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  • 1
    Publication Date: 2019-01-25
    Description: Research activities performed on MAGSAT scalar data over South America, Central America, and the adjacent marine areas are summarized. The geologic utility of magnetic anomalies detected by satellite is demonstrated by focusing on the spherical-Earth interpretation of scalar MAGSAT data in combination with ancillary geological and geophysical data to obtain lithospheric models for these regions related to their contemporary crustal dynamics processes, geologic history, current volcanism seismicity and natural resources.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: E84-10126 , NASA-CR-172801 , NAS 1.26:172801
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  • 2
    Publication Date: 2019-01-25
    Description: A positive magnetic anomaly, which dominates the MAGSAT scalar field over the south-central United States, results from the superposition of magnetic effects from several geologic sources and tectonic structures in the crust. The highly magnetic basement rocks of this region show good correlation with increased crustal thickness, above average crustal velocity and predominantly negative free-air gravity anomalies, all of which are useful constraints for modeling the magnetic sources. The positive anomaly is composed of two primary elements. The western-most segment is related to middle Proterozoic granite intrusions, rhyolite flows and interspersed metamorphic basement rocks in the Texas panhandle and eastern New Mexico. The anomaly and the magnetic crust are bounded to the west by the north-south striking Rio Grande Rift. The anomaly extends eastward over the Grenville age basement rocks of central Texas, and is terminated to the south and east by the buried extension of the Ouachita System. The northern segment of the anomaly extends eastward across Oklahoma and Arkansas to the Mississippi Embayment. It corresponds to a general positive magnetic region associated with the Wichita Mountains igneous complex in south-central Oklahoma and 1.2 to 1.5 Ga. felsic terrane to the north.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: Improving the Geol. Interpretation of Magnetic and Gravity Satellite Anomalies; 1 p
    Format: text
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  • 3
    Publication Date: 2019-01-25
    Description: Current limitations in the quantitative interpretation of satellite-elevation geopotential field data and magnetic anomaly data were investigated along with techniques to overcome them. A major result was the preparation of an improved scalar magnetic anomaly map of South America and adjacent marine areas directly from the original MAGSAT data. In addition, comparisons of South American and Euro-African data show a strong correlation of anomalies along the Atlantic rifted margins of the continents.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: E85-10103 , NASA-CR-175614 , NAS 1.26:175614
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  • 4
    Publication Date: 2013-08-31
    Description: Seismic refraction profiles for the North American continent were compiled. The crustal models compiled data on the upper mantle seismic velocity (P sub n), the crustal thickness (H sub c) and the average seismic velocity of the crystalline crust (V sub p). Compressional wave parameters were compared with shear wave data derived from surface wave dispersion models and indicate an average value for Poisson's ratio of 0.252 for the crust and of 0.273 for the uppermost mantle. Contour maps illustrate lateral variations in crustal thickness, upper mantle velocity and average seismic velocity of the crystalline crust. The distribution of seismic parameters are compared with a smoothed free air anomaly map of North America and indicate that a complidated mechanism of isostatic compensation exists for the North American continent. Several features on the seismic contour maps also correlate with regional magnetic anomalies.
    Keywords: GEOPHYSICS
    Type: NASA-CR-175134 , NAS 1.26:175134
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  • 5
    Publication Date: 2013-08-31
    Description: Efforts are made to identify methods of decreasing magnetic interpretation ambiguity by combined gravity and magnetic analysis, to evaluate these techniques in a preliminary manner, to consider the geologic and geophysical implications of correlation, and to recommend a course of action to evaluate methods of correlating gravity and magnetic anomalies. The major thrust of the study was a search and review of the literature. The literature of geophysics, geology, geography, and statistics was searched for articles dealing with spatial correlation of independent variables. An annotated bibliography referencing the Germane articles and books is presented. The methods of combined gravity and magnetic analysis techniques are identified and reviewed. A more comprehensive evaluation of two types of techniques is presented. Internal correspondence of anomaly amplitudes is examined and a combined analysis is done utilizing Poisson's theorem. The geologic and geophysical implications of gravity and magnetic correlation based on both theoretical and empirical relationships are discussed.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: NASA-CR-144767
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  • 6
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An inversion scheme that determines a velocity model simultaneously from normal-incidence reflection and refraction/wide-angle reflection seismic data has been developed, tested, and applied to real seismic data. Simultaneous inversion minimizes the non-uniqueness of inversion results. A common midpoint (CMP) stacked reflection section provides a detailed image of the structural configuration in time. Refraction or wide-angle reflection data can be used to derive a velocity model. If both refraction data and normal-incidence reflection data are available along coincident profiles, simultaneous inversion of the data sets can produce an improved model. With the extra control of the interface depth provided by reflection data, it is also possible to avoid ambiguity of phase identification. the commonly used damped-least-squares (DLS) inversion method often results in significant inversion artefacts. We provide effective methods for reducing inversion artefacts using both damping and smoothing. From tests using synthetic data, we conclude that the simultaneous inversion scheme can improve resolution and can be applied to velocity models with a reasonably complex structure. the simultaneous inversion scheme has been successfully applied to coincident seismic reflection and refraction data recorded by the SAGE program in 1990–1991 in the western boundary area of the Rio Grande rift near Abiquiu, New Mexico. In this example, the simultaneous inversion derived a velocity model in which stratigraphic units and extensional fault blocks can be resolved.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We propose a quasi-compressional wave (qP-wave) traveltime inversion technique for determination of the 2-D horizontal and vertical qP-wave velocities and depths to interfaces from seismic reflection/refraction data sets. Synthetic test model studies demonstrate that the method is effective if ray-path coverage within the model is adequate, and indicate that horizontal qP-wave velocities and depths to interfaces are better resolved than vertical qP-wave velocities.We applied our inversion technique to the Trans-Alaska Crustal Transect (TACT) profile, southern Alaska, between SP19 and SP11. The results of the inversion suggest that the upper 5 km of the crust is anisotropic. The range of anisotropy for the derived model is between 10 and 25 per cent, consistent with the range determined in previous studies and from laboratory measurements. The cause of anisotropy is thought to be preferred mineral orientations in the foliation planes of the metasedimentary rocks. Based on the test model studies and the applications to actual seismic data sets, it is suggested that future traveltime inversion studies will require particularly high-quality data with dense ray-path coverage in which multiple sources and receivers and both reflection and refraction data are utilized.
    Type of Medium: Electronic Resource
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  • 8
    Publication Date: 2016-03-09
    Description: Scalar magnetic anomaly data from MAGSAT, reduced to vertical polarization and long wavelength pass filtered free air gravity anomaly data of South America and the Caribbean are compared to major crustal features. The continental shields generally are more magnetic than adjacent basins, oceans and orogenic belts. In contrast, the major aulacogens are characterized by negative anomalies. Spherical earth magnetic modeling of the Amazon River and Takatu aulacogens in northeastern South America indicates a less magnetic crust associated with the aulacogens. Spherical earth modeling of both positive gravity and negative magnetic anomalies observed over the Mississippi Embayment indicate the presence of a nonmagnetic zone of high density material within the lower crust associated with the aulacogen. The MAGSAT scalar magnetic anomaly data and available free air gravity anomalies over Euro-Africa indicate several similar relationships.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: NAS 1.26:173485 , NASA-CR-173485 , E84-10115
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  • 9
    Publication Date: 2016-03-09
    Description: The South-Central United States Magnetic Anomaly is the most prominent positive feature in the MAGSAT scalar magnetic field over North America. The anomaly correlates with increased crustal thickness, above average crustal velocity, negative free air gravity anomalies and an extensive zone of Middle Proterozoic anorogenic felsic basement rocks. Spherical dipole source inversion of the MAGSAT scalar data and subsequent calculation of reduced to pole and derivative maps provide constraints for a crustal magnetic model which corresponds geographically to the extensive Middle Proterozoic felsic rocks trending northeasterly across the United States. These felsic rocks contain insufficient magnetization or volume to produce the anomaly, but are rather indicative of a crustal zone which was disturbed during a Middle Proterozoic thermal event which enriched magnetic material deep in the crust.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: NAS 1.26:174048 , E85-10025 , NASA-CR-174048
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  • 10
    Publication Date: 2019-01-25
    Description: Flat-Earth modeling is a desirable alternative to the complex spherical-Earth modeling process. These methods were compared using 2 1/2 dimensional flat-earth and spherical modeling to compute gravity and scalar magnetic anomalies along profiles perpendicular to the strike of variably dimensioned rectangular prisms at altitudes of 150, 300, and 450 km. Comparison was achieved with percent error computations (spherical-flat/spherical) at critical anomaly points. At the peak gravity anomaly value, errors are less than + or - 5% for all prisms. At 1/2 and 1/10 of the peak, errors are generally less than 10% and 40% respectively, increasing to these values with longer and wider prisms at higher altitudes. For magnetics, the errors at critical anomaly points are less than -10% for all prisms, attaining these magnitudes with longer and wider prisms at higher altitudes. In general, in both gravity and magnetic modeling, errors increase greatly for prisms wider than 500 km, although gravity modeling is more sensitive than magnetic modeling to spherical-Earth effects. Preliminary modeling of both satellite gravity and magnetic anomalies using flat-Earth assumptions is justified considering the errors caused by uncertainties in isolating anomalies.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: Improving the Geol. Interpretation of Magnetic and Gravity Satellite Anomalies; 1 p
    Format: text
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