Magnetic Measurements

Magnetic surveying is a passive geophysical technique that measures the strength of the earth’s magnetic field. Magnetic surveying is a useful tool for site studies to locate and identify buried ferrous metal objects such as pipelines, barrels, tanks, etc., as they generally produce a perturbation in the earth’s naturally occurring magnetic field.

In magnetically noisy areas NDT uses magnetic gradiometer measurements to locate ferrous metal objects. Magnetic gradiometer measurements are made with sensors spaced several inches apart. The outputs of the two magnetometer sensors are added to cancel signals resulting from magnetic fields common to both sensors (i.e. the earth’s magnetic field). The residual output signal is the difference in the magnetic field between the sensors, as a result of nearby ferrous metal objects. Magnetic gradiometer measurements can be used in PVC lined boreholes to determine the length of pipe and sheet piling. Magnetic Gradiometer measurements can also be used to locate broken prestressing wires in PCCP pipe.

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