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The problem of inverting or modelling one-dimensional magnetotelluric data can, today, be considered as largely resolved. Attention now focuses on the class of acceptable models. Viewed in the space of model parameters this class occupies a singly connected volume, bound with a surface where the standard deviation ε between measured and calculated response exceeds the minimum εo of the best-fitting model by a constant factor (typically ε ≈ 1.10 εo). This volume of acceptable models is described by its intersections with the parameter axes, and also by the extreme excursions possible for any of the model parameters when all the other parameters are adjusted accordingly. These extreme excursions therefore represent "trade-off" conditions among the model parameters and are summarized in the "trade-off matrix". In a sense this is a generalization of the parameter correlation matrix, which gives only local information in the vicinity of a proposed model. The trade-off matrix, however, is independent of any initial model. Another important question considered deals with the correct choice of the number of layers with which to model a data set. Whereas a single minimum of ε is found with the correct number no, when this number is too small the information contained in the data is spread among several isolated minima. When n > no the problem becomes "illposed ". There are too many degrees of freedom and it becomes possible, then, to move in model space in directions at right angles to the meaningful dimensions without finding a clear minimum. The problem is analogous to a vanishing determinant in linear algebra. To find a regular problem again it is necessary to specify auxiliary constraints provided, for example, by other soundings or prior geological knowledge, to compensate for the increased number of variables.
ARK: https://n2t.net/ark:/88439/y061860
Permalink: https://geophysicsjournal.com/article/289
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In deep-seismic sounding the reflected and refracted waves recorded form a complex system of traveltime curves. The inverse seismic problem consists in determining the function ν (x, z) along the profile for a series of velocity levely ν (x, z) = const and discontinuities as well. This paper described a method which allows the transformation of observed traveltime curves into an intercept-time section with isolines of velocity. Such a section permits discrimination of the various types of waves and gives a qualitative picture of the crustal structure, and is then converted into a depth section. The application of this method is demonstrated by a number of examples from the Ukrainian Shield, the Tien Shan and the Caucasus showing both possibilities and restrictions.
ARK: https://n2t.net/ark:/88439/y052591
Permalink: https://geophysicsjournal.com/article/288
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New characteristics of PiB magnetic pulsations which are associated with the substorm in the local midnight sector are revealed from the recordings of a meridional network of ground stations. PiB's are observed during distortions of the auroral arc which enclose a local region of upward directed field-aligned current. An enhancement at about 0.3–0.4 Hz is shown to be a permanent feature of PiB's. In addition, a secondary amplitude maximum at about 1 Hz sometimes appears in the recordings far from the auroral zone. The generation of PiB pulsations in the Pi1 period range is located at auroral latitudes at a low altitude. Low-altitude electric fields and acceleration of auroral particles, low-altitude resonance cavities, and isotropic propagation of waves is discussed in the interpretation of PiB pulsations.
ARK: https://n2t.net/ark:/88439/y043502
Permalink: https://geophysicsjournal.com/article/287
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An empirical model function of the global electron temperature distribution has been determined based on the measurements of the planar Retarding Potential Analyzer on-board the Aeros-A satellite. The model represents the mean temperature between 300 and 700 km altitude at 0300 LT and 1500 LT depending on latitude, longitude, and height. The model values are compared with all the measured data to show the accuracy achieved and the mean spread of the data for different latitudes. A clear correlation was not found between the electron temperature and geophysical indices such as Kp or sunspot number for the period of low solar activity between January and August, 1973. Seasonal and annual effects could not be detected. The mathematical background and method used to generate the model function is described in the appendix.
ARK: https://n2t.net/ark:/88439/y033123
Permalink: https://geophysicsjournal.com/article/286
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Marine, mainly flat-lying sediments of Late Cretaceous age are exposed throughout a wide area of the Munster Basin (NW-Germany). During the Cenomanian, Turonian, and Campanian fine-grained, grey, marly to pure limestones were deposited. The Campanian limestones carry magnetization components of unknown age due to the presence of secondary goethite and haematite. However, in the Cenomanian and Turonian rocks the natural remanent magnetization (NRM) is due to detrital magnetite and can be associated with the time of deposition. Fold tests confirm a Late Cretaceous age of magnetization in the magnetite-bearing limestones, since the NRM pre-dates latest Cretaceous deformation along the northern margin of the basin. The Munster Basin limestones provide one of the first reliable Cretaceous pole positions (Lat.: 76° N, Long.: 181° E) from stable Europe.
ARK: https://n2t.net/ark:/88439/y023954
Permalink: https://geophysicsjournal.com/article/285
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A palaeomagnetic and rockmagnetic investigation has been carried out in the western Southern Alps. At 31 sites in the Permian volcanics from four regions 349 samples were drilled: 1. the region of Arona (SW of Lago Maggiore), 2. the porphyry district of Lugano and Ganna, 3. the Valle Brembana (N of Bergamo) and 4. the Auccia volcanics (N of Brescia). AF-cleaning as well as continuous and progressive thermal demagnetization reveal, in most of the igneous rocks studied, the presence of a stable characteristic remanent magnetization (ChRM) representing the magnetization acquired at the time of formation of these rocks. Microscopic observations, IRM-acquisition curves and Curie point measurements indicate the ChRM to be carried by Ti-poor magnetite and titanohematite. Stable secondary magnetizations due to oxidation may be present. Their directions, however, are very similar to the primary thermoremanent magnetization (TRM). Therefore it is inferred that the oxidation probably took place shortly after acquisition of the primary TRM. The magnetization directions within individual sites are well grouped (α95 usually < 10°), but the site mean directions are dispersed, due to regional and local tectonic complications. At Arona, Ganna, and Auccia a suitable tilt correction can be made. Since the consistent directions from these sites are very similar to the well defined results from the Bolzano porphyries (Zijderveld et al., 1970), it is suggested that the western and eastern Southern Alps have, on a large scale, behaved as a single tectonic unit. The Southern Alpine block has been rotated anticlockwise by about 50° relative to extra-alpine Europe since the Early Mesozoic. The Permian Southern Alpine palaepoles are situated close to the Permian part of the African polar wander path. Therefore the palaeomagnetic data support geological and sedimentological arguments which consider the Southern Alps as originating on the southern margin of Tethys and forming a parautochthonous extension of the African plate since the Early Mesozoic.
ARK: https://n2t.net/ark:/88439/y013985
Permalink: https://geophysicsjournal.com/article/284
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The postulation of a Mesozoic age for the shield-building basaltic series of Gran Canaria and Tenerife by Storevedt et al. (1978), based upon paleomagnetic data from these islands, is inconsistent with abundant and concordant K/Ar-ages from several laboratories. These latter data leave no doubt that no rocks older than Mid-Miocene have been found on Gran Canaria and Tenerife. Moreover, no volcaniclastic layers older than Miocene were found in Deep Sea Drilling cores near the Canary Islands. Also, the volcanic apron around at least Gran Canaria appears to be Miocene in age judging from seismic reflectors that extend to well-dated drilled sections. There is no evidence for a sialic crustal layer extending beneath all or any of the Canary Islands.
ARK: https://n2t.net/ark:/88439/y002516
Permalink: https://geophysicsjournal.com/article/283
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The nature of long-wavelength magnetic anomalies (λ = 60–300 km) computed for the Ukrainian Shield from the original field by a continuation upward to a height of 10 km is studied. The correlation betweeen the regional anomalies, the crustal thickness and the topography of the Curie isotherm of magnetite is examined. The strongest correlation is established between the regional anomalies and the crustal thickness. Similar results have been obtained by us for the Baltic Shield and by D.H. Hall for the Canadian Shield. It is concluded that the entire lower crust is magnetized, the average magnetization being almost the same for all the ancient shields. These results have been used for the construction of a magnetic model of the Earth's crust. With some exceptions, the magnetization of the lower crust is found to be inhomogeneous and 5-10 times higher than that of the upper crust. Theoretical modelling and experimental results show a high magnetization in the entire sequence of blocks for the case of a thickened crust and, vice versa, weakly magnetized rocks correspond to a smaller thickness of the crust. The present approach may be of potential use for distinguishing and studying crust-upper mantle interaction areas as well as for predicting the topography of the Moho discontinuity.
ARK: https://n2t.net/ark:/88439/y092937
Permalink: https://geophysicsjournal.com/article/282
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Thermal and alternating field demagnetization combined with studies of the convergence points of remagnetization circles have been carried out on a variety of rocks from Fuerteventura. The older (post-Albian) rocks, including the basement plutonics, the sheeted dike complex and the earliest subaerial lava sequence (lava Series I), have a multicomponent remanence while the younger lava Series II of Miocene/Pliocene age has dominantly a one-component magnetization. Comparison of the results with recent palaeomagnetic data from Gran Canaria/Tenerife and with results from continental Africa and Europe suggest a late Cretaceous origin of the basal intrusive rocks while the subaerial volcanism (lava Series I) most likely initiated at around the Cretaceous/Tertiary boundary. This implies that there is a nearly 50-m.y.-long period of volcanic quiescence and erosion between lava Series I and lava Series II. The apparent contradiction between these conclusions and the few K/Ar dates available is discussed.
ARK: https://n2t.net/ark:/88439/y082688
Permalink: https://geophysicsjournal.com/article/281
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The fault-plane solution of the North Italian earthquake of May 6, 1976, has been constructed from long-period records of 37 stations (mainly WWNSS stations), assuming a focus in the crust. It is essentially a thrust type solution, the compressional axis having an azimuth of about 160°, perpendicular to the strike direction of the Eastern Alps. One nodal plane is very well defined from first P-wave motions: it is steeply dipping approximately SSE with a dip angle of 77°. The second nodal plane has been determined from S-wave polarisations at stations in the United States and Japan: it dips roughly NW with a dip angle of about 16°. The dip direction of this nodal plane is less well defined. From the two possible interpretations of the fault-plane solution, either a shallow thrust of Italy under the Alps or a steep thrust of the Alps under Italy, only the first one is geologically plausible. The slip direction of the underthrusting block (or plate?) has an azimuth of 348° and a dip angle of 13°.
ARK: https://n2t.net/ark:/88439/y072059
Permalink: https://geophysicsjournal.com/article/280
