Magnetic Detection
The magnetometer is an ideal auxiliary instrument for the efficient search for deeply buried targets containing even a tiny structural part made of iron or steel.
This device detects and highlights disturbances in the intensity of the magnetic field, also called anomalies.
Like the sonar, it is towed behind the boat.
Principle
The magnetometer detects variations in the magnetic field. These are linked on the one hand to the earth and the sun, on the other hand to the components of the rock or sediment, but also influenced by the presence of metallic objects buried or placed on the bottom.
The earth can be roughly thought of as a bar magnet, with a positive magnetic pole near the geographic North Pole and a negative pole near the South Pole.
The presence of iron objects disrupts this magnetism by creating an anomaly.
Equipment
CERES uses a Geometrics G882 magnetometer alone or mounted in multiples, as a gradiometer.
This association makes it possible to detect the difference between the magnetic fields observed by each device.
This configuration helps to limit the number of corrections needed during processing and improves the positioning of the detected object.
Range: 20,000 to 100,000 nT
Accuracy: < 1 nT
Maximum Depth: 9,000 m
Sensitivity: 0.004 nT/piHz
Resolution: < 2 nT
GEOMETRICS G882
Methodology
Acquisition-Processing
Mapping
The current standard in magnetometry is to perform a pass every 2.5m.
The flight altitude of the sensors is kept below 4m, which allows for the theoretical detection of an object with the ferrous mass of a grenade.
The CERES team provides this information during the technical proposal, often accompanied by a site plan with the projected lines.
Data acquisition and processing are performed in HYPACK. Magnetic-field values are corrected in real time using the IGRF (International Geomagnetic Reference Field), enabling real-time mapping of magnetic anomalies. Magnetometer data are generally acquired at 3 knots.
It is often useful to correlate the magnetometer acquisition map with the sonar mosaic of the same area.
This allows us to identify ferrous objects present on the seabed, estimate their dimensions and determine their positioning.
It is this correlation that allows us to address the UXO problem.
