Guided wave propagation in laterally varying media - II. Lg-waves in north-western Europe

Summary. Lg -wave observations at NORSAR from a sequence of explosions straddling the Central Graben in the North Sea, show that the phase is almost extinguished by passing through this structure. For models of the graben structure, based on seismic refraction experiments, with crustal thinning beneath a zone of thickened sediments, numerical modelling of Lg propagation shows very poor transmission. The inclusion of attenuation simulating the effects of scattering from faults and other complex structures further diminishes the transmittivity. The theoretical effect of a crustal pinch is to redistribute S energy over a wide range of groups velocities, a pattern that fits well with the observations.
Three components recordings at NORSAR of explosive charges fired in water covering a wide range of azimuths, show significant transverse components to the seismograms. The transverse energy builds up with travel time and for Lg is of the same order as the vertical component. These results are consistent with the theoretical prediction of progressive net transfer of energy from Rayleigh to Love modes as the wavetrain propagates through a three-dimensionally heterogeneous medium.     

First Byurakan Spectral Sky Survey. Stars of Late Spectral Types. XI. Zone -7° ≤ δ ≤ -3°

The eleventh list of faint late M and carbon type stars detected on the plates of the First Byurakan Spectral Survey in zone -7° -3° and covering about 1000 square degrees is presented. From 126 detected stars, 88 are newly discovered objects: they are 6 carbon stars, 8 carbon star candidates, and 74 M-type stars; among the latter 38 (26 PSC + 12 FSC) are unclassified IRAS sources, and one object is an unclassified ROSAT source. Distances to the 6 newly discovered early-type carbon stars are estimated. Equatorial coordinates, red magnitudes, and spectral classes determined from the Palomar E-charts are provided. The lack of optical counterparts on Palomar O and E maps for two detected late M-type stars indicates a large variability in brightnesses of these objects (amplitude not smaller than 7.0 magnitude).     

VELOCITY DISPERSION OF SEISMIC WAVES*

In well velocity surveys made to calibrate Sonic (CV) Logs the calibration survey uses frequencies around 50 Hz whereas the Sonic Logging tool uses frequencies around 20 kHz. There thus exists the possibility of making a direct measure of velocity dispersion. In any one survey the disturbing factors, both instrumental and operational, will often mask any dispersive effect that might exist. Consequently this paper reports on a statistical analysis of the velocity differences resulting from calibration surveys and Sonic logs. Only Borehole Compensated Sonic Logs were used. Four areas were investigated: the North Sea, Abu Dhabi, Libya and Alaska. After rejecting logs and calibration records which were obviously in error there remained 424000 feet (about 130 km) of usable log distributed throughout 66 wells. The four areas were analysed separately and in no case was the estimated dispersion significantly different from zero. However, the mean values did correlate with lithology from (? 0.17 ± 0.18)% for the essentially carbonate section in Abu Dhabi to (+ 0.45 ± 0.25)% for the sand-shale section in Alaska, a positive sign meaning that the higher frequencies travelled faster. Except for Alaska the calibration surveys were made with a wall-clamp geophone, and for these areas amplitude measurements were made. After suitable corrections estimates of the absorption parameter Q were obtained. These varied from 20 to 200 with mean values of 63 for Libya, 70 for Abu Dhabi and 88 for the North Sea (excluding the Tertiary). If, as is usually assumed, the absorption mechanism is linear and is described by a Q which is independent of frequency, then these values would necessarily imply dispersion of several percent. As instanced above no such dispersion was observed. It is possible that the expected dispersion was compensated for by invasion of the mud filtrate into the borehole walls, but it is more likely that the absorption mechanism was substantially non-linear.     

The neotectonics and geodynamics of the Lower Oka Region (East European Craton)

The neotectonic structures of the Lower Oka (Nizhneokskii) Region formed under different geodynamic conditions. This is attested by the morphology, orientation, internal structure, and jointing of the structures. The Oka-Tsna arc formed under the effect of tension from an inner source on the one hand and stress from the Alpian belt on the other hand. The latitudinally-oriented structures of the northwestern slope of the Tokmovo arc emerged as a result of uplift and widening. Both types of structure are combined within the limits of the Oka-Murom trough, which is a geodynamically active zone.     

Determination of the optimum period for ciliated protozoa colonizing of an artificial substrate in a tropical aquatic ecosystem

The optimum period for ciliated protozoa colonizing of an artificial substrate, the polyurethane foams have been assessed in a tropical aquatic ecosystem, the Ekozoa stream of the Mfoundi River Basin in Yaounde (Cameroon). 5?days were calculated as the highest period for the biological indicators of pollution to optimally colonize the artificial substrate. This time interval is the same for all the sampling stations assessed from upstream to downstream and the various microhabitats along the water course. The statistical method applied is that of the completely randomized blocks. The colonization of the substrate increases from the first day to the fifth day, before decreasing to the tenth day. The statistical analysis of variance between the maximum day and the other sampling period was significant at 5?% while the calculation of the value between different points of the same station was not significant. The average number of ciliated protozoan ranges from 20 to 23, from upstream to downstream.     

Formalization of assessing radioactive waste burial site seismic regime parameters from seismological and geological data

A model is proposed that shows the relation of the block structure of the crust and earthquake sources (Sadovskii, 1979; Rodionov, 1979, 1984, 1994; Bugaev, 1999, 2011, 2014, 2015). The model can formalize how to assess the prediction of seismic regime parameters depending on the elastic limit and conditions and rate of deformation of the Earth’s crust. The spent nuclear fuel repository site in Olkiluoto (Finland) and a site in the area of the Krasnoyarsk Mining and Chemical Combine are considered as examples. It is demonstrated that the parameters of the prediction graphs limit the location of the points of magnitude repeatability graphs calculated for a site based on samples of earthquakes in the area according to different authors. This makes it possible to recommend predictive assessment of seismic regime parameters for stability monitoring of the seismic regime and safety analysis of a geological environment’s insulation properties for waste sites from the results of seismological monitoring and high-precision observations of modern movements of the Earth’s crust.     

Oxygen and hydrogen isotope fractionation in serpentine-water and talc-water systems from 250 to 450 °C, 50 MPa

Oxygen and hydrogen isotope fractionation factors in the talc-water and serpentine-water systems have been determined by laboratory experiment from 250 to 450 °C at 50 MPa using the partial exchange technique. Talc was synthesized from brucite + quartz, resulting in nearly 100% exchange during reaction at 350 and 450 °C. For serpentine, D-H exchange was much more rapid than ¹⁸O-¹⁶O exchange when natural chrysotile fibers were employed in the initial charge. In experiments with lizardite as the starting charge, recrystallization to chrysotile enhanced the rate of ¹⁸O-¹⁶O exchange with the coexisting aqueous phase. Oxygen isotope fractionation factors in both the talc-water and serpentine-water systems decrease with increasing temperature and can be described from 250 to 450 °C by the relationships: 1000 ln  = 11.70 × 10⁶/T² − 25.49 × 10³/T + 12.48 and 1000 ln  = 3.49 × 10⁶/T² − 9.48 where T is temperature in Kelvin. Over the same temperature interval at 50 MPa, talc-water D-H fractionation is only weakly dependent on temperature, similar to brucite and chlorite, and can be described by the equation: 1000 ln = 10.88 × 10⁶/T² − 41.52 × 10³/T + 5.61 where T is temperature in Kelvin. Our D-H serpentine-water fractionation factors calibrated by experiment decrease with temperature and form a consistent trend with fractionation factors derived from lower temperature field calibrations. By regression of these data, we have refined and extended the D-H fractionation curve from 25 to 450 °C, 50 MPa as follows: 1000 ln  = 3.436 × 10⁶/T² − 34.736 × 10³/T + 21.67 where T is temperature in Kelvin. These new data should improve the application of D-H and ¹⁸O-¹⁶O isotopes to constrain the temperature and origin of hydrothermal fluids responsible for serpentine formation in a variety of geologic settings.     

Amplitudes of long-period PcP and the core-mantle boundary

The amplitudes of the core reflection PcP are sensitive to the wave velocities and densities in the neighborhood of the core-mantle boundary (CMB). We study the amplitude ratio of the long-period phases PcP and P from two South American deep-focus earthquakes with favorable fault-plane solution, depth and magnitude, as recorded by WWNSS and CSN stations in North America.Comparison is made with long-period PcP/P amplitude ratios, derived from theoretical seismograms for a variety of CMB models. Models from previous studies, which were mainly derived from short-period PcP observations and which are characterized by discrete layers above the CMB, are almost all inconsistent with the long-period data. The data also discriminate against low nonzero S velocities below the CMB. Simple first-order-discontinuity models of the CMB, for instance according to the Jeffreys-Bullen earth model or according to recent models based mainly on free oscillations, explain the data reasonably well.Model improvements are attempted by varying the P-velocity gradient above the CMB. The best amplitude fit is obtained for a rather strong decrease in P velocity with depth in this zone which, however, gives no acceptable traveltime fit for PcP. The scatter in body-wave amplitudes is considerable even for long-period waves and may prevent the correct assessment of that part of the amplitude variation of a phase with distance that is due to the variation of velocities and densities with depth alone.