GEOLOGY & GEOMORPHOLOGY

Characteristics of the seabed influence physical, chemical and biological processes in the ocean, both directly and indirectly. The angle and direction of slope guides the currents and affects how and where waves break, and the chemical and biological components of seawater in an area is therefore determined by the horizontal and vertical transport of water. Sediment composition may benefit or exclude biological activity and marine habitats such as coral reefs and seaweed growth, thus controlling food sources and living conditions for different species of benthic dwellers further up the food chain. As with bathymetry, geology and geomorphology are important to map when creating nautical charts, as rocky outcroppings, shallow sandbanks and underwater obstacles may impede navigational safety. Seafloor geology data can be divided into two primary data types: the substrate (surficial geology, sediments) and the subsurface. 

Stakeholders interacting with seafloor geology data are varied; they can focus on sedimentology, paleontology, geochemistry, benthic ecology, or exploratory hydro-acoustics modeling, among others. The marine sediment research community can be organized into three groups based on the focus of their surveys and on how their sampling units are described and structured: “Geological”, “Geotechnical”, and “Geophysical”. The geological approaches regroup the analysis of sediment composition to assess spatial and temporal variations in deposition and erosion over various scales. These approaches express the sediment composition in terms of lithology, mineralogy, and fossil content. The geotechnical approaches focus on sediment morphological changes due to natural or anthropogenic processes. These approaches focus on soil profiles and mineralogy, much like the geological approaches, but give additional attention to sediment structure, such as porosity, shear strength, and mineral stratification. They also focus on how the sediment composition may adjust with future environmental changes. Examples of geotechnical approaches include slope stability analyses and modeling of past and future sediment infrastructure. The geophysical approaches include seismic inversion practices (seismic field and well data) in which subsurface sediment layers are characterized by reflectance, intrinsic attenuation, and propagation of various acoustic wavefields.

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To describe and compare a study region, the chosen sediment classes and resulting sediment transitional regions need to be sampled in a manner that is consistent with the geological scale, providing sufficient sampling events across the study area. In situ sediment sampling is the most common data collection approach for geological and geotechnical studies. Various sampling methods and gear are used for sediment marine analysis, the most common being core samplers, grab samplers, and hand samplers. Each category has distinct benefits and costs for sampling the sediment matrix. The depth and composition of the study site (e.g., fine-grain sediment, coarse sediment, variable organic deposition, rock presence/absence) may often dictate which sampling gear is best suited. The use of acoustic remote sensing is most common for marine geophysical studies. Acoustic backscatter data are often used with in situ samples to describe seafloor substrate type and properties, match acoustic properties to samples, and extrapolate the interpretation of these matches to map the entire acoustically-surveyed area. Sidescan sonars are primarily used for mapping the seafloor and identifying objects or features. However, even if they are not typically used for geological sampling in the traditional sense, they can still be valuable in geological studies as they provide high-resolution images of the seafloor, resolving geological features such as the texture of sediment deposits and geological structures. Sub-bottom profiling is an essential geophysical method to study the sedimentary layers beneath the seafloor in marine environments. It provides valuable information about sediment thickness, stratigraphy, and subsurface structures, which are crucial for geological, archaeological, and environmental studies.