Magnetics

The Method

The magnetic method measures variations in the Earth's magnetic field. Surveys are typically conducted by measuring the magnetic field strength along survey transects, or on closely spaced lines forming a grid over the area of interest. Variations in the Earth’s magnetic field occur for a variety of reasons, and may be strongly influenced by the underlying geology. In addition daily (diurnal) variations occur in the field strength, due to interaction of the solar wind with the ionosphere.

Equipment Used

Geometrics G-858 Cesium-Vapor Gradiometer
Geometrics G-856 Proton Precession Magnetometer
Scintrex ENVI-Mag Proton Precession Magnetometer
Scintrex SM-4 Cesium-Vapor SmartMag

After the data are processed, the data are plotted either as line profiles or contour maps. Localized variations or distortions in the magnetic field within a survey area are considered magnetic anomalies. Anomalies may be caused by subsurface geologic conditions, surface cultural features, or buried ferromagnetic objects. The character of the anomalies is evaluated as to shape, size, and amplitude, and an interpretation is made for the suspected cause.

Compared with other geophysical methods, magnetics can offer an advantage in ease of deployment in rough terrain, and in areas where vegetation may obstruct larger instruments, such as electromagnetic coils.

Field Procedures

It takes extensive experience to know when a magnetic survey is suitable for a given site, and to design the orientation and density of the data points to optimize collection of valid magnetic data. The correct data point density (line and station spacing) is directly correlative to the anticipated size and mass of the expected target and the presence and abundance of surface cultural features. NAEVA employs experienced geophysicists, familiar with the parameters which must be considered to design successful surveys.

Equipment

NAEVA uses the Geometrics G858 gradiometer for UXO, environmental and mineral exploration surveys. Magnetometers can be operated in total field or vertical gradient mode, depending on the specific application. For ordnance detection, a cesium vapor magnetometer provides a clear advantage with rapid data collection and a high degree of resolution. For large survey areas, several units can be run simultaneously to provide more rapid data collection. A base station is commonly used to measure diurnal drift, which is applied to normalize the field survey data.

Data Processing and Presentation

Contour maps are generated on-site to allow for field interpretation of the data, which could identify the need for additional fill-in data collection or further investigation of anomalies with additional geophysical methods, such as ground penetrating radar (GPR). Graphical presentation of magnetic survey results includes a site plan showing the area of investigation, known cultural features, and a course over ground track map. Magnetics data may be presented as color-fill contour maps or as a series of stacked profiles which can overlay the site plan. Contour maps are made by NAEVA’s experienced team of processors, using Geosoft Oasis Montaj software.

Depending on the specific application, sophisticated magnetic modeling programs can be employed to characterize the source of the magnetic anomaly as to size, mass, depth, and possibly orientation. One such program accomplishes this by comparing the field data over discrete anomalies with theoretical model curves to measure the degree of fit.

Applications

  • UXO Surveys: Range Characterization, and Removal Actions
  • Locate underground storage tanks (USTs)
  • Locate buried drums
  • Delineate landfill perimeter
  • Identify locations of historic structures
  • Identify geologic bedrock features such as mafic dikes or geologic contacts
  • Delineate areas of ferromagnetic debris
G-858

The Geometrics G-858 is operated by one person with or without GPS positioning and can cover long distances per day