Page 183 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

162 Maritime Archaeology: A Technical Handbook, Second Edition
on the grounds that the metal detector (a pulsed induction machine; Foster, 1970) could locate both ferrous and nonferrous material. Thus it would be possible to differentiate ferrous material found on the magnetometer survey from the ferrous and nonferrous targets found on the metal detec- tor survey. The results of the survey clearly showed metal targets occurring on the metal detector that did not occur on the magnetometer survey. These items later proved to be the lead sheathing of hull. Other targets which appeared in both surveys proved to be large iron concretions (see Figure 5.1, note metal detector targets Z5–Z10 are nonferrous targets, whereas Z1 and Z4 are ferrous). A similar approach was made with the Amsterdam survey (Figure 5.2), which also turned up large nonferrous targets (Marsden, 1974).
A close-plot metal detector survey was used on the Santa Maria de la Rosa, where the metal detector was used to locate small, buried metal objects, but did not reveal the expected iron and bronze guns on the site (Green and Martin, 1970).
In the Amsterdam survey, the instrument reading was recorded and used in a dot density diagram to indicate targets. In the other two surveys, the instrument was used to delineate the areas where strong signals were recorded. These were plotted on the plan.
III. PROBE SURVEY
Probes have been widely used to determine the extent of a buried site. At Kyrenia, Cyprus (Green et al., 1967), a simple iron-rod probe was used to determine where material was buried (Figure 5.4a). The site was probed at the intersections of the grids, and if there was no contact, the position was re-probed in the neighborhood of the grid intersection. If there was a contact, and usually the operator was able to distinguish if it was ceramic, wood, stone, etc., this was recorded. If there was no contact, this was also recorded. By working systematically, it was possible to clearly delineate the area of the wreck site. Later, at Cape Andreas, a contour probe survey was made to determine the extent of the buried cargo (Green, 1971). In this case, the depth of contact was measured and recorded, so that a contour plot of the site could be made (Figure 5.4b). One of the problems of using a probe under water is that it is often difficult to force into the ground, par- ticularly when the bottom is clay. An effective way of resolving this is to pump air or water down a tubular probe enabling the probe to penetrate quite easily to great depths. The tube can be an ordinary, small-bore, steel water pipe and the air or water pressure does not need to be very high to have the required effect.