Clayton Kunz

Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean

Roughly 60% of the Earth’s outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000u2009m). A pyroclastic deposit has never been observed on the sea floor below 3,000u2009m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85°u2009E, to acquire photographic and video images of ‘zero-age’ volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000u2009m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limuu2009ou2009Pele), covering a large (>10u2009km2) area. At least 13.5u2009wt% CO2 is necessary to fragment magma at these depths, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.

Deep sea underwater robotic exploration in the ice-covered Arctic ocean with AUVs

The Arctic seafloor remains one of the last unexplored areas on Earth. Exploration of this unique environment using standard remotely operated oceanographic tools has been obstructed by the dense Arctic ice cover. In the summer of 2007 the Arctic Gakkel Vents Expedition (AGAVE) was conducted with the express intention of understanding aspects of the marine biology, chemistry and geology associated with hydrothermal venting on the section of the mid-ocean ridge known as the Gakkel Ridge. Unlike previous research expeditions to the Arctic the focus was on high resolution imaging and sampling of the deep seafloor. To accomplish our goals we designed two new Autonomous Underwater Vehicles (AUVs) named Jaguar and Puma, which performed a total of nine dives at depths of up to 4062m. These AUVs were used in combination with a towed vehicle and a conventional CTD (conductivity, temperature and depth) program to characterize the seafloor. This paper describes the design decisions and operational changes required to ensure useful service, and facilitate deployment, operation, and recovery in the unique Arctic environment.

Hemispherical refraction and camera calibration in underwater vision

We examine the challenges to underwater vision that are caused by light refracting through both hemispherical and planar pressure housing interfaces. As light travels through water, into a pressure housing, and finally into a camera, it is bent according to Snells law, rendering the typically-used perspective camera model invalid. Through numerical simulation, we examine the degree to which the incorrect perspective model results in 2-D image distortion, and errors in 3-D scene reconstruction computed using stereo vision or structure from motion. We focus on the use of hemispherical pressure housing interfaces with imperfectly mounted cameras, and include comparisons with cameras behind planar interfaces.We also address the problem of calibrating a camera model which takes into account the physical parameters of the pressure housing interface.

Map Building Fusing Acoustic and Visual Information using Autonomous Underwater Vehicles

We present a system for automatically building three-dimensional (3-D) maps of underwater terrain fusing visual data from a single camera with range data from multibeam sonar. The six-degree-of-freedom location of the camera relative to the navigation frame is derived as part of the mapping process, as are the attitude offsets of the multibeam head and the onboard velocity sensor. The system uses pose graph optimization and the square root information smoothing and mapping framework to simultaneously solve for the robots trajectory, the map, and the camera location in the robots frame. Matched visual features are treated within the pose graph as images of 3-D landmarks, while multibeam bathymetry submap matches are used to impose relative pose constraints linking robot poses from distinct tracklines of the dive trajectory. The navigation and mapping system presented works under a variety of deployment scenarios on robots with diverse sensor suites. The results of using the system to map the structure and the appearance of a section of coral reef are presented using data acquired by the Seabed autonomous underwater vehicle.

Beyond point measurements: sea ice floes characterized in 3-D

A new methodology for coincident floe-scale measurements of the surface elevation, snow depth, and ice draft (the thickness below the water line) of Antarctic sea ice has been demonstrated during two recent research voyages: the Australian-led Sea Ice Physics and Ecosystem Experiment II (SIPEX II) to East Antarctica in September–November 2012 and the United Kingdom–led Ice Mass Balance in the Bellingshausen Sea (ICEBell) voyage to the Weddell and Bellingshausen Seas in November 2010

Stereo self-calibration for seafloor mapping using AUVs

Visual maps of the seafloor should ideally provide the ability to measure individual features of interest in real units. Two-dimensional photomosaics cannot provide this capability without making assumptions that often fail over 3-D terrain, and are generally used for visualization, but not for measurement. Full 3-D structure can be recovered using stereo vision, structure from motion (SFM), or simultaneous localization and mapping (SLAM); of these techniques, only stereo vision is suitable for fully dense (a distance measurement for each imaged pixel) 3-D structure in the absence of significant frame-to-frame overlap. Stereo vision is notoriously dependent on camera calibration, however, which is difficult to compute and maintain in the field. The fewer dependencies an AUV mapping system has on camera calibration, the more reliably it will be able to produce useful maps of the seafloor. We present a system for recovering the 7-DOF relationship between the AUVs estimation frame and the camera rig (Euclidean offsets plus scale), which reconciles the robots odometry-based pose estimate with stereo visual odome-try. The combination of robust frame-to-frame visual feature matching, subpixel stereo correspondence estimation, and high-accuracy on-board vehicle navigation sensors enables us to self-calibrate the extrinsic parameters of the stereo rig including scale, and produce metric maps using only vehicle navigation and the computed camera calibration. Using data acquired in the Bering Sea by the SeaBED AUV in August, 2009, our initial results indicate that accumulated navigation drift is less than 0.5% of distance travelled, suggesting that a visual SLAM system for correcting drift and building a final map would only require the robots path to cross itself every few hundred meters. In addition to providing a large-scale metric 3-D map, the corrected stereo calibration enables scientists to measure the sizes of imaged objects without additional hardware such as laser points or acoustic ranging systems.

Effusive and explosive volcanism on the ultraslow-spreading Gakkel Ridge, 85°E

[1]xa0We use high-definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 Arctic Gakkel Vents Expedition (AGAVE) to evaluate the volcanic characteristics of the 85°E segment of the ultraslow spreading Gakkel Ridge (9xa0mmxa0yr−1full rate). Our seafloor imagery reveals that the axial valley is covered by numerous, small-volume (order ∼1000xa0m3) lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10xa0cm. The valley floor contains two prominent volcanic lineaments made up of axis-parallel ridges and small, cratered volcanic cones. The lava flows appear to have erupted from a number of distinct source vents within the ∼12–15xa0km-wide axial valley. Only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site, and these are associated with the Oden and Loke volcanic cones. We model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of CO2 that accumulated in (possibly deep) crustal melt reservoirs. The energy released during explosive discharge, combined with the buoyant rise of hot fluid, lofted fragmented clasts of rapidly cooling magma into the water column, and they subsequently settled onto the seafloor as fall deposits surrounding the source vent.

Advances in Platforms and Algorithms for High Resolution Mapping in the Marine Environment

A confluence of technologies is changing the manner in which we approach the use of Autonomous Underwater Vehicles (AUVs) in the marine environment. In this paper we review the role of several of these technologies and the way interactions between them will now enable the use of adaptive methodologies for mapping and exploring the underwater environment. We focus primarily on imaging sensors but these methodologies are widely applicable for other types of sensing modalities as well. We look at the role of acoustic telemetry, multi-hop underwater data transmission, in-situ machine learning techniques, and mapping in highly dynamic environments such as under sea ice. In addition, we discuss the role of “hobby” robotics for surface and aerial vehicles in the marine environment.