Scientists deploy 6G LBL to help study plate tectonic monitoring

The precision offered by the Wideband 2 digital signal architecture found in Sonardyne’s sixth generation (6G) positioning equipment, is widely recognised. Structures can be installed on the seabed and mobile targets tracked with millimetric accuracy. A recent trial conducted in the Mediterranean set out to show how this standard, off the-shelf technology could be applied to the science of plate tectonic monitoring.

Earth’s gigantic interlocking, tectonic plates float on molten rock and although we think of them as static, they move continuously – albeit very slowly – at typically just a few centimetres a year. Large events such as earthquakes cause much larger movements of metres, or even tens of metres, and as we know, these can result in underwater landslides triggering Tsunamis causing enormous damage and tragic loss of life. On land, these tiny movements can be tracked with GNSS stations, but tracking oceanic plate boundaries deep subsea to aid understanding of the fundamentals is much more  challenging.

One method used to measure subsea displacement involves using permanent geodetic references consisting of long life acoustic positioning transponders on the seabed. A GNSS positioned survey vessel measures many ranges to the transponders to accurately establish a position. Over subsequent visits to the site (after many months), the acoustic measurements are re-observed and processed to determine the movement of the seabed relative to the GNSS spheroid.

First conceived in the 1980s, the technique is highly dependent upon the precision of the measurements. Variations in the ionosphere, a constantly changing water velocity, a dynamic vessel and instrument errors can all mask these small movements in the seabed references.

Key to success therefore is the precision and repeatability of both the GNSS and the acoustic positioning component of the survey system – the acoustic part of this having improved significantly with the advent of Sonardyne 6G and Wideband 2 acoustic positioning technologies.

Establishing the exact level of improvement was one of the aims of a recent trial conducted by a team of researchers from the University Institute European De La Mer (IUEM).

Mission GEODESEAFor the test, the research vessel Tethys II, operated by the Centre national de la recherche scientifique (CNRS), was mobilised to sail from Nice to a location where the water depth reached 2,400 metres. The vessel sailed with four Compatt 6 transponders, a Pressure Inverted Echo Sounder (PIES) and a deep water optimised GyroUSBL transceiver installed on a temporary over-the-side deployment pole.

Chris Hammersley, Project Engineer at Sonardyne who joined the trial said, “Inside GyroUSBL, we’ve integrated our high grade attitude and heading reference/ INS sensor, Lodestar, with a 6G (Sixth Generation) HPT transceiver. This combination eliminates the alignment errors seen in conventional USBL systems and is proven to deliver unrivalled levels of accuracy and precision – even when installed on Tethys II using a side mount temporary deployment pole.”

At the test site, the Compatts were lowered to the seabed, three forming an equilateral triangle with 2,600 metre baselines and the fourth placed in the triangle’s centre. Each Compatt was mounted in rigid tripods to minimise movement in the current.

The PIES unit was freefall-deployed in the immediate working area to observe change in sound  speed through the water column. It was set to log temperature, pressure and inclination every 10 minutes. The data collected by the PIES was used to independently validate the calculated sound speed along with multiple dips using a CTD.

Transponder positioing and meaurements on the seafloorThe network of transponders was ‘boxed-in’ using Sonardyne’s calibration software to determine their absolute positions and over the course of 36 hours, range observations were logged. At end of the cruise, all acoustic transponders were recovered using their integrated acoustic release mechanism which allows the unit, with tripod, to float back up to the surface.

Analysis of the GPS-acoustic data set (including GPS positions, acoustic ranges and Lodestar attitude data) indicated that the seafloor could be positioned with centimetre-level precision commensurate with the measurement of tectonic plate movements.

Seafloor positioning performance is significantly impacted by the acoustic range precision and GPS accuracy. From the GPS-acoustic data set, the precision of the acoustic range measurement wa estimated to be 5mm one-sigma. Whilst the trial focused on the estimation of tectonic plate movements, the high levels of precision and reliability of the acoustic ranges could support more general seafloor positioning applications.

Sonardyne’s equipment has already been deployed subsea for years to monitor fault zones in the Mediterranean, and off the West coast of North and South America. Hopefully one day, our improved understanding of tectonic plate motion may help computer models better predict earthquake and tsunami risks, so saving lives.