What is GNSS-A?

GNSS-A works by combining satellite positioning with underwater acoustics to track seafloor movement with centimetre-level accuracy. An uncrewed surface vessel (USV) with Sonardyne’s GNSS-A payload patrols above an array of Sonardyne Fetch transponders on the seabed.

Combining its known surface position with acoustic pulses down to each Fetch transponder allows it to calculate the precise position of each transponder – and therefore the position of the seabed it’s sitting on.

By repeating these measurements over time, scientists can track the movement of tectonic plates across faults to better understand and estimate earthquake and tsunami hazard.

One of the organisations using this technique to monitor subduction zones, where one plate is sliding under another, is the USGS.

They first started exploring its use in 2017, working in collaboration with the University of Hawaii and Scripps, and using a Wave Glider and seabed sensors, in order to measure how friction between two tectonic plates restricts sliding and causes a build-up of stress – essentially measuring “how stuck are the plates”.

Since then, they’ve continued to contribute to the development of GNSS-A using Sonardyne GNSS-A modules and Fetch transponders.

Critical insights from the Aleutian subduction zone

One of the tectonic sources of large earthquakes that USGS has been monitoring is the Aleutian Subduction Zone. It was here that the Chignik earthquake struck – and USGS was ready for a post-earthquake response mission.

Just a couple of years before, three GNSS-A monitoring sites had been set up on the seafloor off Alaska, in the Aleutian subduction zone, by a team of scientists funded by US National Science Foundation (NSF).

Several Wave Glider surveys had been carried by the USGS and Scripps prior to the M8.2 Chignik earthquake, monitoring the position of the sites in about 1,200 m water depth.

Within weeks of the earthquake, USGS had their Wave Glider back out to measure what movement there had been during and shortly after the earthquake.

Despite challenging weather conditions, the mission collected high-fidelity GNSS and acoustic data with eye-opening results.

“The tsunami was modest, but the seismic event was the largest in the US for nearly six decades,” says Ericksen, “so we expected a large movement. But it was incredible to know exactly how much – and that was 1.4 m.” This was a critical insight into the co- and post-seismic movement, helping to understand subduction zone dynamics.”

The big question was, did the Chignik earthquake increase the state of stress and tsunami potential on the up-dip portion of the fault or not?

“The measurements showed that the fault moved 2 – 3 m horizontally in a shallow part of the fault, less than 20 km below the seabed, helping us to understand how stress builds up along the fault and is released in an earthquake,” he says

“These results suggested that the cumulative slip had relieved stress on the shallow portion of the fault and therefore, the Chignik earthquake likely did not increase tsunami potential of the shallow fault.

“It also showed the effectiveness of the GNSS-A technique and the utility of rapid response GNSS-A measurements to better assess tsunami and earthquake hazards in the region.”

Read more about the Chignik data here.

The origins of GNSS-A

The ability to measure the movement of plates on the seabed is not that new. It’s based on what was originally called the GNSS-A technique, first developed by Scripps, specifically David Chadwell and Fred Spiess.

“Combining GNSS positioning and acoustic measurements to track seabed movement was a clever idea – and it worked,” says Michelle Barnett, Ocean Science Business Development Manager, at Sonardyne.

“But the cost of using crewed ships to do the positioning made it cost prohibitive. It was also technically challenging.”

“So, working with Scripps, in the early 2010s, we developed a combination of our Fetch long-life sensors and an off-the-shelf GNSS-A payload for Wave Gliders that can go out and do the survey work at a much lower cost than using a crewed ship.”

Worth the wait, even when waiting on weather

The technique is not without its challenges, however. After gathering the positions of the Aleutian subduction zone transponders, Ericksen and his team were naturally keen to see the data.

Due to the significant amounts of data involved – we’re talking 25-30 GB per site (comprising three Fetch) – only sub-samples are sent back to shore from the USV, primarily for quality control.

So, they have to wait until the USV comes back – or is brought back – to shore. Low levels of daylight in the Alaskan winter (when the Chignik survey was carried out) meant limited power availability for the USV.

Combined with bad weather, coordinating its recovery proved challenging, resulting in it taking four-months to recover the Wave Glider and offload the data.

Still, the wait was worthwhile and the results are providing greater insights than we’ve ever had before.

Read more

Sonardyne technology chosen for new Canadian seabed observatory.

TL;DR:

Scientists can now monitor underwater earthquake zones using GNSS-A technology—combining satellite positioning with acoustic sensors on the seabed. This breakthrough, developed through collaboration between Sonardyne, Scripps Institution of Oceanography, and the USGS, allows them to track how tectonic plates move and where stress is building up, which was previously a “blind spot” beneath the ocean.

The technology: Sonardyne’s Fetch transponders sit on the seabed in arrays, while Wave Glider robots equipped with Sonardyne’s GNSS-A payload circle above them, using acoustic signals to precisely measure their positions over time. This makes continuous offshore monitoring both feasible and cost-effective for the first time.

Overview

WSM 6+ is one our most popular USBL transponders, selected by customers for its small size, value and dependable performance in all environments, shallow or deep.

It’s the perfect choice for fitting to observation and work-class ROVs, towfish, submersibles and equipment being deployed onto the seabed for short durations.

WSM 6+ is compatible with all of our Ranger 2 USBL systems and we even include one with every Ranger 2 Standard – as standard.

The compact and rugged design is based on the field proven mechanics from previous generation designs and is available with omni-directional or directional transducers. The omni-directional versions are depth rated to 1,000 m and 4,000 m. The directional transducer is depth rated to 4,000 m.

At a glance

• Rechargeable USBL transponder designed for tracking ROVs, towfish and equipment
• Choice of 1,000 m and 4,000 m depth ratings
• Compatible with our family of Ranger 2 USBL systems: Micro, Mini and Standard
• Transponder or responder operating modes
• Depth sensor for improved USBL positioning performance
• External on/off switch for battery saving

All versions support transponder and responder operating mode if an external trigger from your platform is available. Responder mode provides significantly faster position updates than transponder mode.

WSM 6+ supports Wideband 2 acoustic signals and legacy Wideband 1 and HPR 400. The configuration is programmable using supplied software and a serial link or it can be configured acoustically via our back deck handheld test unit – iWand. This allows the WSM 6+ to be configured for use with all of the popular MF frequency acoustic navigation systems.

All WSM 6+ are also fitted with a depth sensor fitted as standard to aid USBL positioning accuracy and an external on/off switch to save battery power when not in use.

WSM 6+ variants are available with acoustically controlled output lines suitable for external motor drive, burnwire or contact closure releases.