This paper details a methodology for using structured light laser imaging to create high resolution bathymetric maps of the sea floor. The system includes a pair of stereo cameras and an inclined 532nm sheet laser mounted to a remotely operated vehicle (ROV). While a structured light system generally requires a single camera, a stereo vision set up is used here for in-situ calibration of the laser system geometry by triangulating points on the laser line. This allows for quick calibration at the survey site and does not require precise jigs or a controlled environment. A batch procedure to extract the laser line from the images to sub-pixel accuracy is also presented. The method is robust to variations in image quality and moderate amounts of water column turbidity. The final maps are constructed using a reformulation of a previous bathymetric Simultaneous Localization and Mapping (SLAM) algorithm called incremental Smoothing and Mapping (iSAM). The iSAM framework is adapted from previous applications to perform sub-mapping, where segments of previously visited terrain are registered to create relative pose constraints. The resulting maps can be gridded at one centimeter and have significantly higher sample density than similar surveys using high frequency multibeam sonar or stereo vision. Results are presented for sample surveys at a submerged archaeological site and sea floor rock outcrop.
Bottom trawl fishing presents a severe yet largely unquantified threat to shipwreck sites. Here we present a quantification of damage to sites from the Aegean and Black seas through high resolution imaging of 45 shipwrecks discovered by the E/V Nautilus expeditions, 2009-2012. These shipwrecks are part of a modern submarine landscape that is heavily damaged by trawls, which also remove sediment and smooth out natural features of the seabed. We quantify the severity of this threat to archaeological sites through repeat visits to one ancient shipwreck and quantify the change to the seabed over a period of eleven months. The results illustrate the benefits of enforced areas of restricted bottom trawling (Marine Protected Areas) to the in situ preservation of shipwreck sites and to natural seabed features and benthic habitats. Careful marine spatial planning and coordinated management of fishing activity can mitigate this destructive activity. In addition, we counter the claim made by some commercial salvors who use trawl damage as an excuse to salvage artifacts from wrecks, further destroying historically significant sites for profit.
Kick'em Jenny is a frequently erupting, shallow submarine volcano located 7.5 km off the northern coast of Grenada in the Lesser Antilles subduction zone. Focused and diffuse hydrothermal venting is taking place mainly within a small (∼70 × 110 m) depression within the 300 m diameter crater of the volcano at depths of about 265 m. Much of the crater is blanketed with a layer of fine‐grained tephra that has undergone hydrothermal alteration. Clear fluids and gas are being discharged near the center of the depression from mound‐like vents at a maximum temperature of 180°C. The gas consists of 93–96% CO2 with trace amounts of methane and hydrogen. Gas flux measurements of individual bubble streams range from 10 to 100 kg of CO2 per day. Diffuse venting with temperatures 5–35°C above ambient occurs throughout the depression and over large areas of the main crater. These zones are colonized by reddish‐yellow bacteria with the production of Fe‐oxyhydroxides as surface coatings, fragile spires up to several meters in height, and elongated mounds up to tens of centimeters thick. A high‐resolution photomosaic of the inner crater depression shows fluid flow patterns descending the sides of the depression toward the crater floor. We suggest that the negatively buoyant fluid flow is the result of phase separation of hydrothermal fluids at Kick'em Jenny generating a dense saline component that does not rise despite its elevated temperature.
[1] Currently, we lack a systematic and remote method for locating and quantifying diffuse seafloor venting using underwater robotic vehicles. Diffuse flow is characterized by both low temperature and low flux rates, which cannot readily be distinguished using current remote visual, acoustic, or vehiclemounted environmental sensors. The result is a poor understanding of the distribution, contribution, and context of diffuse flow sources. An underwater structured light imaging system, also used for highresolution seafloor bathymetric mapping, has however shown promise in detecting diffuse flows while completing seafloor imaging surveys at a typical altitude of 3 m. The system creates sequential bathymetric profiles by imaging a laser line projected on the seafloor. In the presence of venting fluids, the laser line exhibits a detectable level of distortion due to variations in the refractive index along the optical path. By characterizing the degree of distortion, it is possible to create maps indicating areas of potential venting with sub-meter spatial resolution. Results from three distinct vent fields are presented and discussed. Analysis of these data sets indicates this system is capable of detecting both small point source vents and near bottom diffuse flow.
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