Potential field tilt—a new concept for location of potential field sources

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

A high-resolution technique is developed to image geologic boundaries such as contacts and faults. The outlines of the geologic boundaries can be determined by tracing the maximum amplitudes of an enhanced analytic signal composed of the nth-order vertical derivative values of two horizontal gradients and one vertical gradient. The locations of the maximum amplitudes are independent of the ambient potential field. This technique is particularly suitable when interference effects are considerable and/or when both induced and remanent magnetizations are not negligible. The corresponding depth to each geologic boundary can be estimated from the amplitude ratio of the enhanced and the simple analytic signals, which provides a simple estimation technique. Such a method has been applied to magnetic data acquired in the Ilan Plain of Taiwan located at the southwestern end of the Okinawa Trough. The quantitative analysis shows that the underlying geologic boundaries deepen southward and slightly eastward. The enlargement of the Ilan Plain in the direction of the Okinawa Trough and the existence of north-northwest and west-northwest trending faults near the city of Ilan reveal a discontinuity between the Okinawa Trough backarc extension and the compressional process in Taiwan.

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High‐resolution detection of geologic boundaries from potential‐field anomalies: An enhanced analytic signal technique

The gravity method was the first geophysical technique to be used in oil and gas exploration. Despite being eclipsed by seismology, it has continued to be an important and sometimes crucial constraint in a number of exploration areas. In oil exploration the gravity method is particularly applicable in salt provinces, overthrust and foothills belts, underexplored basins, and targets of interest that underlie high-velocity zones. The gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies. There is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets. Gravimeters have undergone continuous improvement during the past 25 years, particularly in their ability to function in a dynamic environment. This and the advent of

Historical development of the gravity method in exploration

Two new techniques are employed in analysis of aeromagnetic data from the San Juan basin, New Mexico, to enhance the expression of buried basement structure and lithology: (1) the data are analytically continued downward onto the irregular basement surface in order to reduce the effect of variable depth to basement, and (2) magnetization boundaries are delineated by a linear filter based on the gradient of pseudogravity Reduction to the basement involves three procedures that, together, provide a practical method for continuation of potential fields between general surfaces Data are continued from the nonlevel flight-elevation surface onto a level surface (drape-tolevel continuation) by means of a system of successive approximations based on expansion of a Taylor series Data are continued from the new level surface downward to another level surface at the mean elevation of the basement (level-to-level continuation) by analytical downward continuation based on the fast Fourier transform, incorporating a high-cut filter Data are continued from this level onto the nonlevel basement surface (level-to-drape continuation) by direct evaluation of a truncated Taylor series expansion, in the vertical dimension, of the field represented on the level surface Magnetization boundaries are determined by evaluating the magnitude of the horizontal gradient of the pseudogravity transform of the magnetic data Lines drawn along ridges of the horizontal gradient mark inferred basement-magnetization boundaries After application of these techniques, the aeromagnetic data from the San Juan basin, New Mexico, revealed features of the Precambrian basement not evident, or only vaguely so, in the original data Together with scanty drill data, the aeromagnetic data indicate a 70-km-wide belt of predominantly metasupracrustal rocks trending east-northeast across the center of the basin Predominantly granitic terranes border this belt on the northwest and southeast Numerous magnetization boundaries tens of kilometers in length are identified Structural grain is strongly east-northeast; an area of prominent northerly trends occurs in the north-central part of the basin and locally elsewhere Cenozoic intrusive rocks are unquestionably aligned with basement structural grain Laramide and Neogene structures in general are not so aligned There is no evidence of large strike-slip displacement of basement structures within the area surveyed

Mapping Basement Magnetization Zones From Aeromagnetic Data In the San Juan Basin, New Mexico

The horizontal‐gradient method has been used since 1982 to locate density or magnetic boundaries from gravity data (Cordell, 1979) or pseudogravity data (Cordell and Grauch, 1985). The method is based on the principle that a near‐vertical, fault‐like boundary produces a gravity anomaly whose horizontal gradient is largest directly over the top edge of the boundary. Magnetic data can be transformed to pseudogravity data using Fourier techniques (e.g., Hildenbrand, 1983) so that they behave like gravity data; thus the horizontal gradient of pseudogravity also has maximum magnitude directly over the boundary. The method normally is applied to gridded data rather than to profiles. The horizontal‐gradient magnitude is contoured and lines are drawn or calculated (Blakely and Simpson, 1986) along the contour ridges. These lines presumably mark the top edges of magnetic or density boundaries. However, horizontal‐gradient magnitude maxima (gradient maxima) can be offset from a position directly over the boundary f...

Limitations of determining density or magnetic boundaries from the horizontal gradient of gravity or pseudogravity data

In the application of harmonic analysis to potential‐field geophysical studies, relationships derived in terms of the continuous Fourier integral transform are evaluated in terms of the discrete Fourier transform. The discrete transform, obtained by transforming a finite number of equispaced samples of the actual aperiodic continuous function, is too low at the dc level and increasingly too high in the high frequencies, compared with the theoretical integral transform. As a consequence, overly restrictive limitations must be placed on high‐frequency‐amplifying operators such as differentiation and downward continuation. Also, a spurious and troublesome azimuthal distortion occurs in the discrete Fourier analysis of three‐dimensional (3-D) (map) data represented as grids. The discrete transform can be made essentially equivalent to the integral transform if, before sampling, the continuous aperiodic input function is made periodic by shifting the function by integer multiples of the data interval and summi...

V. J. S. Grauch

Papers

Mapping Basement Magnetization Zones From Aeromagnetic Data In the San Juan Basin, New Mexico

Limitations of determining density or magnetic boundaries from the horizontal gradient of gravity or pseudogravity data

Reconciliation of the discrete and integral Fourier transforms