Effects of using inclined parametric echosounding on sub-bottom acoustic imaging and advances in buried object detection

Abstract

This study reports an adaptation of a parametric echosounder system using 15 kHz as secondary frequency to investigate the angular response of sub-bottom backscatter strength of layered mud, providing a new method for enhanced acoustic detection of buried targets. Adaptions to achieve both vertical (0°) and non-vertical inclination (1-15°, 30°, 45°and 60°) comprise mechanical tilting of the acoustic transducer and electronic beam steering. Data were acquired at 18 m water depth at a study site characterized by a flat, muddy seafloor where a 0.1 m diameter power cable lies 1-2 m below the seafloor. Surveying the cable with vertical incidence revealed that the buried cable can hardly be discriminated against the backscatter strength of the layered mud. However, the backscatter strength of layered mud decreases strongly at >3±0.5°incidence and the layered mud echo pattern vanishes beyond 5°. As a consequence, the backscatter pattern of the buried cable is very pronounced in acoustic images gathered at 15°, 30°, 45°and 60°incidence. The size of the cable echo pattern increases linearly with incidence. These effects are attributed to reflection loss from layered mud at larger incidence and to the scattering of the 0.1 m diameter buried cable. Data analyses support the visual impression of superior detection of the cable with an up to 2.6-fold increase of the signal-to-noise ratio at 40°incidence compared to the vertical incidence case.

Multispectral Multibeam Echo Sounder Backscatter as a Tool for Improved Seafloor Characterization

“…The sediment parameters at the site can influence backscatter based on local roughness, which is often unknown or unmeasured [31], which in turn can cause different backscatter responses at different frequencies [16]. Generally speaking, lower frequency signals will penetrate deeper into the substrate than higher frequency signals, which will attenuate over a shorter distance [22,32].…”

Improved Interpretation of Marine Sedimentary Environments Using Multi-Frequency Multibeam Backscatter Data

“…Elevated backscatter levels are chiefly observed in the low frequency mosaics, where acoustic waves penetrated a couple of centimeters into the subsurface. A connection to sorted bedforms that could influence volume scatter by sub-bottom layering [48] is possible based on the seismic data ( Figure 3B). The transparent layer (S1), interpreted as the mobile layer of fine sand [37], is of decreased thickness in the area of elongated HBPs.…”

Recommendations for improved and coherent acquisition and processing of backscatter data from seafloor-mapping sonars

“…The BSWG did not consider other sonar systems of interest in seafloor surveys, such as side-scan sonars (usually unable to provide bathymetry measurements), single-beam echosounders (only receiving vertical echoes) and sub-bottom profilers (designed for sub-seafloor imaging). This could change: the use of SBES as calibrated reference for MBES calibration proves today to be a very reliable and practical solution (Ladroit et al 2017;Lurton et al 2017), while the use of high-resolution sub-bottoms seismic profilers can be of high interest for the investigation of the surficial seafloor layers (Hillman et al 2017;Schneider von Deimling et al 2016) and interpretation of its backscatter.…”