A. M. Bradley

Techniques for Deep Sea Near Bottom Survey Using an Autonomous Underwater Vehicle

This paper reports the development and at-sea deployment of a set of algorithms that have enabled the autonomous underwater vehicle ABE to conduct near-bottom surveys in the deep sea. Algorithms for long baseline acoustic positioning, terrain-following, and automated nested surveys are reported.

Gradient search with autonomous underwater vehicles using scalar measurements

We examine how AUVs can be used to locate features of interest from scalar measurements without exhaustive search. It is first shown how the Autonomous Benthic Explorer (ABE) was used to successfully locate the deepest part of a pond. ABE computed the total water depth by combining the computed vehicle depth with the return of a single beam sonar altimeter. While moving in a straight line, ABE was able to estimate the gradient of total water depth along the vehicle track. By executing circular maneuvers, ABE was further able to estimate the 2D horizontal slope of the bathymetry. These two functions were combined to produce a procedure to locate the deepest point in the search region. This was accomplished without the need for a global navigation system. It is discussed how a vehicle with the capabilities of ABE could be used to locate hydrothermal vents using temperature, optical backscatter or chemical tracers as the search parameter. Data from an actual preprogrammed survey conducted by ABE in an area of sea-bed hydrothermal activity is illustrated. This data has been combined with measurements conducted by moorings, towed vehicles and the manned deep submersible Alvin to create a mathematical model of a vent field as might be seen by an AUV maneuvering through the region. Using this model we will show techniques for locating hydrothermal vents based on gradient following and other optimization methods that exploit the current understanding of the structure and physics of hydrothermal plumes.

Insights into tide-related variability at seafloor hydrothermal vents from time-series temperature measurements

Abstract Thermocouple/thermistor array packages and an in situ gamma detector were deployed in 1994 at two vent sites on the northern Cleft Segment of the Juan de Fuca Ridge. Continuous records of fluid temperatures were obtained in four separate locations over a period of 5.5 months, and these data were supplemented by current meter observations made ∼2.5 km to the south within the axial valley. Temperatures measured at a location of focused high temperature flow showed that: (1) the maximum temperature in the chimney was stable and did not exhibit tide-related variability; (2) temperatures within the chimney wall were variable on time-scales of minutes, indicating rapid shifts in amounts of cold seawater or hot vent fluid flowing across chimney walls; and (3) the stable maximum temperature within the chimney conduit was ∼9°C less than the maximum fluid temperature recorded at the vent site during the same time interval, and thus stable high temperatures within chimneys are not necessarily indicative of the maximum temperature within the vent structure or the hydrothermal system. Time-series records from areas of diffuse flow indicate modulation of temperature and total radioactivity by tidally induced changes in bottom currents. Spectra of the current meter record and temperature records are similar, with spectral peaks observed at 12.4 h, 16–17 h (inertial peak) and 4–5 days. Phases between maxima in current, tide and temperature records are consistent with temperature changes resulting from periodic shifts in currents from north to south, and the subsequent northward or southward advection of warm fluids venting from multiple local sources. Periodic (12.4 h) variability of temperature was also recorded by a thermocouple buried ∼1 cm within one of the deposits and is likely a result of periodic variations in the temperature at the boundary of the highly conductive sulfide deposit. The time-series results presented demonstrate the need for measuring and considering the effects of local currents when investigating causes of temporal variability within seafloor hydrothermal systems.

Power systems for autonomous underwater vehicles

In this paper, we examine the issues involved in designing battery systems and power-transfer (charging) techniques for Autonomous Underwater Vehicles (AUVs) operating within an Autonomous Ocean Sampling Network (AOSN). We focus on three different aspects of the problem, battery chemistry, pack management and in situ charging. We look at a number of choices for battery chemistry and evaluate these based on the requirements of maximizing power density and low temperature operation particular to AUVs. We look at the issues involved in combining individual cells into large battery packs and at the problems associated with battery monitoring, and the charging and discharging of packs in a typical AUV application. Finally, we present a methodology for charging an AUV battery pack in situ in support of long term deployments at remote sites.

Surveying a subsea lava flow using the Autonomous Benthic Explorer (ABE)

This paper summarizes results from the first science deployment of the Autonomous Benthic Explorer (ABE), conducted on the Juan de Fuca Ridge (46°N, 129°W) at depths between 2200 and 2400 m. Using long baseline acoustic transponders, the ABE descended with precision to a preassigned starting point, then executed dead-reckoned tracklines. It followed the bottom at distances between 7 and 20 m using an acoustic fathometer as a reference sensor. The ABE mapped a new subsea lava flow with a magnetometer, imaged the seafloor with a stereo snapshot video system, and mapped a hydro thermal plume with conductivity and temperature sensors. The ABE completed 7 successful dives and covered over 35 km of tracklines. Detailed power records were logged, which permits extrapolation of the ABEs performance to other missions and higher capacity batteries.

In situ measurement of dissolved H2 and H2S in high-temperature hydrothermal vent fluids at the Main Endeavour Field, Juan de Fuca Ridge

The first in situ measurements of dissolved H2 and H2S in high-temperature vent fluids were made at the Main Endeavour Field (Juan de Fuca Ridge) using the submersible Alvin and a newly developed electrochemical sensor. The measurements were successfully conducted in chimneys at sites of venting fluid and in pools of more quiescent hydrothermal fluid that underlie flanges on chimney structures at a depth of 2200 m below the sea surface. Fluid temperatures measured simultaneously with dissolved gas concentrations were up to 370°C. At the highest temperatures, dissolved H2 and H2S concentrations were 0.72 and 17.3 mmol/kg, respectively, which are consistent with data obtained at the same sites through conventional sampling methods. The relatively high concentration of dissolved gases measured by both techniques, however, may be linked to recent tectonic and volcanic activity. The ability to measure in situ dissolved gas concentrations simultaneously with fluid temperature in real time represents a major advance in the approaches available to study the origin and temporal evolution of seafloor hydrothermal systems at mid-ocean ridges. Although the present investigation is primarily based on sensor deployment for relatively short-term measurement of vent fluids, long-term monitoring of vent fluid holds great promise for further applications.

Fine-scale seafloor survey in rugged deep-ocean terrain with an autonomous robot

The rugged, mountainous regions of the deep seafloor hold both great scientific interest as well as a host of difficult challenges for autonomous robots. Exploiting its abilities for precise navigation, trackline following, and bottom-following in rough terrain, the autonomous benthic explorer collected fine-scale bathymetry, magnetic, temperature, optical backscatter and conductivity over a rugged, neo-volcanic and active tectonic zone of the southern east Pacific rise (18/spl deg/S). We combined these data sets to produce a variety of maps showing dramatic terrain in unprecedented detail. We also created the first systematic map view of hydrothermal temperature anomalies over the mid-ocean ridge.

Thickness of a submarine lava flow determined from near‐bottom magnetic field mapping by autonomous underwater vehicle

Magnetic field surveys obtained near the seafloor can map the boundaries of recent volcanic eruptions and can provide thickness estimates of these lava flow units independent of bathymetry differencing methods. Magnetic thickness estimation requires knowledge of the intensity of magnetization of the new lava and surrounding terrain, but this can be satisfactorily obtained by representative sampling of the various volcanic units. While bathymetry differencing requires pre-existing data to assess the thickness of new lava eruptions, magnetic surveys can be obtained after an eruption has occurred. In this study, near-bottom magnetic surveys were obtained using an autonomous underwater vehicle (AUV), which operates without a tether or human intervention. AUV technology offers rapid deployment and an efficient surveying approach for remotely mapping recent lava eruption sites on the seafloor.

Waxing and waning volcanism along the East Pacific Rise on a millennium time scale

Microbathymetric maps of the southern East Pacific Rise reveal subtle field relations between volcanic features and provide new insight on seafloor spreading processes. Along one of the shallowest and broadest sections of ridge at 17°28′S, lavas have erupted from a fissure system and flooded the axis through a network of lava tubes and lava channels. Along the neighboring ridge segment at 18°15′S, the axial area has subsided and formed a broad tectonized trough. A swath of newly accreted crust has since widened that trough; late-stage volcanism consists of small circular pillow mounds. We propose that these contrasting eruptive styles reflect the waxing and waning phases of a common magmatic evolution spanning a few millennia.

Deep submergence synergy: Alvin and ABE explore the Galapagos Rift at 86°W

For over 25 years, hydrothermal vent communities discovered at the Galapagos Rift near 86°W [e.g., Corliss et al., 1979] have provided the foundation of deep-sea vent biology as their study has led to fundamental discoveries of chemoautorophy and novel symbioses in the deep sea [e.g., Cavanaugh et al., 1981]. Since 1979, numerous physiological and geochemical investigations of the Rose Garden vent community [e.g.,Hessler et al., 1988] have been made possible through routine access to this deep sea floor site, provided by the deep submergence vehicle Alvin. This research revolutionized our understanding of basic biological and chemical processes in the deep ocean [e.g. Johnson et al., 1988; Edmond et al., 1979]. In May–June 2002, a sea floor sampling and near-bottom mapping program was conducted using R/V Atlantis (AT7–13),the submersible Alvin, and the autonomous underwater vehicle ABE (Autonomous Benthic Explorer) [Yoerger et al., 1998] to explore and study hydrothermal processes along the Galapagos Spreading Center (GSC) between 86°W and 90°W (Figure 1). This 12-day expedition coincided with the 25th anniversary of the discovery of deep-sea hydrothermal vents at the Galapagos Rift (http://wwwdivediscover.whoi.edu; Expedition 6). It included a planned revisit of the Rose Garden vent field to conduct multidisciplinary time-series observations and sampling that would represent a quarter-century perspective at this longest-studied, active hydrothermal vent field. The fieldwork resulted in the discovery of important geological, hydrothermal, and biological changes that have occurred at the Rose Garden site. During the first few Alvin dives of the cruise, it was discovered that the well-developed faunal communities last documented 13 years ago at Rose Garden were apparently buried by fresh basaltic sheet flows. Notable was the absence of 14 sea floor markers used for past experiments and 7 stacks of Alvin dive weights observed on dive 2224.