Magnetic Technique:

Magnetic techniques measure disturbances in the earth's natural magnetic field. These disturbances are caused by ferromagnetic materials, either magnetic rocks (usually bedrock) or man made objects containing iron or steel.

Magnetic techniques are used extensively in environmental applications to locate ferrous underground storage tanks, drums and other objects. They are used in archeological applications to locate ceramics and fire pits and in a variety of applications where the bedrock needs to be mapped.

Magnetic surveys are conducted by moving a magnetometer over an area while measuring the earths magnetic filed. No contact is required with the ground so large areas can be covered with relative speed. Modern magnetometers usually measure the gradient of the magnetic field as well. This is done by measuring the difference between the magnetic field at two sensors separated vertically by two or three feet. Since the difference between the two measurements is directly related to the distance to the source body, measuring the gradient enables a geophysicist to readily distinguish shallow source bodies from deep ones

Most magnetic disturbances can be attributed to either manmade ferrous objects or bedrock. This is because most other subsurface constituents (soils and sedimentary layers) have negligible magnetization. It possible to distinguish between manmade objects and bedrock based on differences in the magnetic disturbances they create. Manmade objects create magnetic disturbances that are orders of magnitude greater than bedrock of similar size. Manmade objects are also smaller than bedrock and generally shallower in depth. Thus, magnetic disturbances caused by manmade objects tend to be smaller, more intense and from shallower source bodies than disturbances caused by bedrock.

The image above shows a case history of an investigation for shallow manmade objects. The magnetic data is displayed as the analytic signal. The analytic signal is derived from the total magnetic field data. It is presented as a more concise display of that data set than the raw field data. The analytic signal reduces the bipolar nature of magnetic anomalies (typically a negative peak to the north and a positive peak to the south) to a single anomaly centered over the source of the anomaly.

Magnetic methods are very effective in particular types of investigations. They are cost effective for large areas and very reliably locate manmade iron and steel objects and are useful for mapping bedrock as well. Their primary limitation is their inability to locate non ferrous objects such as plastic or unreinforced concrete.