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Feature Article

The Role of AUV Technology in Seafloor Massive Sulfide Exploration

By Dr. Ian Stevenson
Chief Geophysicist
Sean Plunkett
Senior Geophysicist
Nautilus Minerals Inc.
Brisbane, Australia

The Abyss AUV operations loaded in the launch and recovery system.
Seafloor massive sulfides (SMS) deposits are typically located in water deeper than 1,000 meters and in close proximity to tectonic plate boundaries and submarine volcanic activity. Commercial exploration of SMS deposits requires mapping the seafloor at various scales, from regional mapping (greater than 10,000 square kilometers) to detailed prospect delineation (1 to 10 square kilometers). Vessel-based multibeam echosounder (MBES) surveys are commonly used at the regional scale of mapping. Follow-up detailed high-resolution mapping has been done with either ROVs or deep-towed geosurvey platforms.

With the SMS exploration and production industry still in its early stages, significant opportunities exist to reapply existing technologies to this new industry. One technology with significant market benefit and growth potential is AUVs. Nautilus Minerals (Brisbane, Australia), which holds almost 600,000 square kilometers of exploration tenements granted or under application, thinks AUV technology can deliver a faster and improved assessment exploration technique to decrease the cost and delivery time of prospective seafloor sampling and drill targets. The company is developing a copper-gold production operation at Solwara 1, located in the territorial waters of Papua New Guinea, and building an exploration pipeline in the western Pacific.

SMS Exploration Techniques
During the early stages of exploration, vessel-based MBES systems are used to map large areas of the seafloor, typically at 25-meter cell size. This enables identification of broad-scale volcanic and tectonic features associated with SMS mineralization. While this technique is rapid and efficient for surveying large areas, it does not provide the resolution needed to confidently discriminate the mound features and chimney structures associated with SMS formation.

In addition to vessel-based MBES operations, CTD sensor tow-yo operations are undertaken to map plume signatures in the water column from active (venting) hydrothermal systems. This technique involves the deployment of a deep-tow sled behind the survey vessel, which is repeatedly winched in and out as the vessel steams along a predefined course, so that the sled acquires a continuous vertical profile of data within the water column, using CTD, Eh and light-scattering sensors.

Once regional targets have been identified, in order to proceed to prospect delineation, high-resolution surveys must be conducted closer to the seafloor. Historically, this detailed mapping has included use of deep-tow side scan sonar technology, which is limited by reduced survey speed (maximum 2 knots), poor positional control of the tow-body due to the long length of tow cable behind the survey vessel and limitations of tow altitude (100 to 300 meters above the seafloor) required to avoid collision in predominately rugged volcanic terrain. The result is limited resolution of typically 1-to-2-meter cell size.

Another method for detailed seafloor mapping uses ROV technology. Although ROVs can host numerous geophysical mapping sensors and are well-suited to ground intervention and opportunistic sample recovery (geological rock specimens or hydrothermal chimney fragments), they are an inefficient means of mapping the seafloor and limited to maximum survey speeds of 0.5 to 0.7 knots. In addition, ROVs are not well-suited for continually traversing rugged volcanic terrain, and it is difficult to sustain long periods of productivity and minimize equipment breakdown unless two ROVs are utilized.

Despite their limitations, a combination of these techniques has yielded a high success rate in the discovery of SMS prospects for Nautilus Minerals on its exploration tenements.

AUV Technology
AUVs could bridge the gap between target generation from vessel-based MBES acquisition and detailed, prospect-scale ROV intervention for SMS exploration. AUVs provide the operational flexibility to survey with a variety of sensor payload configurations at different scales of resolution, provide higher-resolution mapping than is possible with conventional deep-tow technology and offer significant operational productivity gains over ROVs.

Conducting high-resolution geophysical surveys by AUV would enable ROVs to be deployed more effectively for other tasks to which they are better suited, such as selective follow-up ground truthing, sampling and electromagnetic surveys. To continue this article please click here.

Dr. Ian Stevenson has worked in the marine minerals exploration and mining industry for the past 23 years, specializing in the development and application of high-resolution geophysical techniques and AUV mapping payloads. He has a Ph.D. in geophysics from the University of Reading.

Sean Plunkett is a senior geophysicist for Nautilus Minerals and has been heavily involved in a wide range of seafloor and water-column interrogation techniques utilized in SMS exploration. He holds a bachelor's in geophysics from Curtin University of Technology.

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