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The discovery of massive sulphide deposits along mid-ocean ridges has prompted the development of towed sea floor electromagnetic mapping tools. One suitable configuration of transmitter and receiver is the in-line, coaxial, magnetic dipole-dipole. The step response of this system to a double half-space model has two distinct parts. The position in time of the initial event is indicative of the conductivity of the sea floor. A reduction in dimensionality greatly simplifies the analytic and numerical computation of more complicated cases. The transmitter is reduced to a pair of horizontal line sources carrying equal but opposite currents and separated by a small vertical distance. The transient responses of the simplified system and the coaxial system to the double half-space model are remarkably similar, even though the electromagnetic mode characterised by vertical current flow is excluded by the simplification. The analytic form of the sensitivity function enables a simple expression for a depth of investigation beneath the sea floor to be derived as a function of time. The magnetic effects of currents impressed in a two-dimensional conductive target embedded in the sea floor by a horizontal magnetic point dipole transmitter may be represented approximately by a system of vortex currents only. Since vortex current flow is the type of current flow induced in a two-dimensional target by a two-dimensional magnetic source, the principal characteristics of the three-dimensional problem can be studied by two-dimensional modelling. The scattered fields from a thin conductive dike and a thin conductive sill are evaluated by setting up and solving a boundary element integral equation in the electric field. Transient response curves are presented for a limited range of models. The sea floor conductivity is fixed at 1/30 that of seawater, a typical value for recent basalt. The array size and plate depth extent are fixed at 100 m and 25 m, respectively, while the depth of burial is allowed to vary from 4 m to 25 m. The ratio of the inductive response parameter of the plate to the response parameter of the crust, which determines the degree of influence of the plate conductor on the combined step response, is varied from 0.4 to 10. Increasing the relative effect of the target delays the onset and decreases the gradient of the initial part of the response. Pronounced minima in the response as a function of array location are observed when either the transmitter or the receiver cross over the target. The minimum field over a wide range of times is close to zero for a shallow dike due to the combined shielding effect of the dike and the seawater. The shallow dike may be distinguished from a shallow sill by the shape of the minima.
ARK: https://n2t.net/ark:/88439/y079019
Permalink: https://geophysicsjournal.com/article/123
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Upper Bajocian oolithic limestones and Pliensbachian marls and marly ovoids from 9 sites distributed over 4 exposures at the western border of the Rheingraben (mean coordinates 47.8° N, 7.5° E) were sampled by 92 samples. The carriers of the natural remanent magnetization in the limestones are goethite and magnetite, and in the marls and ovoids the carrier is magnetite. After tectonic correction, the characteristic component of NRM isolated in the cleaning processes was mostly of normal polarity. The mean direction is D = 30.1°, I = 53.2° (N = 7, k = 92, α95 = 6.3°) and the pole position is 63.1° N, 120.1° E. It is close to the Jurassic reference data for the stable European plate.
ARK: https://n2t.net/ark:/88439/y066400
Permalink: https://geophysicsjournal.com/article/122
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Geomagnetic and telluric pulsations were observed at nine stations, partially at the same time, and at two sites with additional recordings of variations. They occupied a 20 x 30 km2 area of high-grade metamorphism. There is a nearly perfect spatial uniformity of the magnetic variation field except for a small local anomaly attributable to the Rheingraben. The telluric field is highly polarized in a N47 ± 7W direction but with local differences in amplitude. In contrast, telluric phases are spatially uniform and, as a function of period, distinctly different for the N47W orientation of the telluric field (= "B-polarisation") and the N43E orientation (= "E-polarisation"). Telluric and magnetic observations are not explainable by one-dimensional (1-D) models for the Schwarzwald alone. Therefore a 2-D model is derived, comprising Schwarzwald and Rheingraben, which can account for the graben Z anomaly and the phase curves in both polarisations. An unscaled 1-D model is obtained from the telluric phases in E-polarisation and then a 2-D model for Schwarzwald and Rheingraben in E- and B-polarisation. This latter model allows the scaling of the 1-D Schwarzwald model and shows a thin conductive layer under the gneiss massif at a depth of 12 km with a conductance of 650 S.
ARK: https://n2t.net/ark:/88439/y056011
Permalink: https://geophysicsjournal.com/article/121
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Strong motion accelerograms recorded at two sites in Lima, Peru, during the earthquake of November 9, 1974, exhibit serious dissimilarities although the sites have nearly the same epicentral distance. The two sites are the Instituto Geofisico del Peru in central Lima and the La Molina sediment-filled valley on the periphery of the city. The anomalously strong and prolonged ground motion at the La Molina site seems to be explained by a combined effect of the complex topography of the bedrock and the presence of low-velocity subsurface sediments. In contrast to an intuitive feeling, a strong velocity contrast along the whole sediment-bedrock interface is not necessary. Because severe earthquake effects in La Molina are of site origin, they should be expected to repeat in the future. As indicated by synthetic accelerograms, the anomaly refers to large areas of the La Molina valley and not only to the immediate vicinity of the recording point. For purposes of seismic microzoning and land-use planning, two microzones in the studied part of the valley will probably be appropriate.
ARK: https://n2t.net/ark:/88439/y042722
Permalink: https://geophysicsjournal.com/article/120
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The propagation of plane waves through statistically layered media is investigated both numerically and with single-scattering theory in the one-dimensional case. Exact apparent or stratigraphic Q, Qs, is determined from synthetic seismograms with the spectral-ratio method. Maximum velocity (impedance) fluctuations up to 30% (~40%) are studied; the fluctuations are uniformly distributed with zero mean. In all cases the trend of Qs as a function of frequency is well described by the analytical Qs, as determined from single-scattering theory under the assumption of an exponential autocorrelation function of the impedance fluctuations. The frequency dependence of the analytical Qs-1 follows a Debye-peak function, its maximum is γ2/2 and corresponds to the wavelength 4πa (γ2 = variance of relative impedance fluctuation, a = correlation distance). In further numerical calculations intrinsic or anelastic Q, Qa, is introduced, and it is shown that total attenuation Q -1 agrees very well with the sum of apparent and anelastic attenuation, Qs-1+Qa-1. Finally, a simple, minimum-phase stratigraphic attenuation operator is derived which describes the amplitude decay and the dispersion in a one-dimensional random medium with good accuracy. Stratigraphic attenuation is similar to the anelastic attenuation of a standard linear solid.
ARK: https://n2t.net/ark:/88439/y038753
Permalink: https://geophysicsjournal.com/article/119
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Palaeomagnetic 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
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Ten fireplaces from Mannheim-Wallstadt (MW) and four ovens from Herrenchiemsee (HC) were studied with regard to their archaeomagnetic properties. The ages of the medieval structures were only poorly known and an age dating was intended with the help of the standard curves for the declination and inclination of the geomagnetic field of the past 2000 years, as set up by Thellier (1981) for France. The stability of the NRM was tested with Thellier's test. Of the ten MW fireplaces, only one passed the test, whereas of the four ovens of HC, all passed the test but one of them had to be excluded because of secondary displacements within the structure. The MW fireplace could be dated to 670-700 A.D., in agreement with other archaeological age determinations for the fireplaces. The ovens from HC yielded an age between 1100 and 1170 A.D. The ovens showed the effect of magnetic refraction. Model calculations carried out on circular ring structures require a susceptibility of the oven material of the order of 0.5 SI units to explain the observed effect. The presently observed mean susceptibility at room temperature is only around 5 x 10-3 SI units. However, heating of the material at 550° C for 2 h in a reducing environment was able to increase the susceptibility by a factor of 20. This effect is explained by the reduction of secondary iron oxides and hydroxides to magnetite. Another increase by a factor of about 1.5 is obtained from the Hopkinson effect (increase of susceptibility with temperature) at the blocking temperature of magnetite. Combining all effects, it is possible to attain values for the susceptibility at the blocking temperature of magnetite as high as 0.2 SI units, which is the order of magnitude required for the explanation of the observed effect of magnetic refraction. This effect demands that special considerations be made during archaeomagnetic sampling from archaeological ovens or kilns.
ARK: https://n2t.net/ark:/88439/y016135
Permalink: https://geophysicsjournal.com/article/117
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The Helios search-coil magnetometer experiments provide a unique potential for studies of electromagnetic plasma wave modes in the solar wind between 0.3–1 A.U. because of excellent background noise levels, high time-resolution and high accuracy. Daily average spectral densities (γ √Hz) in the frequency range from 4.7–220 Hz show a pronounced increase as the sun is approached with complex superposed variations. Their values have a range of more than an order of magnitude. The shock of January 6, 1975 provides an interesting example for the use of high time-resolution spectral data. The fine structure and the long term variations in wave fields after the perpendicular shock are discussed. Also two interesting examples of "magnetic holes" are presented.
ARK: https://n2t.net/ark:/88439/y002056
Permalink: https://geophysicsjournal.com/article/116
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Based on the special geodetic network set up by Niemczyk and Emschermann in 1938 in order to determine horizontal movements of the Earth's crust in the fissure area of the neo-volcanic zone of NE-Iceland, a number of repeated measurements were carried out from 1965 to 1977. In this article, modern geodetic measuring techniques and the (constantly improving) positional accuracy achieved are described; this is followed by an evaluation showing significant crustal movements in the riftzone. In the period of 1965 to 1971, movements in the total area of approximately 110 km from east to west were restricted to compressions, whereas afterwards, and in particular since 1975, expansions of up to 2 m/km have occurred in the central rift area. At present, this expansion is largely compensated by compression in the peripheral areas of up to 0.05 m/km, so that between 1971 and 1977 a total east-west expansion of the neovolcanic zone of only about 0.4 m has resulted over a distance of about 90 km.
ARK: https://n2t.net/ark:/88439/y096107
Permalink: https://geophysicsjournal.com/article/115
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Rayleigh wave phase velocities were obtained in the period range 12.5–83.3 s using the almost linear array of long period seismic stations installed at Bari (BAI), Grosseto (GSO), Bologna (BOL), and Torino (TNO). The Hedgehog inversion gives a crustal thickness in the range 25–37 km. The presence of a low-velocity layer in the crust is allowed, while low velocity material within a few kilometers of the Moho is required. The shear-wave velocities below 60 km are rather higher than usual channel values. If crustal thicknesses of the order of 37 km are rejected as suggested by other geophysical data then the low-velocity layer in the crust is required in order to satisfy the observed dispersion relation.
ARK: https://n2t.net/ark:/88439/y089438
Permalink: https://geophysicsjournal.com/article/114
