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

Abstract

Stones and boulders in shallow waters (0–10 m water depth) form complex geo-habitats, serving as a hardground for many benthic species, and are important contributors to coastal biodiversity and high benthic production. This study focuses on limitations in stone and boulder detection using high-resolution sidescan sonar images in shallow water environments of the southwestern Baltic Sea. Observations were carried out using sidescan sonars operating with frequencies from 450 kHz up to 1 MHz to identify individual stones and boulders within different levels of resolution. In addition, sidescan sonar images were generated using varying survey directions for an assessment of range effects. The comparison of images of different resolutions reveals considerable discrepancies in the numbers of detectable stones and boulders, and in their distribution patterns. Results on the detection of individual stones and boulders at approximately 0.04 m/pixel resolution were compared to common discretizations: it was shown that image resolutions of 0.2 m/pixel may underestimate available hard-ground settlement space by up to 42%. If methodological constraints are known and considered, detailed information about individual stones and boulders, and potential settlement space for marine organisms, can be derived.

Hydroacoustic Mapping of Geogenic Hard Substrates: Challenges and Review of German Approaches

“…Grab sampling and hydroacoustic mapping campaigns have shown that the geogenic hard substrates in the German waters are present as loose scattered cobbles (64-256 mm) and boulders (>256 mm) with maximum sizes up to several meters [20,22,23,34,36,37] including a high amount of flint stones [38]. They originate from Scandinavia and were transported by several Pleistocene glacial advances to the North Sea [38][39][40] and Baltic Sea [41,42], and deposited in end and ground moraines.…”

“…Next to their position in the acoustic swath, the quantity of detected objects is increasing with higher data resolution. Von Rönn et al [23] recognized an underestimation of boulders of up to 42% with lower image resolution. The resolution of backscatter imagery depends on the sonar frequency and pulse length (continuous wave systems) or bandwidth (frequency modulated systems, "Chirp technology"), the across and along track aperture, the range to the sonar and the speed of the ship.…”

“…However, especially when stones (cobbles, boulders and even larger clasts) are loosely scattered on sandy or mixed substrate, delineation rules are important to establish for example special areas of conservation (SAC) or in the course of approval procedures for offshore constructions such as offshore wind farms, cables or pipelines. The most common technique for habitat mapping and the identification of hard substrate areas is the analysis of sidescan sonar (SSS) or multibeam echosounder (MBES) backscatter data [19][20][21][22][23][24]. There are no internationally standardized routines for large-scale data acquisition, processing and interpretation either, albeit recommendations have been developed for the acquisition and processing of MBES derived backscatter data [25].…”

Dynamics of Stone Habitats in Coastal Waters of the Southwestern Baltic Sea (Hohwacht Bay)

“…It is exposed to northwesterly to northeasterly directions. SA2 has been described as a part of a lag sediment deposit, which is the seaward extension of the cliff [50]. Mobile sand layers cover the seafloor landwards while lag sediment deposits extend seawards over the glacial till.…”

Measuring Centimeter-Scale Sand Ripples Using Multibeam Echosounder Backscatter Data from the Brown Bank Area of the Dutch Continental Shelf

“…The sand ripples in that study were larger, with a wavelength of~0.5 m, than the ones encountered in this study. In [16], very high resolution (~0.04 m pixel size) SSS survey data were used to map stones and boulders for habitat mapping. In addition, sand ripples of a similar size were visible as those in this study.…”