Developing a Global Listening Network for Turbidity Currents and Seafloor Processes

This ambitious project can make a major step-change in understanding of seafloor processes and geohazards by developing a listening network based on low-cost hydrophones (via acoustic noise in water column) and geophones (via ground shaking).

This type of low-cost network has unusually widespread applications, but here we aim to answer fundamental questions about how submarine mass-flows (turbidity currents and landslides) are triggered, and then behave. These hazardous and often powerful (2-20 m/s) submarine events form the largest sediment accumulations, deepest canyons, and longest channel systems on our planet. Turbidity currents can runout for hundreds to thousands of kilometres, to break seabed cable networks that carry >95% of global data traffic, including the internet and financial markets, or strategic oil and gas pipelines. These flows play a globally important role in organic carbon and nutrient transfer to the deep ocean, and geochemical cycles; whilst their deposits host valuable oil and gas reserves worldwide.

Submarine mass flows are notoriously difficult to measure in action, and there are very few measurements compared to their subaerial cousins. This means there are fundamental gaps in basic understanding about how submarine mass flows are triggered, their frequency and runout, and how they behave. Recent monitoring has made advances using power-hungry (active source) sensors, such as acoustic Doppler current profilers (ADCPs). But active-source sensors have major disadvantages, and cannot be deployed globally. They can only measure for short periods, are located on moorings anchored inside these powerful flows (which often carry the expensive mooring and sensors away), and they need multiple periods of expensive research vessels to be both deployed and recovered. We will therefore design, build and test passive sensors that can be deployed over widespread areas at far lower cost. These novel sensors will record mass-flow timing and triggers; and changes in front speed (from transit times), and flow power (via strength of acoustic or vibration signal).

We will first determine how submarine mass flows are best recorded by hydrophones and geophones, and how that record varies with flow speed and type, or distance to sensor. Our preliminary work at three sites already shows that hydrophone and geophones do record mass-flows. Here we will determine the best way to capture that mass-flow signal, and to distinguish it from other processes. This will form the basis for then designing and field testing a new generation of low-cost smart sensors, which return data without expensive surface vessels; via pop-up floats and satellite links. Advances in technology make this project timely, as they allow on-board data processing by smart hydrophones to reduce data volumes, which can be triggered to record for short periods at much higher frequency. We will test the new smart sensors, and use them to answer two major science questions. First, do submarine flows in different settings show consistent modes of behaviour? Second, what triggers submarine flows in river-fed systems, and how are they linked to major river floods, earthquakes, and tropical cyclones? To do this, we will place these new sensors along the Congo Canyon (dilute river, passive margin, no cyclones) off West Africa, and the Gaoping Canyon (hyperpycnal river, active margin, frequent cyclones) offshore Taiwan.

These sensors have other widespread applications. Low cost warning sensors would be a major advance for offshore hazard assessment, and leaks from CCS facilities or gas pipelines. Sensors that record landslides would be a step change for tsunami warning systems, or threats to valuable seabed infrastructure. This proposal is also particularly timely, because of advances in technology now allow on-board data processing and communication between smart sensors, which can be triggered to record for short periods at much higher frequency.

Beneficiaries of this work are unusually wide-ranging, both from new insights into submarine mass flows, and the development of low-cost sensor networks with widespread applications.

Seafloor telecommunication cables: Seabed cable networks have strategic importance because they carry >95% of global data traffic, including internet and financial markets. It is important to understand spatial changes in turbidity current frequency and power to optimise the location of future cables. It is also important to understand which types and speeds of turbidity current break cables, and which do not. This project will study the Gaoping Canyon, where flows have previously broken many seafloor cables repreatedly, which is a major pinch point in the global cable routes. We will work with Carter of the ICPC, an umbrella organisation for submarine cable owners and operators, to disseminate results.

Marine cyber-security: There is increasing concern over human tampering with seabed cables using Remotely Operated Vehicles (ROVs), as set out in a 2017 report by UK Member of Parliament, Rishi Sunak. A key recommendation of that report is to develop remote sensing systems that can warn against the approach of a ROVs. We will test whether low-cost hydrophones, which transmit a warning via a pop-up data pod, can successfully detect the approach of an ROV. Our results will be communicated to the Department for Digital, Media, Culture and Sport who have formal responsibility for seafloor cable security; and to the ICPC via Carter.

Hazards to seabed pipelines and other infrastructure: Turbidity current and landslides pose a substantial threat to oil and gas pipelines, and other seabed infrastructure. First, our project will help to understand how frequency, duration and power of flows varies with distance along canyon, and hence where pipelines are best routed. Second, we will test whether low-cost hydrophones and geophones can provide a reliable early warning for hazardous submarine mass flows. We will disseminate results via Clare's Knowledge Exchange Fellowship, and via presentations at the Offshore Technology Conference.

Leaks from pipelines: Hydrophones have successfully located and quantify gas leaks from pipelines in the North Sea (Wiggin et al., 2015). This project will show how low-cost hydrophones can transmit data without an expensive surface vessel, which may warn against leaks.

Leaks from carbon capture and storage (CCS) facilities: Hydrophones can remotely detect and quantify gas leakage over large areas, and thus currently play an important role in monitoring CCS facilities. This project will contribute to such efforts in producing smart hydrophones, which can be triggered to record at higher frequencies, and which return data without the need for a surface vessel.

Oil and Gas Reservoir Characterisation: Deposits of turbidity currents host valuable oil and gas reservoirs in locations worldwide. This project will produce two of the most complete datasets from across two large submarine systems (Congo and Gaoping), where deposits can be compared to direct flow measurements. Laboratory-scale experiments and numerical models underpin many reservoir models, and understanding of these flows more generally. This project will produce a timely and robust test of such models, as predicted flow evolution can be compared to the observed changes in flows.

Marine Biologists: An important use of hydrophones is determine presence and behaviour of marine mammals. Our project helps to develop low-cost hydrophones that return their data without a surface vessel, which have widespread application for marine biologists. Turbidity currents supply important nutrients to submarine canyon ecosystems, which are hot spots of biological diversity. Our work in Gaoping and Congo Canyon will thus help to understand benthic ecosystems function in the deep sea.