Journal of Geophysics
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The maximum entropy approach to inverse problems - spectral analysis of short data records and density structure of the Earth
No full textThe maximum entropy principle as described in the first, introductory part of the paper is applied to 2 problems: the estimation of the power spectrum from a finite number of values of the autocovariance function, and the determination of the density within the Earth from its mass, radius, and moment of inertia. In both cases the available information is given in terms of known values of linear functionals and the maximum entropy principle is used to derive a probability distribution for the values of the unknown function. The expectation value of the probability distribution for the spectral power is shown to be equal to the well-known maximum entropy power spectrum. The expectation value for the density within the Earth is in ― with respect to the few data used ― good agreement with that of accepted Earth models.
ARK: https://n2t.net/ark:/88439/y025164
Permalink: https://geophysicsjournal.com/article/87
 
Helios-1 Faraday rotation experiment: results and interpretations of the Solar occultations in 1975
Get PDFThe polarization angle of the HELIOS-1 downlink signal has been monitored during two solar occultations in 1975 at two widely separated ground stations. Significant Faraday rotation of the signal occurs whenever the signal ray path passes through the solar corona near superior conjunction. Large-scale variations in the data arise both from rotation of the solar corona and from the slowly changing solar offset (point of smallest heliocentric distance along ray path). A simplified model of the solar corona has been developed to simulate the results of the Faraday rotation measurements. In this model the known polarity of the large-scale interplanetary magnetic field is employed as an aid in determination of the product N · B (electron density x magnetic field) as a function of heliographic longitude and heliocentric distance r within 2-10 R☉ . In this distance range N · B is proportional to r-5.5. If the magnetic field can be assumed to follow an inverse square law over this range, the electron density is found to be decreasing as r-3.5, in good agreement with previous results. The derived longitudinal structure for the corona during both occultations is consistent with synoptic coronal white light observations.
ARK: https://n2t.net/ark:/88439/y065590
Permalink: https://geophysicsjournal.com/article/112
 
Geothermal models of the crust and uppermost mantle of the Fennoscandian Shield in South Norway and the Danish Embayment
Get PDFA narrow heat flow transition zone between the Fennoscandian Shield and the North Sea Basin has been investigated along a profile from the Precambrian of South Norway to the Danish Embayment in North Jylland. Along this profile the surface heat flow varies from about 42 mWm-2 (measured) in South Norway to 60–70 mWm-2 (estimated) in Denmark. Geothermal, seismic, gravity and other geophysical and geological data have formed the basis for construction of heat production and thermal conductivity models in the depth interval 0–50 km. The related steady-state temperatures and heat flow distributions are calculated by a numerical solution of the heat conduction equation in two dimensions. Three models are presented, a preferred model and two others which yield temperatures assumed to be close to the lowest and highest possible values. The preferred model gives temperatures of about 350 °C at the crust-mantle boundary in the Shield and approximately 700 °C beneath the Danish Embayment. These differences are associated with considerable variations in the heat flow from the mantle. In the main model variations from 16-17 mWm-2 in the shield region to about 40 mWm-2 in the Danish Embayment have been found. Some geophysical and petrological implications are discussed. In the sedimentary basin partial melting in the lower crust and at shallow depth in the uppermost mantle seems to be likely.
ARK: https://n2t.net/ark:/88439/y020204
Permalink: https://geophysicsjournal.com/article/254
 
An evaluation method combining the Differential Doppler measurements from two stations that enables the calculation of the Electron Content of the Ionosphere
Get PDFDifferential Doppler measurements of signals from NNSS Navigational Satellites can be used to give the electron content of the ionosphere. Measurements carried out up to now using data from one station provide limited information about the structure of the ionosphere, since the method relies on an assumption being made about the prevailing ionospheric conditions. If these conditions are not fulfilled, this method can lead to large errors in the predicted electron content. In the method described in this paper, Differential Doppler data from two stations are combined, resulting in considerably more reliable results, particularly when there is strong horizontal structure in the ionosphere, as is often the case in Polar regions. Examples of model calculations and experimental measurements are also included.
ARK: https://n2t.net/ark:/88439/y034043
Permalink: https://geophysicsjournal.com/article/66
 
Deep structure of the southern Rhinegraben area from seismic refraction investigations
Get PDFA joint interpretation of all seismic-refraction profiles in the southern part of the Rhinegraben area is presented. A time-term analysis of all Pg-arrivals reveals the topography of the crystalline basement and provides an average velocity of 6.0 km/s for the uppermost crust. The crust-mantle boundary is clearly elevated in the Rhinegraben rift system forming an arch with a span of 150-180 km and reaching a depth of only 25 km at the flanks of the graben proper. The velocity of P-waves in the uppermost mantle is 8.0-8.1 km/s. Below the flanks of the graben, the crust-mantle boundary is formed by a first-order discontinuity. Within the graben proper it is replaced by a transition zone of 4 km thickness with the strongest velocity gradient at a depth of 21 km. This transition zone is regarded as region of crust-mantle interaction and seems to be confined to the graben proper.
ARK: https://n2t.net/ark:/88439/y074159
Permalink: https://geophysicsjournal.com/article/71
 
Shock magnetization and demagnetization of basalt by transient stress up to 10 kbar
Get PDFThe effect of stress waves on the magnetization of basalt was studied. The stress waves were generated by impacting cylindrical basalt samples with aluminium projectiles. The 3 mm thick aluminium plates were accelerated in a non-magnetic compressed air gun accelerator to velocities ranging from 20 to 160 m/s, corresponding to peak stresses in the basalt between 2.5 and 10 kbar. The duration of the stress impulse was about several micro-seconds. For the experiments a basalt with well-known magnetic properties was used (Rauher Kulm, Germany). The magnetizing effect of the stress waves was determined as a function of the number of impacts, the intensity and direction of the applied magnetic field ( < 10 Oe) and the peak stress amplitude. In the used stress range the measured shock remanent magnetization (SRM) tends to a final steady value after 5 or 6 impacts. This value is proportional to the intensity of the applied field and increases with the peak stress applied. The produced SRM can be erased with maximum ac-fields of about 150 to 200 Oe. Any dependance of SRM on the direction of the applied magnetic field could not be recognized within the accuracy limits of the experiments. The demagnetizing effect of stress waves on the highfield (1,000 Oe) isothermal remanent magnetization, the low-field (1 Oe) thermoremanent magnetization and the natural remanent magnetization was studied as a function of the number of impacts and the peak stress. A final steady state of magnetization is generally obtained after 4 or 5 impacts. With increasing peak stresses increasingly harder remanent magnetizations can be demagnetized, with stresses of 2.5 kbar corresponding to coercive forces of about 75 Oe, 5.5 kbar to about 125 Oe and 8 kbar to about 175 Oe.
ARK: https://n2t.net/ark:/88439/y023624
Permalink: https://geophysicsjournal.com/article/182
 
A note on the palaeomagnetism of the Late Precambrian Malani Rhyolites near Jodhpur - India
Get PDFPalaeomagnetic properties from a series of mainly rhyolitic lava flows from the Malani volcanic suite in Rajasthan-India, dated at 745±10 my, were studied by means of alternating fields and thermal demagnetization methods. The mean direction of the characteristic magnetization component, of both normal and reversed polarity: D = 354.5°, I = +53.5°, α95 = 8°, N = 10, (Pole: S0.5° N, 43.5° E, dp = 8°, dm = 11.5°) is in good agreement with earlier results obtained by Athavale et al. (1963). The fold test gives a positive result and the above mentioned mean direction from the Malani rhyolites is in agreement with other Precambrian data from the Indian subcontinent. Therefore, this mean direction is interpreted to represent the primary magnetization direction. The position and orientation of the Indian subcontinent about 745 my ago was more or less alike its present-day orientation, however, at that time India was situated at a slightly higher latitude.
ARK: https://n2t.net/ark:/88439/y022604
Permalink: https://geophysicsjournal.com/article/118
 
An anomaly of the upper mantle below the Rhine Graben, studied by the inductive response of natural electromagnetic fields
Get PDFThe methods of Magnetotellurics (MT) and Geomagnetic Deep Sounding (GOS) have been applied to study the electromagnetic response of the rift structure of the Rhine Graben. The measurements at 17 MT and 7 GOS stations were carried out along a profile running perpendicular across the Graben. Fourier analysis and numerical filters were tried for separation of the frequencies and a least squares technique was applied for data reduction. The thus gained transfer functions can be explained well by two-dimensional models under the following assumption: a well conducting layer at the depth between about 80 and 100 km exists at distances 50 km West and 100 km East of the Rhine Graben. Immediately below the Graben, however, a zone of similar good conductivity lies between about 25 and 45 km depth. The lateral extension of this zone is only some tens of km from the Graben's edges.
ARK: https://n2t.net/ark:/88439/y032853
Permalink: https://geophysicsjournal.com/article/276
 
Preliminary polar wander path of central Iran
Get PDFIn 1973, 1974 and 1975 three palaeomagnetic sampling trips were made in Central Iran including the Lut Block for a determination of the polar wander path of Central Iran. This area is believed to be a fragment of Gondwanaland according to paleogeographic evidence and evidence from reconstruction of the Indian Ocean compiled by Forster (1974, 1975) and also according to various geological aspects summarized by Stocklin (1974).
ARK: https://n2t.net/ark:/88439/y024314
Permalink: https://geophysicsjournal.com/article/192
 
Computations of SV waves in realistic Earth models
Get PDFThe reflectivity method for the calculation of theoretical body-wave seismograms is extended to include a double-couple point source. Theoretical seismograms of SV waves from this type of source are presented for models of the Earth's crust and the crust-mantle boundary, and for models of the whole Earth. In the models of the crust-mantle boundary, there are up to four SV head waves, depending on the sharpness of the transition. The most remarkable one is slightly slower and later than Sn and has unusually low frequencies. Theoretical SV-wave seismograms for models of the whole Earth for periods from 15 s to 60 s and in the epicentral distance range from 10° to 160° show as prominent phases S, ScS, SKS and SKKS. SKS, and SKKS are different in wave form, in agreement with observations from long-period WWNSS stations. A diffracted wave SPdiffKS + SKPdiffS is found in the theoretical seismograms whose travel-time curve is tangential to that of SKS at a distance of 107 °. The resulting interference causes the wave form of SKS to change markedly around 120°. Since the theoretical seismograms are the complete response of the Earth models from the crust-mantle boundary down to the inner core, they also include many multiple and converted phases. Besides those associated with the crust-mantle boundary there are phases related to the transition zones in the upper mantle. Their amplitudes depend strongly on the sharpness of these zones.
ARK: https://n2t.net/ark:/88439/y089618
Permalink: https://geophysicsjournal.com/article/249