DeepSurveyCam — A Deep Ocean Optical Mapping System

Underwater photogrammetry and in particular systematic visual surveys of the deep sea are by far less developed than similar techniques on land or in space. The main challenges are the rough conditions with extremely high pressure, the accessibility of target areas (container and ship deployment of robust sensors, then diving for hours to the ocean floor), and the limitations of localization technologies (no GPS). The absence of natural light complicates energy budget considerations for deep diving flash-equipped drones. Refraction effects influence geometric image formation considerations with respect to field of view and focus, while attenuation and scattering degrade the radiometric image quality and limit the effective visibility. As an improvement on the stated issues, we present an AUV-based optical system intended for autonomous visual mapping of large areas of the seafloor (square kilometers) in up to 6000 m water depth. We compare it to existing systems and discuss tradeoffs such as resolution vs. mapped area and show results from a recent deployment with 90,000 mapped square meters of deep ocean floor

Insights into the internal structure and formation of striated fault surfaces of oceanic detachments from in situ observations (13°20'N and 13°30'N, Mid-Atlantic Ridge)

Oceanic detachment faults (ODs) are known to play a significant role in oceanic crustal accretion along slow-spreading ridges, and many display a poorly understood corrugated fault surface. The ODEMAR cruise (Nov-Dec’14) studied the 13°20’N and 13°30’N ODs along the Mid-Atlantic Ridge via extensive microbathymetric surveys with AUV ABYSS (GEOMAR), combined with geological observations and sampling using ROV VICTOR (IFREMER).The 13°20’N OD is largely intact, with an undisrupted corrugated surface. An abrupt, continuous moat where the OD emerges from the seafloor sloping at ~12-18° continuously sheds rubble onto the OD fault plane, blanketing it. An apron surrounds the detachment dipping ~10-14° towards volcanic rift valley floor thus forming a thin wedge above the active OD fault, which uplifts hangingwall material. In contrast, the 13°30’N OD is cut by recent high-angle faults, and is likely inactive. The OD fault is well exposed along these recent high angle fault scarps, and along mass wasting scarps.The OD fault displays individual microbathymetric lineations throughout the >150 m of fault zone thickness, that are traced up to ~2 km in the spreading direction. Flanks of individual lineations display fault planes extending ~20-100 m laterally with well-developed, extension-parallel striae. At 13°20’N. These fault surfaces are primarily basalt fault breccias and minor serpentinite. At 13°30’N the scarps cutting the detachment system reveal highly heterogeneous deformation, with phacoidal blocks of undeformed peridotite, gabbro, and basalt enclosed in anastomosing shear zones. Basalt often shows greenschist grade alteration, and is only present in the upper 50 m of the OD fault zone.In detail, OD faults are characterized by anastomosing zones of localized, strongly anisotropic deformation at different scales (m to km), bounding bodies of largely undeformed rock (basalt, gabbro, peridotite) elongated in the extension direction. Hangingwall material (basalt and dolerite) gets reworked into the fault zone in the green schist facies and effectively accreted to the footwall, both as fault breccia and as large blocks within the fault. Such fine-scale, in situ observations provide unprecedented insights into the three-dimensional and heterogeneous strain localization at OD fault zones

First direct observation of coseismic slip and seafloor rupture along a submarine normal fault and implications for fault slip history

Properly assessing the extent and magnitude of fault ruptures associated with large earthquakes is critical for understanding fault behavior and associated hazard. Submarine faults can trigger tsunamis, whose characteristics are defined by the geometry of seafloor displacement, studied primarily through indirect observations (e.g., seismic event parameters, seismic profiles, shipboard bathymetry, coring) rather than direct ones. Using deep-sea vehicles, we identify for the first time a marker of coseismic slip on a submarine fault plane along the Roseau Fault (Lesser Antilles), and measure its vertical displacement of ∼0.9 m in situ. We also map recent fissuring and faulting of sediments on the hangingwall, along ∼3 km of rupture in close proximity to the fault's base, and document the reactivation of erosion and sedimentation within and downslope of the scarp. These deformation structures were caused by the 2004 Mw 6.3 Les Saintes earthquake, which triggered a subsequent tsunami. Their characterization informs estimates of earthquake recurrence on this fault and provides new constraints on the geometry of fault rupture, which is both shorter and displays locally larger coseismic displacements than available model predictions that lack field constraints. This methodology of detailed field observations coupled with near-bottom geophysical surveying can be readily applied to numerous submarine fault systems, and should prove useful in evaluating seismic and tsunamigenic hazard in all geodynamic contexts