscatter was measured and compared with anacoustic scatter model utilizing the small-roughness perturbation approximation. Results of thelander experiments show that a coverage with epibenthic features of 1.6% increases seafloorroughness at spatial wavelengths between 0.005–0.03 m, increasing both spectral slope andintercept. Despite the fact that a strong impact on backscatter was predicted by the acoustic modelbased on measured roughness parameters, only a minor (1.1 dB) change of backscatter was actuallyobserved during both the lander experiments and the ship-based acoustic survey. The results of thisstudy indicate that benthic coverage of less than 1.6% is insufficient to be detected by currentacoustic remote sensing.

This study investigated the seasonality of acoustic backscatter intensities, exploring three habitats in the southwestern Baltic Sea: 1) a mussel-covered reef, 2) coarse sand and gravel, and 3) seagrass meadows. Backscatter information of different, partly calibrated frequencies (200, 400, 550, and 700 kHz) was collected in three seasons (May, August, and October). The acoustic data were supported by point samples and video profiles for grain size and benthic community analysis. Angular response curves helped to quantify the seasonal backscatter response of the different frequencies. The multifrequency and multiseasonal backscatter maps distinguish the three habitats and reveal variable seasonal differences in acoustic backscatter, but not all changes in the benthic community can be recognized in the acoustic data. 1) The high-backscatter response of the mussel-covered reef shows little seasonal differences and was frequency independent. 2) The ecologically valuable coarse sand and gravel areas show small-scale seasonal alterations in the sediment composition and morphology, mainly caused by changes in local hydrodynamics. Higher frequencies were found best suited to identify coarse sand and gravel. 3) Seagrass meadows seasonality is dominated by growth of seagrass blades, increasing the backscatter response compared to bare sand. The use of multiple frequencies is beneficial as the low frequency is sensitive to changes in the shallow subsurface and benthic features such as seagrass rhizomes, while the higher frequency highlights changes related to coarser sediment.