Page 180 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 5
Subsurface Survey
I. CLOSE-PLOT MAGNETOMETER SURVEY
A close-plot magnetometer predisturbance survey was first tried on the Kyrenia shipwreck in Cyprus (Green et al., 1967) in an attempt to obtain information about the material buried on the site. A grid was set up over the site and the magnetometer detector head was placed at regular inter- vals on the grid to measure the local field strength. By plotting the field intensity, the extent of the magnetic anomalies on the site could be deter- mined. The diver who placed the magnetometer detector head was equipped with aluminum aqualung cylinders and the ferrous component of his equipment was kept to a minimum so that it did not affect the measurements (Figure 5.1).
Tests were made to determine how close the operator could work to the detector before it was influenced by the equipment; the tests showed that the diver needed to be at least 10 m away from the detector head to avoid having any effect. The operator was also equipped with voice communica- tions which enabled the surface operators to advise when a reading had been made, and the diver to advise when the detector was in place.
Initially the detector was set on the seabed in order to take the readings. Later it was found that it was better to have the head about 0.5m above the seabed to filter out small, surface ferrous material. Readings were made at 2-m intervals over an area of 28 ¥ 10 m. A further series of readings was taken at 0, 1, 2, 3, 5, and 10 m above the bottom, so that an estimate of the mass causing the anomaly could be made using the formula discussed in Chapter 3 Section V.A. The results were plotted as a magnetic field inten-
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