Magnetometers are sensors that measure the magnetic field locally. The study of the Earth's magnetic field is carried out as part of certain geophysical exploration campaigns, in addition to bathymetric surveys, seismic acquisitions or gravimetric data recordings.
By example, the study of magnetic anomalies in the oceanic crust plays a decisive role in understanding and describing the phenomenon of plate tectonics. Magnetometers can also be used to search, in small depths, submerged ferromagnetic objects.
Magnetometers of the French Oceanographic Fleet
Magnetometers are available on Pourquoi pas ?, L’Atalante and Marion Dufresne.
An other magnetometer can be loaded onto other vessels depending on the needs of the scientific campaigns.
The magnetometer model installed on the ships is as follows:
- Provide: Marine Magnetics ;
- Model: SeaSpy magnetometer.
The SeaSpy magnetometer provides a measurement of the intensity of the ambient earth’s magnetic field with a very high accuracy through a sensor type Overhauser. This type of magnetometer gives only a measure of the total intensity of the magnetic field, but not of its direction, contrary to the vector magnetometers.
The magnetometer is towed behind the vessel at speeds of 12 knots, at a distance of approximately 3 times the length of the vessel to avoid disturbances of the magnetic field related to the vessel herself. It can also be towed at the end of the seismic flute, 50m behind the tail buoy, when seismic acquisitions are made during the cruise.
Each SeaSpy system is composed of the following elements:
- A towfish (length 1.2 m)
- An electronic interface for supply and communication with towfish
- A Kevlar (300-500 m) tow cable
- An electric winch with rotating contactor
- An acquisition computer unit (TECHSAS system).
The characteristics and performances of the SeaSpy magnetometers are as follows:
- Overhauser magnetometer
- Absolute accuracy: 0,2 nT
- Sensor sensitivity: 0,01 nT
- Resolution : 0,001 nT
- Scale: 18000 - 120000 nT
- Gradient tolerance: > 10000 nT/m
- Sampling range: 0.1 Hz - 4 Hz