The effects of variations in crustal and basin thicknesses on the efficiency of Lg propagation in Central Asia are investigated by comparing the predictions of ray theoretical modeling of Lg as multiple, critically reflected, SmS rays with observations at the KNET, CNET, and ILPA arrays. Ray modeling predicts that strong spatial gradients in crustal thickness within 10 to 50 km of either the source or receiver can affect Lg efficiency over paths exceeding 500 km. Variations in the efficiency of Lg propagation to KNET, including instances of strong variations across the aperture of the array, are consistent with predicted blockage of SmS by high slopes of Moho topography associated with the Hindu Kush and Pamir mountains. At CNET and ILPA the blocking effects of Moho slopes associated with the Caucausus and Zagros mountain belts and Black and Caspian Seas are accentuated by the effects of thick basins. Thick basins (greater than 2000 m) are found to have large effects on the efficiency of Lg propagation from shallow (1 km) crustal sources, blocking either a portion or all of the Lg phase along paths crossing these basins. The basin effects are strongly sensitive to source depth, nearly vanishing for sources 15 km and deeper. 相似文献
We investigated the isotope composition (O, C, Sr, Nd, Pb) in mineral separates of the two Precambrian carbonatite complexes Tiksheozero (1.98 Ga) and Siilinjärvi (2.61 Ga) from the Karelian–Kola region in order to obtain information on Precambrian mantle heterogeneity. All isotope systems yield a large range of variations. The combination of cathodoluminescence imaging with stable and radiogenic isotopes on the same samples and mineral separates indicates various processes that caused shifts in isotope systems. Primary isotope signatures are preserved in most calcites (O, C, Sr, Pb), apatites (O, Sr, Nd), amphiboles (O), magnetites (O), and whole rocks (Sr, Nd).
The primary igneous C and O isotope composition is different for both complexes (Tiksheozero: δ13C = − 5.0‰, δ18O = 6.9‰; Siilinjärvi: δ13C = − 3.7‰, δ18O = 7.4‰) but very uniform and requires homogenization of both carbon and oxygen in the carbonatite melt. The lowest Sr isotope ratios of our carbonates and apatites from the Archaean Siilinjärvi (0.70137) and the Palaeoproterozoic Tiksheozero (0.70228) complexes are in the range of bulk silicate earth (BSE). Positive εNd values of the two carbonatites point to very early Archaean enrichment of Sm/Nd in the Fennoscandian mantle. No HIMU components could be detected in the two complexes, whereas Tiksheozero carbonatites give the first indication of Palaeoproterozoic U depletion for Fennoscandia.
Sub-solidus exchange processes with water during emplacement and cooling of carbonatites caused an increase in the oxygen isotope composition of some carbonates and probably also an increase of their 87Sr/86Sr ratio. A larger increase of initial Sr isotope ratios was found in carbonatized silicic rocks compared to carbonatite bodies. The Svecofennian metamorphic overprint (1.9–1.7 Ga) caused reset of Rb/Sr (mainly mica) and Pb/Pb (mainly apatite) isochron systems. 相似文献
The anisotropy of a periodically layered isotropic medium is numerically modeled in order to study the effect of the scale
of heterogeneity on seismic observations. An important motivation is to delineate the wavelength ranges over which a pulse
propagating obliquely through the structure will be described by either ray (short wavelength) or effective medium (long wavelength)
theory. The same band-limited pulse is propagated obliquely at a variety of incidence angles through a compositionally uniform
layered structure as a function of the layer thicknesses. The resulting seismograms display similar behavior to that encountered
for normal incidence including the effects of stop- and pass-bands. Velocities determined from time picks on these seismograms
show a large difference in velocities between the long and short wavelength limits as has been previously demonstrated for
normal incidence propagation. The bulk of the transition between these two limits is independent of incidence angle and occurs
when the ratio between the wavelength and the layering thickness is near a value of 10. Two more geologically reasonable models
show that these effects are diminished with smaller contrasts between the layers. 相似文献
The 2004 Mid Niigata Prefecture earthquake (MJMA 6.8) and its aftershock sequences generated complicated, i.e., several conjugate fault planes in their source region. In
order to understand the generating process of these earthquakes, we estimated a 3-D distribution of relative scattering coefficients
in the source region. The large slip area during the main shock rupture seems to be bounded by strong heterogeneous zones
with larger scattering coefficients. Hypocenters of the main shock and major large aftershocks with M 5-6 classes tend to be located close to stronger scattering areas. We found that one of these strong heterogeneities already
existed before the occurrence of the M 5.9 aftershock on November 8. We suppose that heterogeneous structures in the source region of this earthquake sequence affected
the initiation and growth of ruptures of the main shock and major large aftershocks. 相似文献
Heterogeneous nucleation and condensation of dibutylphthalate, octadecane, octadecanol, and octadecanoic acid vapors at various pressures on insoluble AgCl and Ag nanoparticles in a turbulent mixing condensation nuclei counter (TMCNC) have been studied theoretically. A method to interpret the particle size distributions measured with a DMA and estimate the parameters for nucleation on single particles is proposed. Based on this semi-empirical method, the Gibbs free energy is calculated and a rate of heterogeneous nucleation on single particles is estimated directly from the experimental “condensation spectra” of inactive and active CN using the DMA data. In some cases, the dependence of the Gibbs nucleation energy on the vapor supersaturation had two maximums and one minimum, instead of one maximum as described by Gibbs' classical thermodynamics of phase transitions. This phenomenon, called “double barrier nucleation” (DBN) is caused by the surface heterogeneity of nano-CN; this is first experimental verification of DBN that had been previously predicted theoretically. Two types of heterogeneity may be present: topographic or energetic. Focusing on energetic heterogeneity, a theoretical model of DBN for spherical geometry is developed. The surface heterogeneity for insoluble nano-sized CN is shown to be critical to explaining the unusual transformation of a monomodal size distribution of inactive CN into a bimodal distribution of activated CN when coagulation is excluded. Future studies will be directed toward more data for further refining the theory and developing a model that simultaneously accounts for both types of surface heterogeneity of nano-CN. 相似文献
High levels of heterogeneity in many carbonate reservoirs have raised concerns about the validity and relevance of small-scale measurements from core plugs and high-resolution logs. While the measurements themselves may be accurate, they may not be representative of the average formation properties. A related question is one of reconciling the measurements made in small volume of investigation data (e.g., core plugs), with the measurements from relatively large volume of investigation data (e.g., wireline logs). This paper presents a technique to quantitatively describe the porosity heterogeneity in a borehole at the scale of several tenths of an inch. The method involves treating high-resolution borehole imagery as a 2D sample from a 3D data volume, and applying geostatistical analysis to these data. We compute the experimental semi-variogram and upscale its range and sill to larger (several inches) scales of measurement to predict the impact of heterogeneity on conventional core plug and logging tool porosity measurements. The resulting dispersion variance between the different measurement scales support the interpretation, application and comparison of these porosity measurements. This technique was applied to an Early Cretaceous carbonate reservoir in Abu Dhabi. We found that the scale of the heterogeneity is typically less than 1– 2in., so that while significant heterogeneity is observed at the core plug and smaller scales of measurement, the larger-volume logging tool measurements smooth out the heterogeneity and show considerably less variability. The differences between porosity measured in core plugs can be completely accounted for by this upscaling effect. 相似文献