Magnetic Surveying in Archaeology - Marine Magnetic Surveys

Marine Magnetic Surveys

Magnetic surveys are extremely useful in the excavation and exploration of underwater archaeological sites. The apparatus used on the water slightly differs from that on land. Marine magnetometers come in two types: Surface towed and near-bottom. Both are towed a sufficient distance (about two ship lengths) away from the ship to allow them to collect data without it being polluted by the ship's magnetic properties. Surface towed magnetometers allow for a wider range of detection at the price of precision accuracy that is afforded by the near-bottom magnetometers.

The most common type of magnetometer used for marine survey is the fluxgate magnetometer. Fluxgate magnetometers utilize two ferromagnetic cores each wound with a primary coil and an outer secondary coil attached to amp meter. When an alternating current (AC) is passed through the primary coils, it creates two opposing magnetic fields that vary in intensity based on the outside magnetic fields. By floating them parallel to the seafloor, they can measure the changes in magnetic fields over the seabed.

Another common type is the newer proton precession magnetometer. This utilizes a container full of hydrogen rich liquids (commonly kerosene or methanol) that, when agitated by a direct current (DC) or Radio Frequency (RF), cause the electrons to become energized and transfer that energy to the protons due to the Overhauser Effect basically turning them into dipole magnets. When the stimulus is removed, the protons precess at a rate that can be interpreted to determine the magnetic forces of the area.

In maritime archaeology, these are often used to map the geology of wreck sites and determine the composition of magnetic materials found on the seafloor. An Overhauser magnetometer (PPM) was used in 2001 to map Sebastos (the harbor of Caesarea Maritima) and helped to identify components of the Roman concrete.