An algorithm that implements Helbig's (1963) integrals for estimating the vector components (m_x, m_y, m_z) of the magnetic dipole moment from the first order moments of the vector magnetic field components (ΔX, ΔY, ΔZ) is tested on real and synthetic data. After a grid of total field aeromagnetic data is converted to vector component grids using Fourier filtering, Helbig's infinite integrals are evaluated as finite integrals in small moving windows using a quadrature algorithm based on the 2-D trapezoidal rule. Prior to integration, best-fit planar surfaces must be removed from the component data within the data windows in order to make the results independent of the coordinate system origin. Two different approaches are described for interpreting the results of the integration. In the “direct” method, results from pairs of different window sizes are compared to identify grid nodes where the angular difference between solutions is small. These solutions provide valid estimates of total magnetization directions for compact sources such as spheres or dipoles, but not for horizontally elongated or 2-D sources. In the “indirect” method, which is more forgiving of source geometry, results of the quadrature analysis are scanned for solutions that are parallel to a specified total magnetization direction.

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A three-dimensional non-uniform magnetic modeling is proposed to obtain information about a magnetization of a seamount, which was divided into many blocks modeled by layered and rectangular prisms, and parameters were assigned to each block describing magnetic three components. Our data were the magnetic total force on the sea. A set of linear observation equations was formulated in terms of three components of magnetization for each block. The solution was obtained by using the conjugate gradients method because of its fast and accurate advantages of calculation. In this inversion, a common set of three components was defined for several blocks to decrease the number of unknown parameters. A computer program has been tested with artificial data and applied to data of Daiichi Kashima Seamount observed during the first phase of the Kaiko project carried out with the R/V Jean Charcot in 1984. In the real application, the crustal structure was divided into three layers (top depth to 5 km depth, 5-6.5 km depth and 6.5-8 km depth). The result of the inversion shows that the top portion and the submerged western half of this seamount are covered with the low magnetization layers, and in the middle layer (5-6.5 km depth) of eastern half side, declinations, inclinations and intensities are almost northward, 15° and 3-5 A/m, respectively. In the third layer (6.5-8 km depth), the reverse magnetizations are revealed in the southeastern and northern sides of Daiichi Kashima Seamount and around Katori Seamount. These reverse magnetizations may reflect part of the magnetic lineations of the Pacific plate.

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Some aquifers of the southwest Colorado Plateau, U.S.A., are deeply buried and overlain by several impermeable units, and thus recharge to the aquifer is probably mainly by seepage down penetrative fracture systems. This purpose of this study was to develop a method to map the location of candidate deep penetrative fractures over a 120, 000 km² area using gravity and aeromagnetic anomaly data together with surficial fracture data. The resulting database constitutes a spatially registered estimate of recharge location. Candidate deep fractures were obtained by spatial correlation of horizontal gradient and analytic signal maxima of gravity and magnetic anomalies vertically with major surficial lineaments obtained from geologic, topographic, side-looking airborne radar, and satellite imagery. The maps define a sub-set of possible penetrative fractures because of limitations of data coverage and the analysis technique. The data and techniques employed do not yield any indication as to whether fractures are open or closed. Correlations were carried out using image processing software in such a way that every pixel on the resulting grids was coded to uniquely identify which datasets correlated. The technique correctly identified known deep fracture systems and many new ones. Maps of the correlations also define in detail the tectonic fabrics of the southwestern Colorado Plateau.

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We produced a crustal magnetization model for the Maud Rise in the southwest Indian Ocean off the coast of East Antarctica using magnetic observations from the @/Os052/@rsted satellite and near-surface surveys complied by the Antarctic Digital Magnetic Anomaly Project (ADMAP). Joint inversion of the two anomaly fields suggests that the magnetic effects due to crustal thickness variations and remanence involving the normal polarity Cretaceous Quiet Zone (KQZ) dominate at satellite altitude (-700 km). The crustal thickness effects were modeled in the @/Os052/@rsted data using crustal thickness variations derived from satellite altitude gravity data. Modeling of the residual @/Os052/@rsted and near-surface magnetic anomalies supports extending the KQZ eastwards to the Astrid Ridge. The remaining near-surface anomalies involve crustal features with relatively high frequency effects that are strongly attenuated at satellite altitudes. The crustal modeling can be extended by the satellite magnetic anomalies across the Indian Ocean Ridge for insight on the crustal properties of the conjugate Agulhas Plateau. The modeling supports the Jurassic reconstruction of Gondwana when the African Limpopo-Zambezi and East Antarctic Princess Astrid coasts were connected as part of a relatively demagnetized crustal block.

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The Geological Survey of Japan (GSJ), AIST has been systematically measuring physical properties of basement rocks in Japan for more than 30 years and has started to construct a petrophysical database called PB-Rock 21 (Petrophysical Database of Basement rocks in Japan for the 21st Century, http://www.aist.go.jp/RIODB/ PB-Rock21/). The petrophysical data has been integrated with aeromagnetic data to determine the subsurface structure of the Himekami and Sobanokami plutons, members of the Kitakami plutonic belt, northeast Japan. For the Himekami pluton, the result of apparent magnetization intensity mapping clearly shows zonation within subplutons as well as regional rock type differences between the North (felsic) and South (felsic and mafic complex) sub-plutons. The bottom depths of the pluton were estimated by a successive 3-D magnetic modeling to be 1.7 km (North sub-pluton) and 1.5 km below sea level (South sub-pluton), respectively. Magnetic modeling of the Sobanokami pluton, which is partly exposed at the eastern edge of the Ishinomaki Plain, implies that the main part of the pluton extends to the northwest below the surface. Its western edge is cut by the Ishinomaki-wan fault, believed to be the upward extension of the fault plane of the 2003 northern Miyagi earthquake as defined by seismic observations and reflection seismic surveys conducted after the earthquake. Integrated magnetic and gravity modeling suggests that the Kakeyama Formation, a Neogene conglomerate deposited in a half-graben along the Ishinomaki-wan fault during Miocene rifting, contains more granitic fragments toward the south than to the north in the Ishinomaki Plain.

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On September 18, 2002, we conducted a high-resolution, low-altitude helicopter-borne aeromagnetic survey at two flight altitudes, using spiral trajectories for the first time, over Unzen Volcano in the framework of the Unzen Scientific Drilling Project (USDP). This study obtained more detailed and new information than the previous aeromagnetic studies in Unzen volcano about the geological features, for understanding the history and eruption mechanism of the Unzen volcano. Therefore, we conducted a magnetization intensity mapping on the volcano, on the assumption that the magnetic anomalies are caused by the terrain magnetized in the same direction as the present Earth's magnetic field and the magnetization intensity varies only laterally. This map shows good agreement with the geologic features, especially the hydrothermal alteration zone and the collapsed pyroclastic deposits. In addition, even in the area covered by lavas, the magnetization intensities show various values corresponding to each eruption event. It may be considered that the differences in magnetic properties reflect different oxygen fugacity in rocks during their cooling time period. Local magnetization lows on Heisei-Shinzan suggest that the Heisei lava produced by the 1991-1995 eruption has not yet been cooled enough.

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The Gebel El-Zeit area is located on the western coast of the Gulf of Suez, Egypt. The areas in/and around the Gulf of Suez are generally important due to their hydrocarbon resources. In this study, we have applied gradient interpretation techniques (Euler deconvolution and analytic signal) to the aeromagnetic data of the Gebel El-Zeit area. The main objective of this study is to identify and delineate the possible subsurface structure of the area that may assist in locating new hydrocarbon prospects. Results of Euler method suggested that, on the eastern and western parts of the area, the basement could be observed on the ground (-50 m over the ground) and became more deeper on the central part to reach depth of 5 km (from the ground level). Results from the analytic signal method indicated that, the depth to the basement has an average value of 156 m on the eastern side and 758 m on the western side. Generally, the area is characterized by a graben structure bounded by major faults striking in the NW-SE direction.

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Exploration in the northern Red Sea of Egypt has had little success in locating hydrocarbon accumulation. The main exploration problems in this region are the complex block faulting and Miocene salt structures. The complex basement block structure arises from the different ages of faults and the difficulty of determining the exact age relations. In this paper, we present a case study of structural mapping using application of the Euler method to high-resolution aeromagnetic data collected in the Quseir area of the northern Red Sea of Egypt. The results indicated that the area is affected by sets of fault systems, which are mainly trending in the NNW-SSE, NW-SE and NE-SW directions. The results also delineated boundaries of a long and wide magnetic body on the offshore part of the study area. This basement intrusion is most probably related to the Red Sea rift and perhaps associated with structures higher up in the sedimentary section. As a result, the area above this anomaly is highly recommended for further oil exploration. This example illustrates that high-resolution aeromagnetic surveys can help greatly in delineating subsurface structure in the northern Red Sea of Egypt.

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The Jutulstraumen ice stream in western Dronning Maud Land may conceal a Jurassic continental rift. Delineating the geometry and the magmatic patterns of this inferred glaciated rift in East Antarctica is important to improve our understanding of the regional tectonic and magmatic processes associated with Gondwana break-up. A high-resolution aeromagnetic survey provides new insights over the largely buried tectonic and magmatic patterns of the Jutulstraumen area. Prominent NE-SW oriented aeromagnetic trends are detected over the Jutulstraumen. These trends delineate major inherited structural boundaries, active in Grenvillian (about 1.1 Ga) and Pan-African times (about 500 Ma), which appear to strongly control the location of the later Jurassic rift. The postulated eastern flank of the rift is marked by a broad positive anomaly over H. U. Sverdrupfjella. Buried Grenvillian age rocks may be the source of the long-wavelength anomaly. However, the higher frequency components correlate with granitoids of late Pan-African age. The inferred western flank of the rift features short-wavelength anomalies over the Borgmassivet and Ahlmannryggen areas, indicating a considerably greater extent of mid-Proterozoic tholeiitic sills than apparent in outcrop. In contrast, aeromagnetic signatures suggest that alkaline plutons, which relate to Jurassic rifting, are restricted to outcrop areas along the eastern rift flank. The prominent magnetic low over the Jutulstraumen indicates either a largely amagmatic rift, or perhaps subglacial sediments within the rift basin.

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High-resolution aeromagnetic surveys over forearc basins can detect faults and folds in weakly magnetized sediments, thus providing geologic constraints on tectonic evolution and improved understanding of seismic hazards in convergent-margin settings. Puget Sound, Washington, and Cook Inlet, Alaska, provide two case histories. In each lowland region, shallow-source magnetic anomalies are related to active folds and/or faults. Mapping these structures is critical for understanding seismic hazards that face the urban regions of Seattle, Washington, and Anchorage, Alaska. Similarities in aeromagnetic anomaly patterns and magnetic stratigraphy between the two regions suggest that we can expect the aeromagnetic method to yield useful structural information that may contribute to earth-hazard and energy resource investigations in other forearc basins.

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