Ships carried equipment made out of iron in many forms; cannon, anchors, chain, fasteners, rigging, and deck machinery. The ability of the magnetometer to detect ferrous material (iron) has made the magnetometer a principal remote sensing tool of marine archaeologists. A magnetometer is a passive device that measures the strength of the local magnetic field. A magnetometer consists of a sensor, the magnetometer unit, an interconnecting cable, and a power supply. Magnetometers can detect ferrous materials, such as iron, but cannot detect non-ferrous materials, such as bronze or silver.
The spinning earth behaves much like a magnet, with north and south magnetic poles. At any point on the earth, there will be natural magnetic field strength. Concentrations of ferrous material, such as iron anchors, cannon, or ships' fittings will alter that field and produce a magnetic anomaly (or variation). It does not matter whether the iron material is buried or exposed. The shape and size of the anomaly give clues to the mass of iron producing it and the depth of the burial.
Magnetic field strength is measured in units called gammas. The earth's natural magnetic field generally ranges from 30,000 to 60,000 gammas, depending upon location. A proton-precession magnetometer can detect anomalies in the local field of less than 1 gamma. The strength of an object's magnetic field decreases with the cubed distance between the sensor and object. This means that the magnetometer sensor must be towed relatively close to the object to detect it. As a rough guide, a large steel wreck can be detected at a range of 120 - 180 meters, a site with iron anchors and cannon at 80 - 100 meters, an isolated cannon at 30 meters (Mazel 1985:87).
This is the magnetic contour map of the Santa Monica, an 18th-century British shipwreck in the Virgin Islands.
The Geometric G-886 magnetometer is the main survey instrument for this project. The G-886 records this data in gammas (g) every two seconds. The local magnetic background in North Carolina is approximately 50,000 gammas. Readings are taken twice a day, in the morning and mid afternoon to assess the local magnetic reading (relative zero). Once data are collected from a given area, it will be reviewed and edited to remove any magnetic noise and incorrect readings using Hypack Max software. The data is then transferred to Golden Software's Surfer Surface Mapping System. Surfer creates interpretive surface and magnetic contour maps from the edited data to provide a complete view of the areas surveyed.
A Geometrics G886 magnetometer
