SOUTHWEST RESEARCH INSTITUTE

Near Surface Geophysics Site Map

	A contour map of magnetometer readings reveals the location of a buried gas pipeline and electric utility line.

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:

	A magnetic anomaly is produced by a local disturbance in the earth's magnetic field, which arises from a local change in magnetization.

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.

	A magnetic contour map of the Amargosa Desert in southern Nevada reveals the alignment of buried volcanic centers.

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:

The observed magnetic data are fit to a 2D geologic model to interpret the location and orientation of faults in the subsurface.	This graphic shows magnetometer readings that illustrate the location of magnetic anomalies associated with faults.

Scientists can collect magnetometer measurements rapidly either on foot or by bicycle. Accurate measurement locations are achieved by interfacing 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.

A comparison of magnetic data collected by foot (magnetic map on left) and by bicycle (magnetic map on right).

	SwRI developed an all-terrain bicycle geomagnetic mapping system to increase the speed and efficiency of geomagnetic mapping.

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.

For more information about our near surface geophysics and magnetics capabilities, or how you can contract with SwRI, please contact Ronald T. Green, Ph.D., at rgreen@swri.org or (210) 522-5305, or James Prikryl at jprikryl@swri.org or

(210) 522-5667.

geophysics.swri.org

| Center for Nuclear Waste Regulatory Analyses | Geosciences and Engineering Division | SwRI Home |

Southwest Research Institute^® (SwRI^®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 11 technical divisions.

August 18, 2011