Page 21 - Marine Magnetometer Processing

The aspect ratio is a factor that can be included in the calculation to overcome differences noted in the mass predicted by the equation and the actual mass of iron causing the anomaly. Why the predicted and actual mass of iron can be different is the subject of research by the SHIPS Project with CISMAS and hydrography students at Plymouth University. However, for the majority of situations the aspect ratio can simply be set to a value of 1. Rearranging we get: Example: For a measured magnetic anomaly of 13nT with a towfish altitude of 20m above the seabed and using an aspect ratio of 1 we get: Mass (kg) = (13nT / 1 / 10) x (20)3 = 1.3 x 8000 = 10400kg or 10.4 tonnes ()=∆ ÷ ÷ 10×3 Note that this estimate of mass assumes that the object is lying on the seabed and directly under the towfish. If the object is buried or to one side of the survey line then it will actually be further away from the towfish and the calculated mass will be underestimated. We can also calculate how close the towfish has to be to detect objects of different sizes. If we assume that the smallest anomaly we can reliably detect is 5nT then the distances are: Example target Mass Minimum detection distance (5 nT anomaly) 20lb round shot 32lb round shot Small anchor Small anchor Small Iron gun (9lb) Medium Iron gun (18lb) Large Iron gun (42lb) Iron ballast Small iron wreck 9 kg 2.7m 14 kg 3m 100 kg 6m 500 kg 10m 1.25 tonne 14m 2 tonne 16m 3.25 tonne 19m 10 tonne 27m 100 tonne 58m 1000 tonne 126m So this means that you need to get the magnetometer towfish within 3m of a single 32 pdr cannonball to be able to detect it. Towing the magnetometer only 3m above the seabed may be unsafe as the chance of hitting something is high. We can reliably tow the magnetometer at 6m above the seabed, so the smallest object we can reliably detect is 500 kg. Iron wreck Table 1: Minimum detection distances Marine Magnetometer Processing \[20\] © 3H Consulting Ltd