Magnetic surveys measure variations in the Earths magnetic field. Regional changes in the magnetic field are typically caused by geologic structure and local geologic changes. Site specific localized variations can be caused by individual magnetic sources or magnetic objects.
Magnetic surveys can be performed over a variety of terrains, usually with just one person, relatively quickly. Magnetic data is usually collected in a grid pattern, but it can also be collected along an planned path. Our magnetic surveys utilize GPS technology to ensure our data location is accurate. Magnetic data is depicted as a contour map.
Magnetic Surveys are able to detect metallic objects not because they are magnetic in nature, but because the Earths magnetic field is enhanced much more greatly in metals than in most other materials (soil, rock). The ability of a material to enhance the local magnetic field is known as magnetic susceptibility, denoted as “k”.
For most materials, k typically is much less than 1, usually on the order of 10-6 for most rock materials. The primary exception is magnetite which has a susceptibility of ~0.3. From a geologic perspective,the distribution of magnetite determines the magnetic properties of most rocks. Steel, iron, and other ferromagnetic alloys have k values one to several orders of magnitude greater than magnetite. Stainless steel is the exception, which has a minimal susceptibility.
Rock materials containing just 1% magnetite can have a susceptibility as large as 10-3, or 100-1,000 times greater than most other rock materials. Typical susceptibility values for some common rock materials are shown in the table below. The range of values given for each sample generally depends on the amount of magnetite in the rock.
In most environmental and engineering investigations, magnetic data collected for sedimentary layers will not show sufficient contrast to be of use in geologic mapping. Magnetic surveys are extremely useful though in the detection of man-made materials, such as municipal trash and industrial waste, which often contain ferrous materials (which have a very high susceptibility). As a result, magnetic surveys can be effective in the detection of:
- Industrial waste
- Buried building foundations
- Archaeological sites such as graveyards (depends on the content of the site)
Magnetic surveys can also be effective in the detection of specific ferrous objects such as:
- Underground storage tanks (USTs)
- Reinforced concrete
- Unexploded ordnance
- Capped wells.