Exploration of hydrothermal venting sites using deep-towed multibeam echosounder data

Abstract

The Federal Institute for Geosciences and Natural Resources (BGR) obtained a contract for the exploration of polymetallic sulphides from the International Seabed Authority (ISA) in an area of 10,000 km2 located in the southwestern Indian Ocean. Since obtaining this contract of exploration in 2015, BGR has conducted annual research cruises to this area and continuously improves the exploration methods for seafloor sulphides deposits. One of the main instruments is HOMESIDE, a deep-towed multibeam echo sounder (MBES) sled. Its water column data allows to visualize and therefore locate discharge sites of hydrothermal fluid – active hydrothermal venting sites (or commonly also referred to as “black smokers”). Additionally, its bathymetric data is used for geological mapping and therefore greater analysis of the seafloor geomorphology. HOMESIDE is typically towed at an altitude of about 100 m above the seabed in a water depth of approximately 3,000 m, allowing a resolution of the derived digital terrain model (DTM) of 2 m. Due to malfunction of individual positioning sensors or operational errors, the navigation data might need to be further corrected in post-processing. Navigation offsets between adjacent lines are mostly apparent in overlapping swath data. The tool mbnavadjust of the open-source software MB-System showed to be a valuable tool to improve the data quality subsequently in areas with data overlap and systematic navigation offsets. Data mismatches of more than 10 m are processed with this tool and can, therefore, be reduced significantly. In this paper, firstly, the role of deep-towed MBES as exploration tool for hydrothermal sites is presented. Secondly, the importance of post-processing MBES navigation based on the swath data especially for this project is highlighted. The workflow as well as the results are presented and show significant improvement.