Transition Zone Magnetometer Array
30 October 2016
The tidal zone is by far the most challenging environment to survey. The ground conditions, tidal range and harsh saline environment constrain access and the type of equipment which can be utilised. In areas with large slow tidal ranges and gentle sloping shores the opportunity to survey the whole survey area may only be presented once or twice a year when the Luna orbit produces atypical tides.
Such a case was presented to Geomatrix earlier this year. In this particular case the client had specified the UXO survey had to be completed by a date. As a result we proposed developing a bespoke telemetry package for deploying an array of 4 x G-880 marine magnetometers off a sledge which could be partially submerged. Many of the system components were already available for rental and the concept behind the system was well understood.
The magnetometers selected for this application were the Geometrics G-880. The G-880s were the original caesium vapour marine magnetometer produced by Geometrics and have since been succeeded by the G-881 and G-882. The form factor of the G-880 resembles that of an airborne magnetometer like the G-823 or G-824 rather than a G-882. They are ideal for this application as the electronics bottle and sensor are housed in independent pressure vessels which keep the weight and size low and also permits sensor geometry to be flexible.
The sledge comprises of two fibreglass hulls which ride smoothly over uneven ground minimising the egress of noise into recorded data. The fibreglass hulls are designed to be light but durable with nylon skids to resist abrasive wear. The hulls are connected by two fibreboard decks held in place with nylon bolts. The platform is designed to be nonmagnetic and nonconductive yet robust and stable. Ports were made in the rear of the hull so additional weight could be added if the force from waves moved the sledge. Our main concern was that the elliptical wave motions would pick the sledge up and move it off line.
In order to keep the sensors isolated from the magnetic field of the vehicle and battery the telemetry system had to be able to provide power to the magnetometers and convey data back to the recording laptop over a 30m tow cable, and run from a 24V DC supply. With this in mind the telemetry system comprises a top end power data junction box and wet end electronics package which is placed on the sledge. The wet end electronics package includes a DC-DC inverter to ensure adequate power is received by all of the magnetometers. Serial data from the sensors is converted to UDP before being passed to the data recording device. As well as the Magnetometer data additional channels were included for GPS and a Gyro.
We have subsequently added the system to the rental pool. Please contact sales@geomatrix.co.uk for further information.
