HMS Colossus: debris field survey 2004
ADU magnetometer survey
In 2002 the ADU conducted a magnetometer survey over part of the designated area. The survey was conducted using a Geometrics caesium vapour magnetometer. The data was collected on two separate days; on the 9th June the stern area was surveyed and on 14th of June part of the bow area was covered. The data set as presented was a single tab delimited text file. The data set includes corrected positions for tow fish layback, tow fish depth, time/date and field strength in nT. There is no record of water depth in the data set.
The data was first split by the two separate dates collected, then split into separate files for the individual tracks. These were then imported into Excel and graphs of magnetic field strength were plotted against latitude (the track lines were approximately north-south). The resultant graphs are reproduced below, as is the plan of the individual track lines.
Determining the likely size of the iron objects causing the anomalies was problematical. As there was no associated bathymetric data recorded and the recorded tow fish height varied between 1.6m – 8.8m, it was necessary to estimate the depth of water using the chart depths and the tide height at the time of the magnetometer reading. Using the recorded tow fish depth it was then possible to estimate the tow fish to seabed distance. The approximate predicted weight of iron was then derived using the algorithms outlined in Hacon, 19801. The estimated weights are reproduced in tables 1 and 2 below. It is clear from the predicted weights that where the original weight of the object is known, the predicted weights are considerably lower. An example of this are guns 8, 9 and 10, all 32lb Blomefield guns which would have weighed 55cwt2 (2794kg) when manufactured. Reference to the table shows that predicted weights for these guns vary between 100-500kg. There are a number of technical reasons why these estimates of weight derived from the magnitude of the magnetic anomaly can only be approximations, especially when the tow fish to target distance is itself an approximation, as is the case here. Nevertheless the estimates give some indication of the relative sizes of the objects which caused the magnetic anomalies detected.
1 M.P. Postle Hacon. The Proton Procession Magnetometer and its role in Marine Magnetic Searches. The Hydrographic Journal No 17, 1980.
2 Adrian B. Caruana. The History of English Sea Ordnance 1523-1875. England 1997