Modes of Acquiring Magnetic Observations
Magnetic observations are routinely collected using any one of three different field operational strategies.
- Airborne - Both fluxgate and proton precession magnetometers can be mounted within or towed behind aircraft, including helicopters. These so-called aeromagnetic surveys are rapid and cost effective. When relatively large areas are involved, the cost of acquiring 1 km of data from an aeromagnetic survey is about 40% less than the cost of acquiring the same data on the ground. In addition, data can be obtained from areas that are otherwise inaccessible. Among the most difficult problems associated with aeromagnetic surveys is fixing the position of the aircraft at any time. With the development of realtime, differential GPS systems, however, this difficulty has rapidly disappearing.
- Shipborne - Magnetic surveys can also be completed over water by towing a magnetometer behind a ship. Obviously, marine magnetic surveying is slower than airborne surveying. When other geophysical methods are being conducted by ship, however, it may make sense to acquire magnetic data simultaneously.
- Ground Based - Like gravity surveys, magnetic surveys are also commonly conducted on foot or with a vehicle. Ground-based surveys may be necessary when the target of interest requires more closely-spaced readings than are possible to acquire from the air. In the next discussion we will concentrate on ground-based surveys. All of this discussion, however, could be applied to air- and shipborne surveys also.
Because magnetic surveying is generally far cheaper than other geophysical methods, magnetic observations are commonly used for reconnaissance. These surveys can cover large areas and are used to identify the locations of targets for more detailed investigations. Because of their cost effectiveness, magnetic surveys usually consist of areal distributions of data instead of single lines of data. We will refer to the collection of geophysical observations over a geographic area as two-dimensional surveys. Data that is collected along a single line of observations will be referred to as one-dimensional surveys.