A STUDY OF THE SEISMIC SIGNATURES OF SEDIMENTATION MODELS USING SYNTHETIC SEISMOGRAMS*

In modern exploration for hydrocarbons there is a great emphasis on the location of stratigraphic traps and estimation of lithologic information like sand-shale ratios from seismic data. In order to investigate the possibilities of success in this endeavour we have studied the synthetic seismograms for wave form and spectral characteristic for four basic sedimentation models: (I) interbedded sand-shale model representing the sediments of generally fluviatile origin, (2) interbedded coal-shale model representing deltaic deposits, (3) sedimentary models representing transgression and regression of shore lines, and (4) a basal sand model. The results have shown that for the first two models a change in the sand-shale or coal-shale ratio results in a characteristically different seismogram. The nature of the seismogram, however, is also strongly dependent on how the sand-shale or coal shale layers are arranged to ultimately give the same number of total layers, thus implying the same coal-shale or sand-shale ratios. The transgression, regression, and basal sand models also produce characteristically different seismic signatures. The spectra of these seismograms show attendant characteristic changes. However, it seems that in the case of real data which are disturbed by noise and the effects of overlying layers these characteristic features may not always be distinguishable.     

Regularized derivatives of potential fields and their role in semi-automated interpretation methods

Evaluation of higher derivatives (gradients) of potential fields plays an important role in geophysical interpretation (qualitative and/or quantitative), as has been demonstrated in many approaches and methods. On the other hand, numerical evaluation of higher derivatives is an unstable process – it has the tendency to enlarge the noise content in the original data (to degrade the signal-to-noise ratio). One way to stabilize higher derivative evaluation is the utilization of the Tikhonov regularization. In the submitted contribution we present the derivation of the regularized derivative filter in the Fourier domain as a minimization task by means of using the classical calculus of variations. A very important part of the presented approach is the selection of the optimum regularization parameter – we are using the analysis of the C-norm function (constructed from the difference between two adjacent solutions, obtained for different values of regularization parameter). We show the influence of regularized derivatives on the properties of the classical 3D Euler deconvolution algorithm and apply it to high-sensitivity magnetometry data obtained from an unexploded ordnance detection survey. The solution obtained with regularized derivatives gives better focused depth-estimates, which are closer to the real position of sources (verified by excavation of unexploded projectiles).     

Synchrotron spectra of short-period pulsars

A model with synchrotron radiation near the light cylinder is proposed to explain the observed spectra of short-period pulsars (P≤0.1 s). These spectra can be described if a power-law energy distribution of the emitting electrons with exponent γ=2–8 is assumed. For most pulsars, the peak frequency ν_m is below 10 MHz. The ν_m(γ) dependence is derived, and shows that the peak frequencies for pulsars with spectral indices α<1.5 may fall in the observable range. In particular, ν_m may be ν_m ～ 100 MHz for PSR J0751 + 1807 and PSR J1640 + 2224. The observed radio spectrum of Geminga (PSR J0633 + 1746) can be described by a synchrotron model with a monoenergetic or Maxwellian distribution of relativistic electrons and a small angle β between the spin axis and magnetic moment (β ～ 10°).     

Observation of the solar eclipse 1966 May 20 in the region between 3.2 cm to 2.7 m wavelength

The solar eclipse 1966 May 20 was observed at 3.2, 10, 15, 20, 54, 59 cm, 1.05, 1.34 and 2.7 m wavelength by the observatories of the Heinrich-Hertz-Institut. The meter-wavelength observations were superposed by a noise storm. The result shows a region of enhanced radiation from the plages near the central meridian at the short wavelengths. Consulting additionally the observations made at Kiel, NERA, and Ondejov in the region of about 1.2 m wavelength it is possible to localize the source of the noise storm.     

The cooling of flare produced plasmas in the solar corona

Solar flare X-rays, at energies less than 10 keV, are emitted by hot plasmas located in the corona. Three plasma cooling models are examined in detail. The cooling of the electrons by Coulomb collisions with ions at a lower temperature would require the observed material to occupy very large volumes. Cooling could take place by conduction or by radiation and observations are proposed which would allow the dominant cooling mechanism to be established.On leave during a portion of this work as University Research Fellow in Astronomy, University of Leicester, England.     

Magnesium-iron compositional variance in hornblende pyroxene granulites

Hornblende-orthopyroxene-calcic pyroxene-plagioclase (andesine-labradorite) — magnetite-ilmenite±quartz assemblages are evidently the most common granulite facies representatives of basic rocks throughout the world, and they must represent a variance of more than one. Ramberg (1948) invoked an additional degree of freedom arising out of unequal fractionation of Mg and Fe in the ferromagnesian silicates in such rocks. This is examined in detail on the basis of chemical data on twentytwo hornblende-orthopyroxenecalcic pyroxene triads, half of them from the type charnockite area near Madras.Theoretical consideration of a magnesium-iron distribution model shows that in quartz-bearing hornblende pyroxene granulites, the Mg/Mg+Fe ratios of hornblende, orthopyroxene and calcic pyroxene are uniquely determined by temperature at constant pressure. But these assemblages contain quartz rarely and Mg-Fe distribution among the three can be best described by three exchange equilibria where, at constant temperature, there is a fixed relation between the Mg/Mg+Fe ratios of the minerals. Among these, the exchange equilibria between hornblende-orthopyroxene and hornblende-calcic pyroxene appear to be interdependent; however, they are known to be significantly modified by varying tetrahedral aluminium contents of hornblendes. Comparison of molecular Mg/(Mg+Fe) ratios of hornblendes and parent rocks strongly points to an absence of Mg-Fe compositional variance in the total system, a petrologically important corollary being that the hornblendes in these rocks are highly unlikely to be secondary after pyroxenes. The general variance of the assemblages is also discussed.     

Scaling of response spectrum and duration for aftershocks

Aftershocks have the potential to cause collapse of a structure that has been already damaged by the preceding main shock. Seismic safety of a structure should therefore be ascertained through a damage analysis using the anticipated main shock and few larger-aftershock motions. Simulation of aftershock motions needs characterization of the seismic hazard due to aftershocks, and therefore it will be useful to develop a conditional scaling model that can predict the response spectrum of an anticipated aftershock motion consistent with the design spectrum of the main shock motion anticipated at the same station. In this study an attempt is made to develop a conditional scaling model for the pseudo spectral velocity spectrum via linear regression analysis on the aftershock and main shock recordings for the 1999 Chi–Chi earthquake. It is shown that it may be possible to obtain a simpler and approximate version of the conditional model from an unconditional model. Damage-causing potential of a ground motion also depends on its strong motion duration (SMD) and therefore a conditional scaling model is developed for SMD of the aftershock motion in several narrow frequency-bands. The model is developed for the larger-aftershock motions and it is shown that a reasonable replacement of such a model may be obtainable directly from an unconditional model. Finally, a simple weighted averaging scheme is proposed to obtain the composite SMD from the SMDs for different frequency bands by using the pseudo spectral acceleration spectrum of the motion.     

Understanding the surface hydrology of the Lake Eyre Basin: Part 1—Rainfall

This is the first of two papers that describe the surface hydrology of the Lake Eyre Basin (LEB) (1,140,000 km²) in central Australia and compares some key characteristics with those observed from arid regions globally. This paper concentrates on annual rainfall, whereas the second paper is devoted to streamflow. The first part describes the LEB's climate (arid to semi-arid), which is dominated by a subtropical high pressure ridge stretching latitudinally across central Australia. Then follow major analyses that include the characteristics of rainfall, wet and dry spell lengths and cumulative surpluses and deficiencies, rainfall trends and intra- and inter-decadal fluctuations, and the relationship between rainfall and El Niño-Southern Oscillation (ENSO). The paper concludes with six conclusions, the key ones being: the variability of the annual rainfall (based on the coefficient of variation) in the LEB is approximately 60% greater than that found for stations located in arid regions in the rest of the world; there is a bias towards longer lengths of dry years than observed in the rest of Australia; and, there is a significant lag correlation between rainfall and ENSO, particularly in the east and in the latter part of a year.