The magnetic method, perhaps the oldest of geophysical exploration techniques, blossomed after the advent of airborne surveys in World War II. With improvements in instrumentation, navigation, and platform compensation, it is now possible to map the entire crustal section at a variety of scales, from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale. Methods of data filtering, display, and interpretation have also advanced, especially with the availability of low-cost, high-performance personal computers and color raster graphics. The magnetic method is the primary exploration tool in the search for minerals. In other arenas, the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications, such as locating intrasedimentary faults, defining subtle lithologic contacts, mapping salt domes in weakly magnetic sediments, and better defining targets through 3D inversion. These new applications have increased the method’s utility in all realms of exploration — in the search for minerals, oil and gas, geothermal resources, and groundwater, and for a variety of other purposes such as natural hazards assessment, mapping impact structures, and engineering and environmental studies.

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The historical development of the magnetic method in exploration

The 3D analytic signal amplitude of a total magnetic intensity (TMI) map, introduced by Roest et al. (1992), is widely used in magnetic interpretation as a means of positioning anomalies directly over their sources. This technique is most important at low magnetic latitudes, where reduction to the pole distorts anomalies to the point where they often become uninterpretable: the reduction operator does not converge if the magnetization and regional field are truly horizontal (Baranov, 1957). Methods have been devised to suppress the artifacts appearing in low-latitude reduction to the pole, but no method can reduce such data without distortion (e.g., Silva, 1986; Hansen and Pawlowski, 1989), which becomes severe for inclinations less than 20°. The amplitude of the analytic signal, denoted by | A |, has the added advantage of being independent of the orientation of magnetization of the source bodies. It reaches a maximum over magnetic contacts, and thus, in theory, can be used to trace the outline of magnetic bodies. In practice, especially in the case of aeromagnetic data at high instrument-source separation, | A | is high over magnetic bodies, but is not sufficient to resolve body edges. This appears to be true even with higher-order analytic signal derivatives (Debeglia and Corpel, 1997, their Figure 11).

Theta map: Edge detection in magnetic data

The interpretation of magnetic field data at low magnetic latitudes is difficult because the vector nature of the magnetic field increases the complexity of anomalies from magnetic rocks. The most obvious approach to this problem is to reduce the data to the magnetic pole (RTP), where the presumably vertical magnetisation vector will simplify observed anomalies. However, RTP requires special treatment of north-south features in data observed in low magnetic latitudes due to high amplitude corrections of such features. Furthermore, RTP requires the assumption of induced magnetisation with the result that anomalies from remanently and anisotropically magnetised bodies can be severely disturbed. The amplitude of the 3-D analytic signal of the total magnetic field produces maxima over magnetic contacts regardless of the direction of magnetisation. The absence of magnetisation direction in the shape of analytic signal anomalies is a particularly attractive characteristic for the interpretation of magnetic field data near the magnetic equator. Although the amplitude of the analytic signal is dependent on magneti­sation strength and the direction of geological strike with respect to the magnetisation vector, this dependency is easier to deal with in the interpretation of analytic signal amplitude than in the original total field data or pole-reduced magnetic field. It is also straightforward to determine the depth to sources from the distance between inflection points of analytic signal anomalies.

3-D analytic signal in the interpretation of total magnetic field data at low magnetic latitudes

Three-dimensional 3D inversion of magnetic data to recover a distribution of magnetic susceptibility has been successfullyusedformineralexplorationduringthelastdecade. However, the unknown direction of magnetization has limited the use of this technique when significant remanence is present. We have developed a comprehensive methodology for solving this problem by examining two classes of approaches and have formulated a suite of methods of practical utility. The first class focuses on estimating total magnetization direction and then incorporating the resultant direction into an inversion algorithm that assumes a known direction. Thesecondclassfocusesondirectinversionoftheamplitude of the magnetic anomaly vector. Amplitude data depend weaklyuponmagnetizationdirectionandareamenabletodirect inversion for the magnitude of magnetization vector in 3D subsurface. Two sets of high-resolution aeromagnetic dataacquiredfordiamondexplorationintheCanadianArctic areusedtoillustratethemethods’usefulness.

Comprehensive approaches to 3D inversion of magnetic data affected by remanent magnetization

The characterization and interpretation of magnetic anomalies rely upon knowledge of the total magnetization direction. Magnetization is usually assumed to consist solely, or primarily, of induced magnetization. The presence of strong remanent magnetization can alter the direction significantly and consequently adversely affect the interpretation, leading to erroneous sizes or shapes of causative bodies. Therefore, it is imperative to have some understanding of the total magnetization direction. We propose a method based upon the correlation between two quantities in magnetic data interpretation: the vertical gradient and the total gradient of the reduced-to-pole (RTP) field. This method is tested on both synthetic and field data sets. The results show that the method is effective in a variety of situations, including those with two-dimensional and three-dimensional dipping bodies and a field example that has a large deviation between the inducing field direction and the total magnetization direction.

A new method for determination of magnetization direction

Using simple estimates of the signal and noise power from gridded magnetic data, we design regulated frequency-domain operators for reduction to the pole at low magnetic latitudes. These operators suppress the artifacts along the direction of the magnetic declination associated with the conventional reduction-to-the-pole procedure, with negligible increase in computational load. The new procedure is applied to produce high-quality reductions to the pole for noisy low-latitude synthetic data and for magnetic data from the Dixon Seamount.

R. O. Hansen

Papers

Reduction to the pole at low latitudes by Wiener filtering