Acoustic quantitative analysis of ferromanganese nodules and cobalt-rich crusts distribution areas using EM122 multibeam backscatter data from deep-sea basin to seamount in Western Pacific Ocean

Abstract

EM122 multibeam backscatter data acquired from the Caiwei seamounts and the deep-sea basin to its west in the western Pacific Ocean, is used to identify the distributions of ferromanganese nodules with different abundance, cobalt-rich crusts, pelagic calcareous and clay sediments, etc. Box and drill sample and video are used for ground truth. The backscatter data are processed by CARIS HIPS&SIPS 11.1, including transmission losses, insonified area, AVG, despeckling and mosaicking. The qualitative and quantitative relationship between backscatter and nodules, crusts and different sediments are established successfully. Significantly, low-backscatter values are observed on areas of pelagic clay sediments (C1) in the deep-sea basin and calcareous pelagic sediments (C2) on the summit of the seamount, whereas obvious high-backscatter values are observed on areas of nodules with high abundance around 40 kg/m2 (C4) in the deep sea basin and cobalt-rich crusts (C5) on the seamount, the important difference in backscatter between soft pelagic sediments (C1 and C2) and hard minerals (C4 and C5) is nearly 20dB. Quantitatively, the GSAB improved model is used to identify several parameters for different seafloor geological classes, e.g., BS45°, specular level, specular angular extent, Lambert reference and Lambert decrement. The parameters for nodules with different abundance, cobalt-rich crusts and different pelagic sediments provide insight into reflection coefficient, specular regime, seafloor roughness and impedance, etc. The profiles of angular response of nodules with high abundance (C4) in the basin and crusts (C5) on the seamount show high values in BS45°, Lambert reference but low specular level, and hence can be interpreted as representative areas with hard or relatively rough substrates. Contrarily, the profiles of the pelagic clay sediment (C1) and calcareous pelagic sediment (C2) both show low values in BS45°, Lambert references but high values in specular level, indicating similar high coherent reflection coefficient, low slope variance and smooth sediment interfaces for these two types of pelagic sediments.