Journal of Geophysics
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Spherical-Earth gravity and magnetic anomaly modeling by Gauss-Legendre quadrature integration
Get PDFThe usefulness of long-wavelength potential field anomalies in lithospheric interpretation is greatly increased with spherical Earth modeling techniques. Gauss-Legendre quadrature integration is used to calculate the anomalous potential of gravity and magnetic fields and their spatial derivatives on a spherical Earth for an arbitrary body represented by an equivalent point source distribution of gravity poles or magnetic dipoles. The distribution of equivalent point sources is determined directly from the coordinate limits of the source volume. Variable integration limits for an arbitrarily shaped body are derived from interpolation of points which approximate the body's surface envelope. The versatility of the method is enhanced by the ability to treat physical property variations within the source volume and to consider variable magnetic fields over the source and observation surface. A number of examples verify and illustrate the capabilities of the technique, including preliminary modeling of potential field signatures for Mississippi embayment crustal structure at satellite elevations.
ARK: https://n2t.net/ark:/88439/y030173
Permalink: https://geophysicsjournal.com/article/163
 
Correction of: High precision measurement of the frequency of mode 0S0
No full textThe corrected frequency of mode 0S0.
ARK: https://n2t.net/ark:/88439/y000005
Notes: Corrected article
Permalink: https://geophysicsjournal.com/article/96
 
Temperature derivatives of compressional and shear wave velocities in crustal and mantle rocks at 6 kbar confining pressure
Get PDFMeasurements of compressional and shear wave velocities, VP and VS, were made in a cubic anvil apparatus up to 700° C at 6 kbar in igneous and metamorphic rocks typical of the crust and mantle. Samples range in composition from acidic to ultramafic, with bulk density of 2.67–3.46 g/cm3 at 6 kbar. Mean atomic weights of the rocks vary between 20.37 and 23.03. A rough dependence of wave velocity on bulk density is apparent. However, there is considerable scatter, and the velocities do not generally follow lines of constant mean atomic weight. Both VP and VS increase with increasing amphibole, garnet, pyroxene and olivine content. High quartz content produces relatively low P-wave and high S-wave velocities, giving significantly low Poisson's ratios. In contrast, a high feldspar content is related to relatively high P-wave and low S-wave velocities and consequent high Poisson's ratios. The calculated temperature derivatives of VP and VS for the range 20-500° C at 6 kbar confining pressure, using best-fit solutions, cover the range –1.60 to –4.94 x 10–4 km/s° C and –1.39 to –3.93 x 10–4 km/s° C, respectively. The results compare fairly well with the few data published so far. For the dunite and peridotite rocks the (dVP/dT)p-values are in excellent agreement with published Voigt-Reuss-Hill values calculated from single crystal data. Using the experimental results, P-velocity profiles were calculated along a geotherm of a cold Precambrian shield crust and a warm continental crust. The temperature gradients for the latter cause velocity inversion in numerous rocks, rich in olivine or quartz.
ARK: https://n2t.net/ark:/88439/y069600
Permalink: https://geophysicsjournal.com/article/145
 
Multi-method observations and modelling of the three-dimensional currents associated with a very strong Ps6 event
Get PDFA pulsation event has been studied which showed Ps6-type characteristics with a period of about 20 min. Data from a number of magnetometer and riometer measurements over a large area have been combined with balloon measurements of the electric field to obtain a detailed description of the ionospheric current system during the event. The pulsations reported here occured over an interval longer than 3 h and showed large magnetic variations, up to 700 nT. A model of the three-dimensional current flow has been derived which agrees with the data. It is concluded that most of the magnetic disturbance measured on the ground is caused by Hall-currents.
ARK: https://n2t.net/ark:/88439/y016745
Permalink: https://geophysicsjournal.com/article/211
 
Densities and magnetic susceptibilities of Precambrian rocks of different metamorphic grade (Southern Indian Shield)
Get PDFDensities of 1294 Precambrian rocks from the Southern Indian Shield, confined to low-grade granite-greenstone (Karnataka) and high-grade granulite (Eastern Ghats Belt) terrains are presented and categorized into 16 metamorphic and igneous varieties. In Karnataka, densities of granites (2.65 g/cm3), gneisses (2.68 g/cm3) and granodiorites (2.73 g/cm3) are considerably less than those of greenstone belts (2.84 g/cm3) and dolerites (3.03 g/cm3). An average density of 2.75 g/cm3 is obtained for surface rocks from this terrain. Over the Eastern Ghats belt, densities of charnockites range from 2.71 g/cm3 to 3.12 g/cm3 as they vary from acidic to ultrabasic composition. An average density of 2.85 g/cm 3 is obtained for surface rocks from this belt. The contrast in the average densities of surface rocks from the lowand high-grade terrains may play a significant role in a proper assessment of the regional gravity fields over these terrains. The results compare well with the data from similar terrains in other shield regions. Magnetic susceptibilities for 482 of these samples are also presented. All the rock types include a wide range of susceptibilities, as is to be anticipated in metamorphic terrains. A plot of density versus magnetic susceptibility for thirteen of these rock types suggests a linear relationship.
ARK: https://n2t.net/ark:/88439/y009916
Permalink: https://geophysicsjournal.com/article/251
 
Geomagnetic induction studies in Scandinavia. II. Geomagnetic depth sounding, induction vectors and coast-effect
Get PDFIn this paper an event of very favourable structure for induction purposes, which was observed by the Scandinavian magnetometer array, is discussed and analysed in detail. The responses derived, in both the time and frequency domains, display a large coast effect at all coastal stations, both on the sea coasts and on the Gulf of Bothnia. Two relatively large inland anomalies are also delineated. The more significant of the two, the Storavan anomaly, is apparent using all the analysis techniques employed, and may be associated with a remnant of the Svionian island arc system. The less dominant anomaly, in the vicinity of Mieron, has no obvious correlation with geology or tectonic formations, and appears to be polarisation sensitive - it is only energised by east-west magnetic fields. First approximation modelling of the coast effect observed by the northwestern stations illustrated that the responses are well satisfied by the conductivity contrast between sea and land. Finally, the validity of the derived induction vectors, and the possible effects of induction for source field studies, are discussed.
ARK: https://n2t.net/ark:/88439/y095007
Permalink: https://geophysicsjournal.com/article/105
 
Anisotropy of vp and vs in rock-forming minerals
Get PDFThe compressional and shear wave anisotropy coefficients of 33 minerals of igneous and metamorphic rocks were calculated from published elastic constants and tabulated together with the orientation of velocity extremes in single crystals. The most abundant minerals of crustal crystalline rocks - alkali feldspars, plagioclases, quartz, micas and hornblende - have higher anisotropy coefficients than the upper mantle minerals - olivine, pyroxenes and garnets. Due to the orientation of mineral grains and their velocity extremes in a stress field, however, the olivine-rich ultramafites belong to the most anisotropic rocks and, in contrast, the crack-free anisotropy of crustal crystalline rocks is generally low, with the exception of metamorphic rocks rich in micas, hornblende and calcite.
ARK: https://n2t.net/ark:/88439/y095987
Permalink: https://geophysicsjournal.com/article/209
 
A geotectonic paradox: Has the Earth expanded?
No full textFrom the recognition of common apparent polar-wander (CAPW) paths for Africa, Australia, Greenland, and North America in the early Proterozoic, we have deduced that these continents today occupy approximately the same relative locations on the globe as they did in the early Proterozoic. However, there is abundant geochemical, geological, geochronological and tectonic evidence for landmasses having been much less dispersed in the Precambrian than they are now. It is shown in this paper that an Earth of about half the present radius accomodates the present continents in such a manner that this paradox can be satisfactorily resolved, and we propose that between about 1,600 Myr and 1,000 Myr ago, the Earth expanded to approximately its present dimensions. A change from Proterozoic to Phanerozoic tectonic styles is supported.
ARK: https://n2t.net/ark:/88439/y034403
Permalink: https://geophysicsjournal.com/article/34
 
Velocity variations in systems of anisotropic symmetry
Get PDFAngular variations of seismic velocities have been observed in the Earth and attributed to some form of anisotropy caused by aligned crystals, orientated cracks and inclusions, and laminated strata. The exact analytical expressions for the velocities in each particular symmetry-system, derived from the Kelvin-Christoffel equations, are complicated functions of the elastic constants and cannot be easily manipulated. This paper examines the form of the velocity variations for the several systems of elastic symmetry; five of these seven symmetry-systems have been suggested for possible Earth structures. We shall demonstrate that the approximate equations of Backus (1965) and Crampin (1977 a) are good estimates for the velocity variations in symmetry planes of all symmetry systems, but not in general for off-symmetry planes. These equations are linear in the elastic constants, and provide a convenient link between velocity variations and elastic constants, if used judiciously. The behaviour of shear waves in off-symmetry directions is complicated by pinches, caused by the proximity of shear-wave singularities, where the two shear-waves exchange polarizations. Despite the restrictions to their use, the equations are the fundamental relationship for a number of modelling studies.
ARK: https://n2t.net/ark:/88439/y036503
Permalink: https://geophysicsjournal.com/article/109
 
Numerical simulation of vertical seismic profiling
Get PDFMethods for the calculation of synthetic seismograms along vertical profiles through horizontally layered media are presented; the aim is to simulate numerically the technique of vertical seismic profiling (VSP), which is sometimes used for improved resolution of deep primary reflections. We present the theory for vertically travelling plane waves and for spherical waves, generated by a single force or an explosive point source. The solution is restricted to the acoustic case. The theoretical seismograms are complete, i.e., they contain all surface and internal multiples. Anelastic effects are taken into account with the method of complex velocities. In order to save computing time, seismogram segments are calculated, using the method of complex frequencies. These methods are applied to a layered model of the coal-bearing carboniferous in the Ruhr district of W. Germany. Different assumptions on overburden structure, recording geometry, and anelasticity are made, and the VSP seismograms are compared with seismograms calculated for horizontal profiles at the surface. It is suggested that VSP field surveys be preceeded by similar numerical studies for an estimate of the information that can be obtained by this expensive technique.
ARK: https://n2t.net/ark:/88439/y091727
Permalink: https://geophysicsjournal.com/article/271