Page 83 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

62 Maritime Archaeology: A Technical Handbook, Second Edition
V. ELECTRONIC TECHNIQUES
A. MAGNETOMETER
1. Principles of Operation
The use of the magnetometer for locating archaeological shipwreck sites was developed by Professor E.T. Hall at the Research Laboratory for Archaeology, Oxford, England. In this pioneering work, he showed that a marine proton magnetometer could be used to locate shipwrecks (Hall, 1966). A number of marine archaeological magnetometer surveys have been conducted, notably off Padre Island, TX (Arnold, 1976, 1981; Arnold and Clausen, 1975), in the Bell project (Arnold, 1996a,b), and in the Cana- dian Great Lakes (Nelson, 1979, 1983). They are an excellent guide to the logistics of mounting large-scale magnetometer surveys. Particular case studies of interest include those by Cusnahan and Staniforth (1982), Green (1987a), and Green et al. (1984).
Although the instrument is ideally suited for locating iron ships, it can, in certain circumstances, be used for locating nonferrous shipwrecks. The marine magnetometer has been widely used for archaeological survey work and the principals and application are well understood. The proton preces- sion absolute magnetic field intensity instrument is the most commonly used magnetometer for marine work, although differential proton magne- tometers, fluxgate, and cesium vapor instruments have also been used. There are three main problems with using a proton magnetometer (which is generally towed behind a survey vessel) in the marine environment: (1) determining the height of the detector head above the seabed; (2) the limitation of the sensitivity of the system when operating in seawater; and (3) determining layback.
The magnetometer measures the intensity of the Earth’s magnetic field at the sensor head. The presence of ferromagnetic material influences this field; the local effect increasing the intensity in some areas and decreasing it in others. Assuming that the Earth’s magnetic field intensity is uniform over small geographic distances (compared to the site being searched for), and that an iron object behaves like a short bar magnet in this field, the object will create an anomaly. Figure 3.24 shows, in a simplified form, the cross section of a magnetized object in the plane of the magnetic meridian. Areas where the local field intensity is enhanced and diminished are shown together with a contour map showing an idealized intensity plot at a fixed distance above the object.
The intensity of the anomaly varies as the inverse cube of the distance, and an approximate formula for the intensity (the Hall equation) is as follows