Principles of Magnetic Surveying
Magnetic surveying is used to measure the spatial variations of the magnetic field. The results reflect the variations in the magnetic properties of the underlying rocks, and provide valuable information about their compositions and the structure of the earth’s crust. The measurements are typically shown as maps of magnetic anomalies, namely, areas in which the recorded intensities of the magnetic field is higher or lower than the regional norm. Because most present-day surveys acquire data in the digital form, the results can be treated by a range of different mathematical processes in order to create different types of highly useful maps.
Most rocks contain at least a small amount of magnetic minerals, such as iron oxides or ilmenite. The Earth's magnetic field induces secondary fields in the rocks in the direction of the main magnetic field, and the anomalous fields can be measured at the surface and used to make inferences about the nature of the underlying lithologies and structures. There may also be permanent remanent magnetization in the rocks, which can have an orientation that differs from the present magnetic field direction. At temperatures greater than the Curie temperature, which is about 550-580°C, materials change their magnetic behaviors from ferromagnetic to paramagnetic. This corresponds to depths in the lower crust or upper mantle, below which the rocks should not contribute to the observed magnetic anomaly field. The strongest magnetic effects are generally associated with igneous lithologies, especially basic rocks, such as basalts, whereas, sedimentary rocks may give a low-level magnetic response that can be measured by modern techniques. The unit of measurement of the strength of the magnetic field is the nanotesla (nT), previously known as the gamma. The magnetic field strength in central Saudi Arabia is about 41,000 nT. In the field, it is set or measured to 0. Minor anomalies of geological interest may be less than 1 nT in amplitude. Cesium vapor or similar magnetometers are used in aerial surveys using aircrafts. In ground surveys, the proton precession magnetometers may be employed.
On the ground, magnetic surveys are used for the detailed studies of local areas, such as in mining areas. On an aircraft, two or more magnetometers at a fixed separation can be used to measure the field gradient directly over large areas using a systematic pattern of flight lines, which may be useful in delineating complex magnetic patterns. Since the amplitude of an anomaly tends to decrease rapidly with distance from the source, the observations should be taken as close to the surface as possible. Some recent aeromagnetic surveys elsewhere have, therefore, been flown at altitudes as low as 20 m above the ground and with sample intervals of 0.1 second or even less, or about 6 m. Earlier surveys also tended to have significant navigational problems, but with the advent of the GPS, it is now possible to obtain very accurate flight line patterns, which, in conjunction with improved processing techniques, has resulted in the great improvement of data quality.
The Earth's idealized International Geomagnetic Reference Field (IGRF) is generally subtracted from the measured field intensity to remove the regional trends, which results to an anomaly or residual field that is largely an expression of the local geologic structure. In addition to image enhancements to reveal the details in magnetic anomaly images, forward modeling of the magnetic grid or profile data is often employed to determine the geological structure. Automated inversion techniques may be used to model the magnetic data over large areas, including the mapping of the source depths and outlining the structural boundaries.
