UT-2, a publication of the Society of Underwater Technology highlights RM Hypermag

RM Hypermag covered in the most recent issue of UT-2, Society of Underwater Technology. Source

Canadian company Ocean Floor Geophysics Inc. (OFG) has developed the RM Hypermag, a multi-vector magnetic gradiometer suitable for UXO detection and for locating cables and pipelines and measuring their depth of burial.

Sensitive total field magnetometers are often towed behind vessels or AUVs. OFG originally developed a self-compensating magnetometer that compensated for static and dynamic magnetic field created by the vehicle. This self-compensating magnetometer typically goes inside AUVs, ROVs, and more recently, USVs for shallow water detection. Now, OFG has developed the much more sensitive RM Hypermag. The system is currently being integrated into an EIVA Scanfish ROTV (actively controlled towfish) for deployment in the USA in May.

"With the new RM Hypermag, it is possible to deploy multiple magnetometers in an array that gives a combined sensitivity of at 4pTrms/rthz, similar to a caesium vapour towed magnetometer," said Matthew Kowalczyk. “It is a combination of subsea magnetometer sensor arrays, processor/data logger, calibration procedures, and proprietary software algorithms to provide compensated and corrected magnetic data in real-time. Individual sensors are tightly synchronized to provide magnetic gradiometer measurements. The gradiometer directions are dependent on installation configuration and can be designed for the application, be it vertical, horizontal or inline gradients. Total field and vector field data, without dead zones, is also produced for a comprehensive magnetic data product.

An RM HyperMag array comprises four individual 3-vectormagnetometers, each residing within their own subsea bottle, connected to a central subsea interface bottle, which acquires, processes, and communicates via a high bandwidth protocol to a central receiver or switch. The individual sensing units are very small, and multiple arrays can be combined into larger arrays making the whole system easily scalable.

It also features a 1kHz bandwidth for fast sampling, simple connectivity (Ethernet or RS 232) and very low power, less than 4 W per Hypermag unit.

Because they don’t have the same power draw as other marine magnetometers, they are well suited to AUVs and other low power systems. As AUVs and active towfish house increasingly more instrumentation, the low hotel load from the RM Hypermag conserves valuable power and enables easy integration into the vehicle systems.

Other options include a single towed body system with an integrated IMU, depth, altitude and USBL responder trigger passthrough for improved positioning at lower costs. When trying to identify magnetic seafloor objects the absolute sensitivity of a magnetometer is not the only important measure. Having vector and gradient information potentially allows for better discrimination of objects such as UXO, cables or pipelines. An important consideration is the control and measurement of the sensor position. For example, if a magnetometer array is towed on lines behind an ROTV the magnetometers may fly at different or changing heights so measured magnetic field from the objects of interest will change in magnitude.This can cause errors and artefacts in the data. Operating the RM Hypermag as a strap down instrument on a well-positioned subsea vehicle or towfish and coupling better position control with the additional data from multiple vector and gradient measurements provides excellent target discrimination. It may also allow for wider line spacing.Good data, well controlled, can drive down survey costs while maintaining client confidence that the survey objectives have been met.