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

The dynamic history of the Trans-Mexican Volcanic Belt and the Mexico subduction zone

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.

/pdf/the-historical-development-of-the-magnetic-method-in-2o9psruy4m.pdf

The historical development of the magnetic method in exploration

Satellite telemetry studies of 20 adult and sub- adult white sharks (360-530 cm estimated total length (TL)) in the eastern North Pacific during 1999-2005 re- vealed long distance seasonal migrations from the coast of California to an offshore focal area 2,500 km west of the Baja Peninsula, as well as the Hawaii Islands. Three tags were recovered allowing detailed behavioral analyses, including one shark's migration cycle from the coast to the offshore focal area and back. While near pinniped rook- eries in autumn and winter, sharks avoided the surface and used water to 50 m depth, consistent with a silhouette- based hunting strategy. Offshore migrations were initiated during November-March and followed periods of decreasing pinniped abundance. Migrations were highly directed, taking 23 ± 5 days to reach the offshore focal area along similar paths among sharks and years, defining a migration corridor. Sharks exhibited a broad depth distri- bution (0-644 m) in the offshore focal area, and remained there for up to 167 days during spring and summer, though primary productivity and fishery data suggest that forage resources are scarcer there than in other regions of the eastern North Pacific. Archival data from one shark re- vealed intensive oscillatory movements while in the off- shore focal area, a behavior that may be related to foraging or mating. Sharks traveling to Hawaii remained near the islands up to 122 days, potentially feeding on pelagic fishes and marine mammals that concentrate around the islands.

/pdf/migration-and-habitat-of-white-sharks-carcharodon-carcharias-32m90022ww.pdf

Migration and habitat of white sharks (Carcharodon carcharias) in the eastern Pacific Ocean

Tensor Euler deconvolution has been developed to help interpret gravity tensor gradient data in terms of 3-D subsurface geological structure. Two forms of Euler deconvolution have been used in this study: conventional Euler deconvolution using three gradients of the vertical component of the gravity vector and tensor Euler deconvolution using all tensor gradients. These methods have been tested on point, prism, and cylindrical mass models using line and gridded data forms. The methods were then applied to measured gravity tensor gradient data for the Eugene Island area of the Gulf of Mexico using gridded and ungridded data forms. The results from the model and measured data show significantly improved performance of the tensor Euler deconvolution method, which exploits all measured tensor gradients and hence provides additional constraints on the Euler solutions.

Euler deconvolution of gravity tensor gradient data

Data presented on the magnetic anomaly map in the continental areas have largely been derived from published map sources. However, these maps for the most part have been digitized and subjected to various processing steps in preparation for compositing. The marine areas of the map consist mostly of digital data track, except in the Gulf of Mexico, Caribbean Sea and the continental shelf areas of Alaska where the data were obtained from digitized maps.

Magnetic anomaly map of North America

The magnetic anomalies caused by such diverse model shapes as the finite strike length thick dike, the vertical prism, thes loping step, the parallelepiped body, etc., may be obtained through an appropriate numerical integration of the expression for the magnetic effect produced by a finite thin plate. Using models generated in this manner, an automatic computer method has been developed at the Geological Survey of Canada for the interpretation of magnetic data. Because the magnetic anomalies produced by the various model shapes are nonlinear in parameters of shape and position, it is necessary to use an iterative procedure to obtain the values for the various model parameters which yield a least‐squares best‐fit anomaly curve to a set of discrete observed data. The interpretation method described in this paper uses the Powell algorithm for this purpose. The procedure can sometimes be made more efficient using a Marquardt modification to the Powell algorithm. Examples of the use of the method are presente...

An automatic least‐squares multimodel method for magnetic interpretation

The first instrument to measure the magnetic field while being flown in an aircraft was the Earth inductor magnetometer. This was done in the USSR in 1936. But the system's sensitivity of 100 nT, called gammas in those days, was hardly suitable for routine surveys of large areas. Hans Lundberg built, after World War II, a helicopter-borne system for mineral prospecting in the Canadian Precambrian Shield. The rotor was spun by compressed air at 1800 Hz between a set of gimbal-mounted Helmholtz coils whose axis was aligned vertically. Thus, the system measured variations of the Earth's vertical field with a sensitivity of about 25 nT.

History of aeromagnetic surveying in Canada

Publisher Summary Developments in airborne geophysics have occurred so rapidly in the past several years that it is a difficult task to follow and record them all. Geophysics covers a broad spectrum of scientific endeavors and workers in one specialized field are often unaware of important events in another. Unlike any other science, geophysics has become identified not with a particular technology or discipline but with its objective—the study of the earth—and for this reason it embraces many different disciplines and traverses many established lines of communication. This chapter attempts to describe generally and discuss critically some of the airborne geophysical methods that have seen successful application in recent years. In geophysics, “methods” are not nearly as important as “applications.” Too often a method is used holus-bolus without any very precise idea of what it is supposed to indicate. One way of subordinating methods is to discuss them as they appear under the headings of applications or objectives. The applications covered in this chapter are more numerous than the methods, to avoid complicated cross-referencing; the methods of airborne geophysics have been grouped under the following headings: (1) Aeromagnetic, (2) low-frequency electromagnetic, (3) radiometric, (4) gravity, and (5) other remote-sensing techniques.

Airborne Geophysical Methods

The Magnetic Anomaly Map of North America; A new tool for regional geologic mapping

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