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  • 1
    Electronic Resource
    Electronic Resource
    DEPTH ESTIMATION OF MAGNETIC SOURCES BY MEANS OF FOURIER AMPLITUDE SPECTRA (1976)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 24 (1976), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The Fourier transform of a square-shaped section of a magnetic survey, digitized in a square grid, forms a rectangular matrix of coefficients which can be condensed to a series of average amplitudes dependent only on their frequency and no longer on the direction of the respective partial waves. These average amplitudes together represent a spectrum which–when plotted in a semilogarithmic coordinate system (log amplitude versus frequency)–often shows straight segments which decrease with increasing frequency. By continuing the given field downwards these straight segments become horizontal at a certain depth, the so-called “white depth”. This white depth may be used as a first estimate for the depth of magnetic sources producing the respective part of the field. It is shown that the sources which correspond to such use of the white depth can be expected to be “randomly distributed with some positive autocorrelation”.As an example for such a depth estimation the interpretation of the aeromagnetic survey of NW-Germany by a relief in 8–16 km depth is given. The relief divides the subsurface in an upper nonmagnetic layer and a lower layer with magnetization M= 2 Am−1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1976.tb00926.x
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  • 2
    Electronic Resource
    Electronic Resource
    REPLY TO COMMENT ON “TWO-DIMENSIONAL POWER SPECTRAL ANALYSIS OF AEROMAGNETIC FIELDS” BY A. SPECTOR AND F. S. GRANT (1976)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 24 (1976), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1976.tb00935.x
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  • 3
    Electronic Resource
    Electronic Resource
    HARMONIC INVERSION OF AN AEROMAGNETIC ANOMALY FROM A REMANENTLY MAGNETIZED PRECAMBRIAN SCHIST BELT (KARNATAKA, INDIA) (1978)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 26 (1978), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The conventional methods of magnetic interpretation consider in general the induced magnetization only. However, in a computer oriented inverse method, the direction of magnetization can be taken as arbitrary. Following this procedure, this study aims to present a realistic interpretation of an aeromagnetic anomaly from Chitaldrug Schist belt (Mysore, India) assuming a three-dimensional configuration of the causative source possessing remanent magnetization. It further provides the direction of remanent magnetization of the source rock as obtained from the analysis of the magnetic anomaly. The total intensity map obtained delineates the surface and sub-surface structures in the region and provides informations regarding the characteristics of primordial crust.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1978.tb01617.x
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  • 4
    Electronic Resource
    Electronic Resource
    TWO-DIMENSIONAL POWER SPECTRAL ANALYSIS OF AEROMAGNETIC FIELDS (1974)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 22 (1974), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A study of probability distribution function and spectrum of the Airborne Total Intensity Map for an area of 3000 Squ. miles covered by the Bundelkhand granite in India has revealed a close correlation between the Gaussian or non-Gaussian nature of distribution and the shape of unite value contour on the spectrum plot. Further studies of radial spectrum plots suggest that the surface magnetic sources are mainly responsible for the non-Gaussian character of the distribution function and as well as for irregular spectrum shape. The histogram of depth values for magnetic sources obtained from radial spectrum plots show three horizons, namely (i) surface (ii) 410 m(1400′) and (iii) 1000 m (3400′).Based on these informations blocks with ferromagnetic composition at the surface can be identified from those which are non magnetic at the surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1974.tb00090.x
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  • 5
    Electronic Resource
    Electronic Resource
    Results of airborne magnetometer profile from offshore Mangalore to offshore Madras (India) along the 13th degree parallel (1969)
    Springer
    Pure and applied geophysics 75 (1969), S. 133-139 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary A study is made of an airborne magnetometer profile from 130 km offshore Mangalore to 60 km offshore Madras approximately along the 13th degree parallel obtained in May 1967 by the National Geophysical Research Institute. The total length of the profile is about 770 km. A Bouguer gravity anomaly profile along same traverse over the land section has been also studied along with the magnetic profile. Eight major anomalies on the magnetic profile were chosen for depth calculations. Depths of the magnetic bodies were determined by elementary approximation method ofSmellie. It is found that in most of the cases the sources of the magnetic anomalies lie between 5 to 20 km (approximately). Only one anomaly yielded a depth of 2.7 km for its source. These anomalies are accounted for in terms of possible basic intrusions, faults, zones of weakness and ridge-like structures in deeper Crustal levels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00875050
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  • 6
    Electronic Resource
    Electronic Resource
    Crustal structure in the North Arabian Sea from magnetic surveys (1981)
    Springer
    Marine geophysical researches 4 (1981), S. 427-436 
    Details
    ISSN: 1573-0581
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The spectral study of the aero-magnetic map of the North Arabian Sea (above 20°N) has delineated three horizons at average depths of 45 km, 21 km, and 8 km. Spectral estimates from smaller blocks of data drawn from the original map suggest that the 21 km horizon varies in depth from 14 km on the abyssal plain (oceanic crust) to 24 km towards the north and 28 km towards the east onto the continental shelf. This appears to correspond to the crust-mantle interface (Moho). The 8 km horizon corresponds to the top of the igneous basement. The significance of the deepest layer (45 km) is discussed as the maximum depth of the Curie point geotherm in this region. The spectral estimate of the block of data on the continental shelf off the west coast of India (above 20°N) has brought out some magnetic inhomogeneity at a shallower depth of 4 km. This appears to be connected with the sea-floor spreading phenomenon from the Carlsberg ridge. The presence of such a magnetic inhomogeneity at a depth of 4 km is further confirmed by the spectral estimate of a marine magnetic map off the west coast of India around Bombay. The depth of the basement inferred from this study is in close agreement with that obtained from other studies in this region, such as seismics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00286037
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  • 7
    Electronic Resource
    Electronic Resource
    Spectral study of the Bouguer anomaly map of a rift valley and adjacent areas in Central India (1981)
    Springer
    Pure and applied geophysics 119 (1981), S. 1051-1062 
    Details
    ISSN: 1420-9136
    Keywords: Spectral analysis ; Bouguer anomaly ; Rift valley
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The spectral study of the Bouguer anomally map of Central India suggests an uplifted crust-mantle interface under the Mahandi graben. This study has delineated three subsurface levels of anomalous masses at the respective depths of 23 km, 8 km, and 2 km apparently representing the Moho, an intermediate discontinuity in the sialic part of the crust and the basement, respectively. Model study of the Bouguer anomaly along a profile suggests a typical continental graben type subsurface structure with a low density depression in the sialic part of the crust between 8 and 18 km supported by an elevated upper mantle of intermediate density (∼3.4 g/cm3) varying in depth from 25 km to 55 km. The depths of the inferred interfaces in case of Bundelkhand granite are 32 km, 11 km, and 1.5 km, which might represent the Moho, the base of intruded granite massif, and some shallow compositional variation. Similar studies in case of Vindhyan basin have brought out three discontinuities at the respective depths of 16 km, 6–4.5 km, and 2.4 km. The first horizon at the depth of 16 km probably represents the interface between the granitic and the basaltic part of the crust. The 6–4.5 km is the depth of the basement, with the 2.4 km interface separating Bijawar rocks from Vindhyans wherever they are present. A generalized inversion of a profile across a positive belt of Bouguer anomaly representing the subsurface Bijawar rocks support the above result.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00878969
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  • 8
    Electronic Resource
    Electronic Resource
    Crustal model based on aeromagnetic profiles, gravity studies and wave velocities in rocks and crust (1970)
    Springer
    Pure and applied geophysics 81 (1970), S. 5-16 
    Details
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary Regional airborne magnetic profiles from India and U.S.A. are analyzed. Profiles are i) 130 km offshore Manglore to 60 km offshore Madras (India) along 13th parallel; ii) Washington to San Francisco (U.S.A.): iii) Brownsville (Texas) to Guatemala City (Mexico). Depth to the sources of magnetic anomalies along Manglore-Madras profile and Washington-San Francisco profiles is calculated either by elementary approximation ofSmellie or Prism model method ofVacquier et al. It is significant that depth values for some of the anomalies obtained by these methods are in very good agreement with those based on drilling data. The magnetic pictures along these profiles are compared with Bouguer gravity anomaly maps and it is shown that in almost all cases where magnetic bodies lie below 5 km (approximately) from sea level they are not reflected in gravity maps whereas all the magnetic bodies which are above 5 km (approximately) produce a markable feature in Bouguer gravity anomaly. This indicates that density of material below this level is almost equal to that of normal basic rocks (2.80 gm/cm3) and those above 5 km have a density less than this. Based on these results the top most layer in crust is considered to be metasedimentary including intrusive rocks and below this it is tentatively taken as Quartz-diorite accounting for the quartz rich Archean formations. Curves representing the variation of compressional wave velocity in i) granite; ii) quartz-diorite; iii) gabbro and iv) dunite, with pressure and temperature as reported from measurements in laboratory, are studied in the light of the general variation of P-wave velocity in the earth's crust reported from seismic sounding studies. It is found that a change in composition from metasedimentary zone to quartz diorite at about 5 km below sea level is supported by this study. It is found that further increase in compressional wave velocity in earth's crust can be explained by a compositional change from quartz diorite to gabbro. At certain places an unusual high velocity for compressional wave at the base of the crust is reported. This can be explained by considering that gabbro merges to Dunite in those areas. Based on this crustal model a probable explanation for the origin of granite masses is attempted.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00875006
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