Seafloor geological mapping and interferometric synthetic aperture sonar: from mid-ocean ridges to continental shelves

Abstract

The Oceans cover 70% of the Earth's surface, yet only 25% have been mapped at a resolution of at least ~100 m, a resolution that is coarser than the size of many important geological features on the seafloor, such as hydrothermal vents or volcanoes. On land, geological maps typically end at the shoreline due to the inaccessibility of the seafloor and challenges of working underwater (i.e., pressure, light attenuation). Seafloor surveys are carried out using acoustic techniques, such as sonars, to obtain bathymetric maps and acoustic imagery of the seafloor by measuring the travel time and intensity of an emitted acoustic signal reflected at the seafloor, respectively. Seafloor mapping, however, involves a trade-off between data resolution and coverage, with higher resolutions obtained at the cost of coverage area. Typically, the study of fine-scale (e.g., 10s of m or less) geological features relies on a multi-scale and multi-tool approach, combining large-scale acoustic mapping (meter to sub-meter resolution) with fine-scale optical surveying (millimetre resolution) and sampling of the substrate. This thesis presents an investigation of the application of different seafloor survey technologies and approaches for geological characterization of the seafloor. The research focuses specifically on the study of seafloor hydrothermal vents, which are hot springs on the seafloor that support life in the deep-oceans, and where metals, including societally relevant base and precious metals, accumulate as mineral deposits. As a case study, the classic approach to the geological characterization of a hydrothermal deposit is applied to the Fåvne vent field, located on the Mohns Ridge in the Norwegian Sea, using 1 m resolution bathymetry, optical imagery, rock samples, and sediment cores. This study improves our understanding of metal enrichment processes at the seafloor and extends our knowledge on the global diversity of hydrothermal deposits. With the development of interferometric synthetic aperture sonar (InSAS), a new high-resolution survey technology, larger areas of the seafloor can be surveyed at much higher resolutions than using conventional acoustic techniques. While InSAS has mostly been applied to survey flat terrain, in this study a comparison of using InSAS on a flat continental shelf to a rugged mid-ocean ridge terrain is presented. Identification of survey parameters necessary to optimize InSAS data quality for effective use for scientific applications, shows that InSAS is an appropriate tool for surveys even in morphologically complex terrain. Through the identification of bedrock structures and overburden classes from InSAS data, InSAS is evaluated for geological applications, such as bedrock stratigraphy and tectonics, glacial reconstructions, and marine litter distribution. Overall, this thesis shows that, compared to conventional techniques, InSAS is an effective tool for high-resolution characterization of seafloor geological features in various geomorphological environments, thus opening the door for a significant increase in potential applications, including for the survey of hydrothermal deposits.