Detection of Stones in Marine Habitats Combining Simultaneous Hydroacoustic Surveys

Abstract

Exposed stones in sandy sublittoral environments are hotspots for marine biodiversity, especially for benthic communities. The detection of single stones is principally possible using sidescan-sonar (SSS) backscatter data. The data resolution has to be high to visualize the acoustic shadows of the stones. Otherwise, stony substrates will not be differentiable from other high backscatter substrates (e.g., gravel). Acquiring adequate sonar data and identifying stones in backscatter images is time consuming because it usually requires visual-manual procedures. To develop a more efficient identification and demarcation procedure of stone fields, sidescan sonar and parametric echo sound data were recorded within the marine protected area of "Sylt Outer Reef" (German Bight, North Sea). The investigated area (~5.900 km 2 ) is characterized by dispersed heterogeneous moraine and marine deposits. Data from parametric sediment echo sounder indicate hyperbolas at the sediment surface in stony areas, which can easily be exported. By combining simultaneous recorded low backscatter data and parametric single beam data, stony grounds were demarcated faster, less complex and reproducible from gravelly substrates indicating similar high backscatter in the SSS data.

Detection of Boulders in Side Scan Sonar Mosaics by a Neural Network

“…In current practice, boulder detection often requires time-consuming manual interpretation of sidescan-sonar or multibeam echo sounder backscatter mosaics and/or confirmation by underwater video footage [8]. In principle, it has to be differentiated between the detection of buried boulders, and boulders exposed at the seafloor.…”

“…In principle, it has to be differentiated between the detection of buried boulders, and boulders exposed at the seafloor. Buried boulders may be detected by seismic methods [8]. While the detection of slightly buried boulders is important, especially considering dynamic areas such as parts of the North Sea, where boulders may be frequently exposed or covered based on environmental conditions [9], the focus of this study is on the detection of stable surficial boulders widespread in the Baltic Sea.…”

Limitations of Boulder Detection in Shallow Water Habitats Using High-Resolution Sidescan Sonar Images

“…The difference in spatial scale between individual stones and the large extents of boulder assemblages, their spatial heterogeneity and their temporal development require remote sensing observation techniques. Whereas airborne techniques and satellite remote sensing may be limited by resolution, vegetation and water turbidity, ship based hydro-acoustic techniques, such as sidescan sonar (SSS hereafter) or multibeam echosounder (MBES), allow the investigation of the seafloor in high-resolution and at turbid water conditions [14][15][16][17][18]. In coastal waters of the Baltic Sea, a great variety of sediment types and geomorphological features characterize the seafloor.…”

“…Many surveys have been carried out to localize ecologically important marine habitats (>10 m water depth) using SSS [13,16,18,20] and have successfully demonstrated the suitability of SSS to map and identify areas with stone and boulder occurrence. Various authors used a wide range of instruments, with different characteristics and survey settings, resulting in quantitative assessments of habitats [16,[21][22][23][24][25]. The identification of objects using multibeam echo sounders (MBES) has been shown for, e.g., mine detection [20,26].…”

“…The identification of objects using multibeam echo sounders (MBES) has been shown for, e.g., mine detection [20,26]. Area-wide habitat mapping is often performed by using acoustic frequencies ranging from 300 to 500 kHz, survey speeds vary between 2-4 m/s and across-track ranges between 50 m and 180 m [13,16,18,27]. Those survey setups generate SSS image resolutions ranging from 0.25 m/pixel to 1 m/pixel, thus are insufficient to identify individual stones or boulders, especially in areas with coarse, mixed sediments [16].…”