Magnetic Search and Survey in Shallow Water and Beach Areas
Not all shipwrecks of interest to the nautical archaeologist are found in depths requiring diving techniques. Shipwrecks can be found in beaches, under sand dunes, behind barrier islands, and in sand flats, bays, and rivers. The use of magnetic survey in dealing with locations in extremely shallow water or on beaches requires a different approach, as the standard marine magnetometer tow system cannot be used efficiently. Under certain circumstances, the marine sensor can be lifted by net buoys attached by lines behind the sensor, but one should be aware of problems encountered with this technique. Buoys generally cause the sensor to fishtail while under tow. These changes in orientation cause gradient changes in the data, as the sensor observes the magnetic field in different directions. If the fishtailing is rapid enough, the data often appear to show constant anomalies or an unacceptable level of noise. The addition of fins helps to stabilize the sensor; one can also use a drogue chute or small changes in speed or length of tow cable to achieve good data.