Page 22 - Marine Magnetometer Processing

Problems with the Hall equation Practical tests on land using magnetometers over selected iron objects has shown that the Hall equation does not always give an accurate prediction of the mass of a target. Field trials using targets detected during a magnetometer survey then identified by divers shows that some targets have a good prediction of mass while others can be wrong by up to a factor of three, so the Hall equation can be used as an approximate indicator of mass but not a precise one. Tests on a number of cannons of the same type on a shipwreck site showed that they had magnetic fields of different sizes and shapes. One of the reasons that there are differences between predicted and actual mass may be because of the magnetisation of the object itself, known as remanent or permanent magnetisation. The remanent magnetisation is the magnetic field that an object has if the Earth’s magnetic field was not present. The magnetic field around an object is the sum of the magnetic field induced in it by the Earth’s magnetic field plus any remanent field the object has. The remanent field makes the object, say a cannon or an anchor, behave like a magnet with a north and a south pole. If the remanent field is aligned with the Earth’s magnetic field then the effects will add up making the field distortion measured by the magnetometer larger than expected. If the alignment is opposite then the remanent field will counteract some of the induced field and make the signal recorded by the magnetometer smaller than expected. The alignment of the remanent field of an object will vary according to a number of factors, including how the object was aligned in the Earth’s field when it was made. Tests on this idea are still being done so the results are not yet published. Figure 15: Testing two types of magnetometer; Geometrics G882 caesium vs. Marine Magnetics SeaSPY Overhauser Marine Magnetometer Processing \[21\] © 3H Consulting Ltd