The goal of magnetic surveys is to measure small variations in the Earth’s magnetic field produced by shallow sources under the ground. The magnetic properties of ferrous objects, or naturally occurring materials such as ore bodies allows them to be detected and mapped by magnetic surveys. Magnetometer surveys are commonly used to find oil and gas pipes, abandoned steel well casings, storage tanks, drums, unexploded ordnance and metallic debris by detecting the magnetic anomalies they produce. It is the preferred method in mapping old waste sites and landfill boundaries, mapping basement faults and basic igneous intrusives or investigating archaeological sites.
In a magnetic survey, data points are distributed along profiles at regular intervals on a survey grid. Modern magnetometers and gradiometers allow to collect data with a high sampling rate with excellent accuracy (below the nT level). Acquisition systems are usually complemented with a GPS unit or a total station to ensure the correct location of acquired data.
To make accurate magnetic anomaly maps, temporal changes in the Earth’s field during the period of the survey must be considered. Normal changes during a day, sometimes called diurnal drift, are a few tens of nT but changes of hundreds or thousands of nT may occur over a few hours during magnetic storms. During severe magnetic storms, which occur infrequently, magnetic surveys should not be made. The correction for diurnal drift can be made by repeat measurements of a base station at frequent intervals. The measurements at field stations are then corrected for temporal variations by assuming a linear change of the field between repeat base station readings. Continuously recording magnetometers can also be used at fixed base sites to monitor the temporal changes. If time is accurately recorded at both base site and field location, the field data can be corrected by subtraction of the variations at the base site. After all corrections have been made, magnetic survey data are usually displayed as individual profiles or as contour maps. Identification of anomalies caused by cultural features, such as railroads, pipelines, and bridges are commonly made using field observations and maps showing such features.