Interpretation theory in applied geophysics: Grant, F S ... Interpretation theory in applied geophysics Unknown Binding – January 1, 1965 5.0 out of 5 stars 1 rating. See all formats and editions Hide other formats and editions. The Amazon Book Review Book recommendations, author interviews, editors' picks, and more. Read it now. Enter your mobile number or email address below and we'll send you a ...

Interpretation Theory in Applied Geophysics Grant (McGraw-Hill, 140s.)

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

stress provinces indicate that these transitions can be abrupt, occurring over <75 km in places. In the western United States, a region of active tectonism characterized by high levels of seismicity and generally high heat flow, the stress pattern is complex, but numerous stress provinces can be well delineated. Despite relative tectonic quiescence in the eastern and central United States, a major variation in principal stress orientation is apparent between the Atlantic Coast and midcontinent areas. Most of the eastern United States is marked by predominantly compressional tectonism (combined thrust and strike slip faulting), whereas much of the region west of the southern Great Plains is characterized by predominantly extensional tectonism (combined normal and strike slip faulting). Deformation along the San Andreas fault and in parts of the Sierra Nevada is nearly pure strike slip. Exceptions to this general pattern include areas of compressional tectonics in the western United States (the Pacific Northwest, the Colorado Plateau interior, and the Big Bend segment of the San Andreas fault) and the normal growth faulting along the Gulf Coastal Plain. Sources of stress are constrained not only by the orientation and relative magnitude of the stresses within a given province but also by the manner of transition of the stress field from one province to another. Much of the modem pattern of stress in the western United States can be attributed to present transform motion and residual thermal and dynamic effects of Tertiary subduction along the western edge of the North American plate. Abrupt stress transitions around actively extending regions in the western United States probably reflect shallow sources of stress and anomalously thin lithosphere. Large areas characterized by a uniform stress field in the central and eastern United States suggest broad scale plate tectonic forces. In the midcontinent region, both ridge push and asthenospheric viscous drag resistance to lithospheric motion can explain the NE-SW compression in the cold, thick lithosphere of the craton, although drag-induced stress directions (resistance to absolute plate motion) correlate better with the data than do the ridge push directions. Asthenospheric counterflow models do not apply in this region, as predicted stress orientations are about 90 o off. A region of compression oriented approximately perpendicular to the continental margin and Appalachian fold belt is defined by the stress data along the Atlantic Coast. This NW-SE compression is in direct contrast with previous models predicting extension perpendicular to passive continental margins due to lateral density contrasts at the continental-oceanic crust interface. Ridge push forces, while capable of producing a component of compression across the coastal area, do not explain the observed orientation of the stress. Two speculative mechanisms are suggested to explain the observed orientations: (1) rotation of stress (or strain) due to anisotropic Appalachian basement structure and (2) flexural effects associated with erosion and isostatic rebound of the Appalachians.

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State of stress in the conterminous United States

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 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

Given gridded gravity anomaly values and certain limiting restrictions on the mass distribution, a three‐dimensional structural model can be calculated automatically from gravity anomaly data by successive approximations. The causative body is optionally assumed to be either flat‐topped, flat‐bottomed, or symmetrical about a horizontal plane. Density contrast and the position of a horizontal plane delimiting either the top, base, or midsection of the causative body must be specified. A first approximation of structure is obtained by means of the Bouguer slab relationship. The gravity field of this first model is calculated and at each grid point the ratio of observed to calculated gravity is used to modify the first structural model, thus leading to a second approximation of structure. The process is iterated until a satisfactory agreement between observed and calculated gravity is obtained.

Iterative three-dimensional solution of gravity anomaly data using a digital computer

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 analysis of gravity data over thick sedimentary basins, density contrast can sometimes be approximated by a continuous function decreasing exponentially with depth. Typical values of the exponential decrement are in the order of 0.3 to 1.5 km (super -1) . The gravity effect of an infinite (Bouguer) slab in this system tends to a finite limit as the slab becomes infinitely thick, leading to quantitative, and sometimes stringent, limitations on the resolving power of gravity analysis over deep structures. The exponential density term acts as an integrating factor in deriving the gravity effect of a prismatic building element, leading to a simple expression involving both the gravity field and its vertical derivative. A recursive computer algorithm giving the inverse solution automatically in terms of assigned density parameters is applied to a gravity profile over a graben structure at San Jacinto, California. The computed basin configuration is consistent with the seismically determined basement depth of 2.4 km and the estimated density layering of the graben fill.

Gravity analysis using an exponential density-depth function; San Jacinto Graben, California

The Rio Grande rift encompasses uplifts of the southern Rocky Mountains and their southern extension as well as axial fault blocks. The rift widens irregularly southward from a narrow horst in Colorado into a broad collapsed vault, characterized by grabens, in southern New Mexico. Whether manifested by horsts or grabens, primarily extensional strain is involved which increases in magnitude southward. Extensional faulting along the rift occurred in Neogene to Quaternary time, but the rift follows an axis of Laramide, Pennsylvanian, and possibly earlier uplifts. Gravity gradients due to the low density of graben fill delineate major faults of the rift system, which show a gridded or en echelon pattern over distances of tens of kilometers. Aeromagnetic data show these faults to be aligned with basement structural grain. Zigzags hundreds of kilometers long in the trend of the rift may also be related to basement grain. Basement trends in the Colorado Plateau to the west seem to differ in direction from those in the High Plains to the east. Seismic data also show that the rift occurs in an area of transition between anomalous crustal and upper mantle structure typical of the Cordillera and crustal structure typical of the High Plains.more » Deep seismic data are sparse within the rift, but high heat flow, high elevation, high electrical conductivity, and both residual positive (shallow source) and negative (deep source) gravity anomalies suggest the presence of symmetrical anomalous crustal and upper-mantle structure along the axis of the rift. In the Socorro area, where the rift has been studied intensively, available data indicate relatively low compressional velocity, rapid Holocene uplift, and the presence of magma within the crust. In view of geomorphic evidence for widespread Holocene faulting, the seismicity of the rift is surprisingly low.« less

Lindrith Cordell

Papers

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

Iterative three-dimensional solution of gravity anomaly data using a digital computer

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

Gravity analysis using an exponential density-depth function; San Jacinto Graben, California

Regional geophysical setting of the Rio Grande rift