A contour map of magnetometer readings reveals the location of a buried gas pipeline and electric utility line.
A magnetic contour map of the Amargosa Desert in southern Nevada reveals the alignment of buried volcanic centers.
SwRI developed an all-terrain bicycle geomagnetic mapping system to increase the speed and efficiency of geomagnetic mapping.
To measure the earth's magnetic field, Southwest Research Institute (SwRI) uses cesium-vapor rover magnetometers. The primary applications for magnetic measurements are:
- Locating buried ferrous metals
- Mapping geologic features
Locating Buried Ferrous Metals
The presence of buried ferrous metal objects creates a local variation in the strength of the earth's magnetic field. Buried objects include:
- Underground structures
Total field measurements employing one magnetometer or gradient measurements employing two magnetometers can be used to map local magnetic variations. Scientists use gradient measurements to enhance the detection of magnetic anomalies produced by shallow buried metal objects.
Mapping Geologic Features
Magnetic measurements are used for geologic mapping by detecting contrasts in the magnetic susceptibility of soil and rock. Geologic strata with high remnant magnetism (chiefly caused by the presence of hematite, the most common magnetic mineral) are more magnetically susceptible, which causes a local variation in the earth's magnetic field. Displacement or disruption of a uniformly magnetic soil or rock layer can also create local magnetic variations that can be detected by a magnetometer.
Total field magnetic measurements are generally used in geologic mapping surveys.
Magnetic contrasts in soil and rock can be applied to:
- Mineral exploration
- Delineation of geologic structure (location and mapping of faults and karst features)
- Delineation of stratigraphic relationships (rock unit contacts and orientations)
- Archaeological exploration or prospecting (locating ancient inhabited sites, graves, or buried walls and structures)
Sophisticated two- and three-dimensional (2 and 3D) magnetic modeling software allows scientists to interactively create and manipulate geologic models to fit observed magnetic data. These models are powerful interpretive tools that help determine the characteristics of geologic features, such as location, depth, and orientation.
Scientists can rapidly collect magnetometer measurements by interfacing either a hand-carried or bike-mounted rover magnetometer with a differential global positioning system (DGPS). Accurate and comprehensive plan-view contour maps or vertical profiles can be generated using this survey technique.
Advantages of Magnetics
- Rapid data collection
- Integration with differential global positioning system (GPS), which allows accurate measurement location
Limitations of Magnetics
- Susceptible to interference from cultural features such as steel pipes, vehicles, fences, and buildings
Total field measurements are susceptible to natural fluctuations in Earth's magnetic field. SwRI scientists collect base station magnetometer readings when conducting long-duration magnetic surveys to correct for these fluctuations.
A magnetic anomaly is produced by a local disturbance in the earth's magnetic field, which arises from a local change in magnetization.
A comparison of magnetic data collected by foot (magnetic map on left) and by bicycle (magnetic map on right).
electrical resistivity • electromagnetics • environmental geophysics • geophysics • gravity • ground conductivity • ground-penetrating radar • induced polarization • magnetics • Near-Surface geophysics resistivity • surface-based geophysics • transient electromagnetics