No abstract

Hydrological data from a repeated cross‐shore transect obtained using Teledyne Webb Research Slocum Electric gliders offshore Two Rocks in south‐western Australia over 13 months are presented. The data revealed that formation of dense water inshore and its transport across the shelf as a near bed gravity current (defined as Dense Shelf Water Cascade, DSWC) was a regular occurrence, particularly during autumn and winter months. In autumn, the dense water is mainly formed through changes in salinity resulting from evaporation, whilst in winter; temperature change through surface cooling was the dominant factor. The mean wind speeds also decrease during the transition during autumn. The speed of the DSWC was estimated to be 0.01–0.02 m s−1, and similar to that measured in other selected regions globally. The offshore transport from the shelf is a significant component of the alongshore wind‐driven transport.

Underwater robots are subject to position drift due to the effect of ocean currents and the lack of accurate localisation while submerged. We are interested in exploiting such position drift to estimate the ocean current in the surrounding area, thereby assisting navigation and planning. We present a Gaussian process (GP)-based expectation-maximisation (EM) algorithm that estimates the underlying ocean current using sparse GPS data obtained on the surface and dead-reckoned position estimates. We first develop a specialised GP regression scheme that exploits the incompressibility of ocean currents to counteract the underdetermined nature of the problem. We then use the proposed regression scheme in an EM algorithm that estimates the best-fitting ocean current in between each GPS fix. The proposed algorithm is validated in simulation and on a real dataset, and is shown to be capable of reconstructing the underlying ocean current field. We expect to use this algorithm to close the loop between planning and estimation for underwater navigation in unknown ocean currents.

Since 2008, 26 glider missions have been undertaken along the continental shelf of southeastern Australia. Typically these missions have spanned the continental shelf on the inshore edge of the East Australian Current from 29.5–33.5°S. This comprehensive dataset of over 33,600 CTD profiles from the surface to within 10 m of the bottom in water depths ranging 25–200 m provides new and unprecedented high resolution observations of the properties of the continental shelf waters adjacent to a western boundary current, straddling the region where it separates from the coast. The region is both physically and biologically significant, and is also in a hotspot of ocean warming. We present gridded mean fields for temperature, salinity and density, but also dissolved oxygen and chlorophyll-a fluorescence indicative of phytoplankton biomass. This data will be invaluable for understanding shelf stratification, circulation, biophysical and bio-geochemical interactions, as well as for the validation of high-resolution ocean models or serving as teaching material.

Offshore exploration commonly uses geochemical sniffer technologies to detect hydrocarbon seepage. Advancements in sniffer technology have seen the development of submersible in-situ methane sensors. By integrating a Franatech laser methane sensor onto an autonomous underwater glider platform, geochemical survey durations can be increased, and associated exploration costs reduced. This paper analyses the effectiveness of methane detection using the integrated system and assesses its practical application to offshore hydrocarbon seep detection methods. Blue Ocean Monitoring surveyed the Yampi Shelf, an area with known oil and gas accumulations, and observed hydrocarbon seeps on the North West Shelf of Australia. Results from the survey showed a background dissolved methane concentration of 3 to 4 volumes per million (vpm). A distinct plume of methane between 30 to 84 vpm measured over 24 km2 was detected early in the survey. Three smaller plumes were also identified. Within a small plume, the highest concentration of methane was detected at 160 vpm. Methane above background levels was observed within 8 km of previously identified seeps; however, these seeps were unable to be pinpointed. Comparisons with data from previous surveys suggest similar oceanographic influences on the behaviour of the seeps, including tidal variations and the position of the thermocline. The results demonstrated that the integrated system may be used to effectively ground truth remote sensing interpretations and survey areas of interest over long durations, providing methane presence or absence results. To this effect, the integrated system may be implemented as a supporting technology for assessing the risks of further funding hydrocarbon detection surveys and focusing the area of interest before the deployment of vessel-based surveys.