Magnetic
- Main field is internal (i.e., from a source within the Earth that varies slowly in time and space)
- Secondary field is external to the Earth and varies rapidly in time
- Small internal fields constant in time and space are caused by local magnetic anomalies in the near-surface crust.
Of interest to the geophysicist are the localized anomalies. These anomalies are either caused by magnetic minerals, mainly magnetite or pyrrhotite, or buried steel and are the result of contrasts in the magnetic susceptibility (k) with respect to the background sediments. The average values for k are typically less than 1 for sedimentary formations and upwards to 20,000 for magnetite minerals.
The purpose for this type of survey is to collect magnetic data that will help characterize the subsurface through non-intrusive means. For environmental, hydrologic or minerals exploration investigations, magnetic surveying can help characterize subsurface conditions by the following:
- Locate shallow buried man-made objects such as ferrous UST’s, pipelines, rebar reinforced cement and utility lines.
- Delineate landfill perimeters and characterize in-fill patterns.
- Locate other buried ferrous objects such as UXO, firearms and well casings.
- Identify shallowly buried subsurface geologic structures due to mineralogy.
- Identify archaeological sites by detecting magnetic objects or firepits.
- Delineate lithologic contacts due to mineral content (mainly magnetite).
- Identify buried ore deposits.
- Characterize shallow subsurface drainage channels through preferential magnetite deposition.
Magnometers permit rapid, non-contact surveys to locate buried metallic objects and features. Portable (one person) field units can be used virtually anywhere that a person can walk, although, they may be sensitive to local interferences, such as fences and overhead wires. Airborne magnetometers are towed by aircraft and are used to measure regional anomalies.The method reads contrasts in magnetic susceptibility between subsurface rocks or other features. By mapping magnetic distortions in the magnetic field, various subsurface conditions may be inferred.
Field-portable magnetometers may be single- or dual-coil. Dual-sensor mags are called gradiometers; they measure gradient of the magnetic field and their use does not require a base station. Single-sensor magnetometers measure total field, they require the use of a base station to correct for daily variations in the earth’s magnetic field.
MAG surveys are typically run along a predetermined linear grid laid out at the site. Readings are virtually continuous and results can be monitored in the field as the survey proceeds. Field data are typically collected using a magnetometer/gradiometer. Basically, a receiving sensor at a fixed distance from the ground surface measures the amplitude of the earth’s magnetic field. If two sensors are used, the unit is referred to as a gradiometer Two main types of ground surface magnetometers are used: proton precession and cesium vapor. Both have their benefits and are specifically recommended for certain types of surveys or target objects. Both types detect and measure the magnetic field in a discrete location and values are usually reported as nanoTeslas (1nT = 10-9 Teslas = 10-5 gammas).
The method reads contrasts in magnetic susceptibility between subsurface rocks or other features. By mapping magnetic distortions in the magnetic field, various subsurface conditions may be inferred.
A typical survey
A typical survey consists of collecting magnetic data along pre-determined survey lines or evenly spaced grid lines. Preferably, the survey lines or grid corner(s) are referenced to known points such as building structures, triangulation stations, bench marks or section corners. This allows the collected data to be fiducialized (x and y coordinate positioned) and by it being geo-referenced, the grid is recoverable at some future date.
Some magnetometers can be outfitted with a GPS system. The combination system can then record magnetic data along with real time positioning coordinates. This is especially useful in remote areas or for large-scale surveying.
Results from an example magnetic survey.