GPS-Acoustics unlock a new approach to seabed geodetics
Guest Author: Liquid Robotics
Autonomous and unmanned marine platforms are transforming the way in which we acquire and analyse data from our oceans. We take a closer look at seafloor geodesy, an emerging scientiﬁc ﬁeld that is making the real-time study of continental plate tectonics a cost-effective and viable option.
Seafloor geodesy projects are underway across the globe, all in pursuit of scientiﬁc advances that will help us crack the code on earthquake and tsunami risk.
What is the shared goal? To better understand earthquakes, tectonic processes and tsunami hazards, and ultimately save lives. In addition, the technology is being applied within the offshore oil and gas industry to mitigate risk through better ongoing surveillance during the producing life of a ﬁeld.
But how can you track seafloor and oilﬁeld infrastructure movement at minute scale resolution through two miles of seawater and deliver the results straight to an analyst’s desk anywhere in the world?
The satellite-based GPS and laser methods used on land don’t work in the ocean, so researchers are turning to a technique that’s referred to as GPS-Acoustics (GPS-A). While it’s possible to do these surveys using a vessel, to make GPS-A measurements cost-effective, unmanned surface vehicles like the Liquid Robotics Wave Glider and long endurance Sonardyne surveillance technology offer a solution.
Instruments such as Sonardyne’s Autonomous Monitoring Transponder (AMT) are designed to autonomously and precisely measure horizontal and vertical displacement using thousands of range (distance between pairs of transponders), pressure (depth), sound velocity , and inclination measurements. Each unit runs a fully automatic data gathering and logging regime and can remain continuously deployed for up to 10 years.
Operating at the surface, the wave and solar powered Wave Glider can be used to precisely position each AMT in depths exceeding 5,000 metres. By returning to the location on a regular basis, the absolute change in position of each AMT can be measured with millimetric accuracy to reveal their movement due to plate tectonic action. When required, it also serves as a robust communications gateway – acoustically harvesting the data logged inside each unit and transmitting it to shore in real-time.
Wave Glider payload
Sonardyne’s GPS-A module, carried in the hull of the Wave Glider, takes advantage of developments within low power electronics to provide a data acquisition, processing and seafloor-to-surface-to-shore link.
Inside is a dual core processor mounted onto a custom designed interface board which brings together survey-grade accuracy GPS, MEMS-based pitch, roll and heading in addition to GPS-derived heading. The tight coupling of these sensors, combined with Sonardyne’s 6G wideband acoustics and the ability to feed in additional external inputs, delivers the capability – after post-processing –to detect the smallest of movements in a fault line or structure.
Orbital path correction
To be able to accurately position a subsea transponder, you must ﬁrst be able to accurately position the unmanned surface vessel. So during a mission, the Wave Glider GPS-A payload records GPS logs in Receiver Independent Exchange (RINEX) format; a data interchange format for raw satellite navigation system data. This is a critical and unique part of the system’s ability to achieve millimetric precision.
By taking the GPS RINEX ﬁles and post processing this data with the corrected orbital paths of the GPS satellites themselves, it is possible to reduce the RMS (Root Mean Square) of the positioning by up to 30 times compared to typical GPS receiver accuracy.
Table 1 shows an Easting and Northing position scatter plot for an unmanned surface platform before (blue) and after (red) GPS-A post processing. The data was captured during a trial at Sonardyne’s research facility in Plymouth.
Similarly in Table 2, the plot shows a surface height comparison over a complete tidal cycle. Once again, the data in blue is the raw observations, whilst the data in red is the RINEX post processed results.
With these advances in technology , seafloor geodesy projects have sprung up around the world, in particular around the Ring of Fire in the Paciﬁc basin where some of the most powerful earthquakes originate.
The Cascadia Subduction Zone
Dr. David Chadwell of Scripps Institute of Oceanography is working with the United States Geological Survey (USGS) to better understand the Cascadia Subduction Zone in order to better predict when a major event is more likely to occur.
After selecting Sonardyne’s Fetch instrument (functionally equivalent to the AMT , but with a much bigger battery that enables deployments of up to 10 years) for the seabed component of the study and validating their abilities using a research vessel, Dr. Chadwell was looking for a more cost-effective platform to collect their data. His original plan was to use a diesel powered buoy but he soon recognised the mobility and longevity advantages the Wave Gliders could offer. Dr. Chadwell has also been recognised by further funding to extend this seafloor geodesy research north to the Aleutian Islands.
The Mentawai Seismic Gap
At the Earth Observatory of Singapore, Dr. Sylvain Barbot, Dr. Emma Hill, and Dr. Sharadha Sathiakumar are working to better understand seismic hazards in Indonesia.<
The 2004 Sumatra earthquake and tsunami triggered a series of earthquakes along the Sunda subduction zone. The Mentawai seismic gap is one of the remaining regions that did not experience a large earthquake in the last decade.
An extensive land-based network for geodetic measurements has been installed on islands along the fault line, but there are gaps offshore that prevent understanding tsunami generation dynamics. Seafloor geodesy can ﬁll that observation gap.
Here, researchers are also equipping Wave Gliders with GPS-A technology to monitor seafloor deformation off the coast of Sumatra. An unmanned platform is essential,as regular surveys using research vessels are just too expensive. The ultimate goal is to move towards persistent ocean laboratories (or towards extensive offshore geodetic networks) that can cover a large spatial footprint and build a multi-decade time-series of data on seafloor deformation.
Nazca-South American Plate Boundary
Off the coast of northern Chile, where some of the most powerful earthquakes on the planet originate, scientists from GEOMAR Helmholtz Centre for Ocean Research Kiel have installed a seafloor geodetic network called GeoSEA (Geodetic Earthquake Observatory on the SEAfloor) at depths ranging from 2,600 – 6,000 metres. In this case, rather than using absolute GPS-A measurements, the relative movement of the AMTs to each other is measured using the on-board pressure sensors and acoustic ranging between the AMTs. Once installed, the next challenge was how to retrieve the seafloor data frequently and cost-effectively.
As reported in Issue 18 of our Baseline magazine, the GeoSEA network consists of AMTs installed in three areas along the NazcaSouth American plate boundary , an area identiﬁed to be in the latest stage of the seismic cycle. The other key component of the network is a GPS-A equipped Wave Glider.
Operating autonomously at the surface, the vehicle holds position above the seafloor stations, monitors system health, uploads data from the seafloor node, and transfers it back to shore via satellite – allowing the research vessel to focus on other more valuable tasks.
Dr. Heidrun Kopp, Chief Scientist at GEOMAR, said retrieving data with a Wave Glider was an important ﬁrst step to proving the capability of the network. “In the future, as we think about other seafloor geodesy projects in remote places, these would not be possible without the Wave Glider.”
GPS-A is also being applied to oilﬁeld asset monitoring. For example, if a pipeline is suspected of creeping due to axial strain, the survey choices are somewhat limited. You can send out a vessel of opportunity then launch an ROV to visually observe and measure the creep. However, it’s not cost effective or practical to do this very often because it can cost $1-$2 million alone just to get to the location. Alternatively, it can be monitored with data loggers, but until the data is downloaded, it’s impossible to know what is really going on.
Instead, imagine using AMTs deployed on and near the pipe communicating with a GPS-A Wave Glider patrolling above, enabling asset teams thousands of miles away to be alerted to movement in real-time.
Reimagining ocean monitoring
Gathering seabed geodetic data is difﬁcult, slow, expensive and not without risk to the people sent out to get the job done. With long endurance instruments, such as Sonardyne’s Ambient-Zero-Ambient (AZA), which overcomes the inherent problem of pressure sensor drift, and a Wave Glider at the surface to position the transponders and transmit the data, there is now a viable alternative that provides near real-time awareness of plate tectonic activity.
The impact of the technologies developed by Sonardyne and Liquid Robotics goes far beyond simply providing a cost-effective alternative to crewed vessels, as researchers pursue breakthroughs in earthquake and tsunami early warning systems that may ultimately save lives. These solutions are proven and ready for deployment today, so if you’re working on a seafloor geodesy , asset monitoring or subsea communications gateway project, get in touch with our organisation at: www.liquid-robotics.com
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