The magnetic method measures variations in the earth's magnetic field. A survey is typically conducted by measuring the magnetic field strength at evenly-spaced points (a grid) over the area of interest. 
 

A magnetic survey at a four-acre industrial site produced these anomalies.  Subsequent excavation discovered buried drums in disposal pits.

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

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.

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.

Contour maps are generated on-site to allow for field interpretation of the data, which could demonstrate 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 the results of a magnetic survey includes a site plan showing the area of investigation, known cultural features, and data collection points. Magnetic data may be presented as color or color-fill contour maps or stacked profiles which can overlay the site plan.

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 perhaps orientation. One such program accomplishes this by comparing the field data over discrete anomalies with theoretical model curves to measure the degree of fit.
 
 

NAEVA'S experienced personnel procure data rapidly, providing economical field surveys.