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A method of seismic response analysis of 2 — D inhomogeneous structures, based on the ray method and on the application of the Debye procedure to include slight absorption, is presented. Program package RESPO, designed for such an analysis of the seismic response on the free surface of a general 2 — D laterally varying, geological near-surface structure assuming P, SV or SH plane-wave incidence from below, is briefly described. The package is tested on the classical model of a sedimentary basin. The study differs from previous applications of the ray method to the basin model in the following aspects. The frequency-domain approach is used. A comparison of the ray method results with the results of the discrete wavenumber method for long periods is made. More attention is paid to the analysis of the formation of the wave field. This analysis reveals two main types of wave propagation inside the basin: the dominant horizontally propagating local interference waves and less expressive vertically propagating waves in the central part of the basin. Effects of slight absorption (Futterman's causal absorption) are considered. The absorption causes a decrease in amplitudes and time delays at later times. The decrease in the amplitudes is not so expressive because of large periods and relatively short travel times of the waves investigated.
ARK: https://n2t.net/ark:/88439/y070849
Permalink: https://geophysicsjournal.com/article/136
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A data-processing method is applied which includes a rotation of the three components, normalization and delay-and-sum of broadband records of earthquakes from a large distance and azimuth distribution, recorded at a single station (or an array). Clear P-to-S converted phases at the mantle discontinuities are observed in the Grafenberg records, after this data processing. Theoretical seismograms are computed for the PREM model and processed in the same way as the observed data. A comparison with the data shows that the depth interval between the two discontinuities in the mantle transition zone (those at 400 and 670 km depth in PREM) is around 240 km. The 670-km discontinuity is sharper than the 400-km discontinuity and is comparable in sharpness with the crust-mantle transition, as far as it is possible to judge from the broadband data used. There are indications of pronounced lateral heterogeneity of the 400-km transition, underneath GRF. We have also observed converted and multiply reflected shear waves in the crust, which set sensitive limits to the average crustal model underneath the array. These data suggest that the velocity jump at the Moho is smaller than indicated by refraction studies.
ARK: https://n2t.net/ark:/88439/y067070
Permalink: https://geophysicsjournal.com/article/135
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The July 1978 deflation of Krafla volcano in the volcanic rift zone of NE-Iceland was in most respects typical of the many deflation events that have occurred at Krafla since December 1975. Separated by periods of slow inflation, the deflation events are characterized both by rapid subsidence and volcanic tremor in the caldera region, as well as extensive rifting in the fault swarm that transects the volcano. Earthquakes increase in the caldera region shortly after deflation starts and propagate along the fault swarm away from the central part of the volcano, sometimes as far as 65 km. The deflation events are interpreted as the result of subsurface magmatic movements, when magma from the Krafla reservoir is injected laterally into the fault swarm to form a dyke. In the July 1978 event, magma was injected a total distance of 30 km into the northern fault swarm. The dyke tip propagated with a velocity of 0.4-0.5 m/s during the first 9 h, but the velocity decreased as the length of the dyke increased. Combined with surface deformation data, these data can be used to estimate the cross-sectional area of the dyke and the driving pressure of the magma. The cross-sectional area is variable along the dyke and is largest in the regions of maximum seismic energy release. The average value is about 1,200 m2. The pressure difference between the magma reservoir and the dyke tip was of the order of 10-40 bars and did not change much during the injection.
ARK: https://n2t.net/ark:/88439/y057911
Permalink: https://geophysicsjournal.com/article/134
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Two methods for tracing seismic rays between 2 given end points through three dimensional, continuously varying velocity structures are available. This paper describes and compares them for problems of practical interest and for analytical ray paths through an idealized velocity structure. One method involves "shooting" the ray from one point with a given starting direction and then modifying this starting direction until the ray emerges at the desired target, while the other method involves "bending" an initial path between the end points until it satisfies the principle of stationary time. For most of the models investigated, "bending" is computationally faster than "shooting" by a factor of 10 or more. The "bending" method can be modified to deal with discontinuities in the velocity model, and can also be adapted for use in conjunction with a table of distances as a function of ray parameter when the three dimensional anomaly influences only a small fraction of the total ray path. The geometrical spreading effect on the amplitude of the ray may be retrieved easily from the "bending" solution.
ARK: https://n2t.net/ark:/88439/y041002
Permalink: https://geophysicsjournal.com/article/133
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A method for the interpretation of the magnetotelluric (MT) impedance tensor, the telluric-vector technique, is presented. The phase information of all impedance tensor elements is used to distinguish between local telluric distortion and regional induction. A model incorporating a superposition of the effects of local surface anomalies and a regional 1-D, 2-D, or 3-D conductivity distribution is applied. In 2-D regional structures, a complete separation of the contributions of local and regional anomalies is possible if additional information from geomagnetic depth sounding (GOS) is used. A new skewness parameter derived from phases alone is introduced to measure the three-dimensionality of the regional structure independent of local distortions.
ARK: https://n2t.net/ark:/88439/y031103
Permalink: https://geophysicsjournal.com/article/129
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Data from arrays of magnetometers along lines of constant magnetic latitude and longitude supplemented by all-sky camera and riometer data are used to infer the characteristics of the temporal development and the typical scale size of westward travelling surges which occur during magnetospheric substorms. It is found that the motion of the head of the surge can be quite irregular, and that in extreme cases the surge form may grow and decay in a confined longitudinal sector without suffering any significant westward displacement. The positive D-component perturbation, known to be the characteristic signature of a surge, is generally confined within a longitude range of ~6-10° at ~70° N and is thought to be generated by a filamentary southward ionospheric current flowing at the head of the surge. A comprehensive model three-dimensional current system involving this equatorward current and northwestward current flow in the region to the east of the head of the surge is presented through a detailed comparison of model and observed latitude and longitude profiles of the magnetic disturbance. It is found that best agreement is obtained when the entire electrojet system flows from southeast to northwest relative to the lines of constant magnetic latitude.
ARK: https://n2t.net/ark:/88439/y029714
Permalink: https://geophysicsjournal.com/article/128
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An array of 19 broad-band seismometers with flat velocity response between 0.05 Hz and 5 Hz combined with a binary gain-ranging data acquisition system of 138 db dynamic range is being installed in Southern Germany. The array has a maximum extension of 80 km, it consists mainly of three triangular subarrays, the dimensions of which are chosen so as to reduce the microseisms by simple summation. The dynamic range is accomplished by digitizing the output at the seismometer sites and transmitting the digital information on telephone lines to a central control station. Single station data are available since 1975 and continuous recordings of the first subarray exist since 1976. They provide extended information of the fine structure of the seismic wave field in the medium frequency range between 0.5 Hz and 0.05 Hz and allow the quantitative investigation of frequency-dependent seismic phenomena over a wide range of amplitude and frequency. An elementary problem is the numerical analysis of broad-band seismograph systems especially seismometer-galvanometer systems and the determination of the true ground displacement or velocity. This simulation and restitution problem can be solved by digital recursive filters applying the z-form method. The simulation may include high sensitive short-period detection instruments.
ARK: https://n2t.net/ark:/88439/y012005
Permalink: https://geophysicsjournal.com/article/127
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Ten sites from a dike swarm of early Cretaceous/Late Jurassic age in northeast Brazil (5.7°S, 36.6°W) yielded a pole at 80.6°N, 95°E with A95 = 9.5°, K = 26.7 after AF cleaning. Rock-magnetic investigations and chemical analyses revealed titanomagnetites and maghemites, both with low titanium content showing ilmenite exsolution lamellae (oxidation class III). These are due to an internal high-temperature oxidation during cooling of the large dikes, follwed by low-temperature oxidation and/or hydrothermal alterations. There is evidence that with low-temperature oxidation the Ti-to-Fe ratio increases, a finding that is consistent with previous studies. Hysteresis parameters and susceptibility versus temperature curves can be interpreted in terms of pseudosingle-domain behavior with a trend toward multidomain behavior in accordance with moderate-to-weak stability of the remanence.
ARK: https://n2t.net/ark:/88439/y008126
Permalink: https://geophysicsjournal.com/article/126
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Upper Carboniferous and Permian volcanites from Sudetes were sampled in 11 localities (mean site coordinates: 50.8°N, 16.3°E). Thermal demagnetizations of the samples showed several magnetic components. But in one site, all high-temperature components are reversed. The interpretation of the results shows a first, primary magnetization acquired probably in the Upper Carboniferous before tilting (N = 8, D = 192°, I = -2°, k = 27, α95 = 11°, pole: 39°N, 181°E) and a remagnetization acquired later, during or after tilting (N = 5, D = 190°, I = -19°, k = 18, α95 = 18°, pole: 48°N, 181°E).
ARK: https://n2t.net/ark:/88439/y097077
Permalink: https://geophysicsjournal.com/article/125
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Model calculations are carried out for the temperature distribution in an aquifer, into which water is injected. The models are simplified so that the temperature distribution can be given by an analytical expression. The one-dimensional simulation shows the well-known fact that effects of conduction can generally be neglected. In two-dimensional models the case of a doublet, in particular, is analysed. A new approach is used for solving the heat transport equation. The distributions of temperature, streamlines and potential, even if a natural uniform flow of groundwater exists, can be easily calculated and plotted. The computation of the temperature in the extraction well shows that the economical working time of a doublet can be longer than the thermal breakthrough time.
ARK: https://n2t.net/ark:/88439/y088418
Permalink: https://geophysicsjournal.com/article/124
