May 25 2014
A magnetic gradient survey is a type of geophysical survey that is conducted with the help of a pair of magnetometers. The two magnetometers are held apart at a fixed distance, and the difference in magnetic field at a height above the ground is measured.
A magnetic gradient survey is done for measuring the contrast in the value of magnetic field strength across a survey area.
This method is predominantly used to map archeological features at a survey site. Concentrations of iron, ceramics, fire-altered soils, hearths, magnetic pits, and ditches, as well as fire-altered stone and disturbances in the surface soils, can be measured through this survey.
A geophysical survey yields a map of the subsurface archeological features. The arrangement of two magnetometers held apart is referred to as a gradiometer. The gradiometer acts like a high-pass filter and removes the regional anomalies from the survey result.
Every material has its own unique magnetic characteristic; different materials below the ground affect the Earth’s magnetic field in different ways. Magnetometers are sensitive to these disturbances in the Earth’s magnetic field.
Some of the materials to which magnetometers show large deflections are iron, steel, brick, rocks, and other archeological features. Even minute disturbances caused by unsettled soils or decayed organic materials can be picked up by magnetometers.
Geoscan FM36 is a commonly used gradiometer for magnetic surveys. For conducting a magnetic gradient survey, the surveyor carries the gradiometer and walks around the survey site. Magnetic field disturbances are indicated by a sound emitted by the instrument. The data is collected by the surveyor at these points.
Care must be exercised in carrying the gradiometer, such that both the magnetometers are aligned perpendicular to the ground, in order to get error free readings.
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