Optimising shallow water positioning for combined magnetometer and hydrographic surveys

Seabed surveys are now on the critical path towards meeting global renewable energy generation targets. That’s why Ocean Floor Geophysics are working on ways to improve how pipelines and cables are detected and monitored. Our positioning and navigation technologies were used to support a demonstration of their Hypermag gradiometer with Covelya Group company EIVA.

The Challenge

Detecting cables or pipelines on the seabed and checking or monitoring cable depth are essential requirements in the development and management of most offshore energy projects. With a global target of 80 GW of offshore wind to be built annually by 2030, these tasks are also now on the critical path.

Magnetometers or gradiometers (comprising an in-line array of magnetometers) are a key tool for these operations. Accurately knowing the position of these instruments in the water throughout a survey ensures that target detection resolution is as high as possible. So, Canada-based Ocean Floor Geophysics’ (OFG) has developed the Hypermag gradiometer.

Rather than towing a magnetometer or gradiometer from a vessel or ROV, OFG’s Hypermag has been designed with multiple vector magnetometers installed in a scalable array that can be integrated directly onto a remotely operated towed vehicle (ROTV), such as EIVA’s ScanFish. This removes any positional uncertainties seen when using a layback offset when towing a magnetic sensor. The multi-vector gradiometer could then acquire compensated magnetic data in real-time

OFG possesses advanced multiphysics capability, particularly in fusing acoustic data with magnetic and electromagnetic field data, in this case magnetic field interpretation across multiple three-vector sensors. The team, therefore, required optimum shallow water positioning of the ROTV to demonstrate the gradiometer at its highest capability. “A high-quality, accurate navigation solution is a key component in achieving the full potential from our next generation of magnetic vector gradiometer sensors,” says OFG CEO Matthew Kowalczyk. “In shallow water environments, such as near-shore windfarms and UXO survey sites, it can also be one of the most difficult datasets to produce.”

The solution

Covelya Group companies EIVA and Sonardyne supported a demonstration of the Hypermag in the vicinity of a wind farm in the North Sea. EIVA provided ScanFish support, including integration of the Hypermag configuration, a Norbit multibeam and a NaviPac software solution. Sonardyne provided a SPRINT-Nav Mini INS system alongside Ranger 2 equipped with a Gyro USBL transceiver, providing OFG with comprehensive position and behaviour accuracy of the ScanFish platform and it’s offset Hypermag sensors.

The SPRINT-Nav Mini Navigator is our smallest form factor hybrid navigation instrument, combining an INS, AHRS, Doppler velocity log (DVL) and pressure sensor.

By using this integrated INS system, updated by the Ranger 2 Gyro USBL, positioning errors or any erroneous USBL pings or dropouts could be mitigated. The INS can, in real-time, reject outliers in the data and output AHRS data, velocity, depth and altitude, as well as outputting real-world positional data in real-time, mitigating the need for post-processing. In addition, the SPRINT-Nav Mini could support multibeam data acquisition when a combined multibeam and magnetic gradiometer solution is used.

To put the approach to the test, a vessel of opportunity, the 22 m Marshall Art survey vessel, was mobilised. We used our Ranger 2 Gyro USBL system, which comes pre-calibrated, for easy mobilisation, with a Norbit iWBMSh multibeam echosounder (MBES) on Marshall Art providing RTK GNSS into the Gyro USBL.

On the ScanFish there were two OFG Hypermag panels, comprising a total of eight magnetometers, our SPRINT-Nav Mini Navigator, an omnidirectional Wideband Sub Mini 6+ transponder and a Norbit multibeam. An OceanEnviro fibreoptic winch system was installed on the vessel to tow the ScanFish. Orientation and position data was provided by the SPRINT-Nav Mini.

The results

“This was a great collaboration,” says Matthew Kowalczyk. “The combination of Sonardyne’s Ranger 2 Gyro USBL system and SPRINT-Nav Mini allowed us to quickly produce a navigation solution for the Hypermag system that exceeded our expectations.

“A high-quality navigation solution is necessary for achieving the full potential from our next generation of marine magnetic vector gradiometer sensors. Sonardyne’s support contributed to the trial’s overwhelming success.”

Another benefit of using SPRINT-Nav Mini on the ScanFish is that it can support its 3D steer algorithm, which can make surveying more efficient, reducing overall survey costs.

Pipelines and cables are often surveyed using a zig-zag pattern to decrease survey time. However, the most efficient way to survey a pipe would be to run parallel to the pipe, which EIVA 3D Steering allows with a suitable navigation solution because the position of the data measurement can be known with higher certainty.