Abstract
Seafloor characterization using multibeam echosounder (MBES) backscatter data is an active field of research. The observed backscatter curve (OBC) is used in an inversion algorithm with available physics-based models to determine the seafloor geoacoustic parameters. A complication is that the OBC cannot directly be coupled to the modeled backscatter curve (MBC) due to the correction of uncalibrated sonars. Grab samples at reference areas are usually required to estimate the angular calibration curve (ACC) prior to the inversion. We first attempt to estimate the MBES ACC without grab sampling by using the least squares cubic spline approximation method implemented in a differential evolution optimization algorithm. The geoacoustic parameters are then inverted over the entire area using the OBCs corrected for the estimated ACC. The results indicate that a search for at least three geoacoustic parameters is required, which includes the sediment mean grain size, roughness parameter, and volume scattering parameter. The inverted mean grain sizes are in agreement with grab samples, indicating reliability and stability of the proposed method. Furthermore, the interaction between the geoacoustic parameters and Bayesian acoustic classes is investigated. It is observed that higher backscatter values, and thereby higher acoustic classes, should not only be attributed to (slightly) coarser sediment, especially in a homogeneous sedimentary environment such as the Brown Bank, North Sea. Higher acoustic classes should also be attributed to larger seafloor roughness and volume scattering parameters, which are not likely intrinsic to only sediment characteristics but also to other contributing factors.
Highlights
High-resolution knowledge of the morphology and sediment composition of the seafloor is in high demand for many offshore activities
Having a small surface patch will not change the statistical characteristics of backscatter values significantly and its averaging is allowed for our application
Seafloor sediment characterization using multibeam echosounder (MBES) backscatter data has been the subject of intensive research in the last two decades
Summary
High-resolution knowledge of the morphology and sediment composition of the seafloor is in high demand for many offshore activities. The ACC is usually determined by the calibration of the MBES in flat areas, having homogenous sediment types of known grain size values This is achieved through the application of the angular range analysis (ARA) to the measured backscatter data [6], as applied in seafloor characterization by [22,23]. Given the frequency of the multibeam system, the angular response curve is affected by two main factors: (1) the sediment type, and (2) the angular calibration effect as a function of incident angle (angular calibration curve) The former is not restricted to the sediment grain size distribution only and to other geoacoustic parameters such as the interface roughness parameter and volume scattering parameter.
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