Library

Notes: In this study we derive expressions for particle displacement or particle velocity anywhere inside a stratified earth and at its surface due to horizontal torque source located in the top layer. Equivalently, invoking Green's function reciprocity theorem, the solution applies also to the case of a surface or subsurface source when the resulting displacement or velocity is measured within the top layer.In order to evaluate the closed-form analytical solution economically and accurately it is advisable to introduce inelastic attenuation. Causal inelastic attenuation also lends the necessary realism to the computed seismic trace. To provide proof that the analytical solution is indeed correct and applicable to the multilayer case, a thick uniform overburden was assumed to consist of many thin layers. The correctness of the computed particle velocity response can be very simply verified by inspection. The computed response can also serve as a check on other less accurate methods of producing synthetic seismograms, such as the techniques of finite differences, finite elements, and various sophisticated ray-tracing techniques.It is not difficult to construct horizontal surface torque source. It appears that such source is well suited for seismic exploration in areas with a high-velocity surface layer. A realistic source function is analyzed in detail and normalized displacement response evaluated at different incidence angles in the near and the far fields.In an effort to distinguish the features of an SH torque seismogram from a pressure seismogram two models with identical layerings and layer parameters have been set up. As expected the torque seismogram is very different from the compressional seismogram. One desirable feature of a torque seismogram is the fast decay of multiples.Exact synthetic seismograms have many uses; some of them, such as the study of complex interference phenomena, phase change at wide angle reflection, channeling effects, dispersion (geometrical and material), absolute gain, and inelastic attenuation, can be carried out accurately and effortlessly. They can also be used to improve basic processing techniques such as deconvolution and velocity analysis.The numerical evaluation of the analytical solution of the wave equation as described in this paper has a long history. Most of the work leading to this paper was carried out by one of us (M. J. K.) in the years 1957 to 1968 at the Geophysical Research Corporation. However, the full testing of the various computer codes was carried out only very recently at the Phillips Petroleum Company.

Notes: A technique is presented that allows measurement of both the thermal effusivity and diffusivity of the specimen. It employs the thermal excitation of a sample with a plane-surface heater at frequency ω and the observation of the 3ω thermal response of this metal element. In addition the temperature decay of the surface is observed, after the periodic heating current is switched off. Using this technique, the temperature dependence of the thermal conductivity and the specific heat of fused silica have been determined over a wide temperature range. For measurements on electrically conducting solids a technique for preparation of a dielectric insulating magnesiumoxide (MgO) film has been developed. A theoretical model is discussed, that allows to investigate not only bulk materials, but layered structures, also. Applying this model, the thermal effusivity of the upper polymethylmetacrylate (PMMA) layer of a PMMA-aluminum system and the effusivity of the substrate in a MgO stainless-steel system has been measured.

Notes: Abstract Young volcanic rocks from different sections of the Aleutian Islands-Alaska Peninsula Arc have been measured for 87Sr/86Sr, 143Nd/144Nd and some trace elements. We found the 143Nd/144Nd to be highly restricted in range (ɛ Nd=6 to 7) and low as compared to midocean ridge ba-salts (MORB). This indicates that the source of the Aleutian Arc magmas is different from MORB and remarkably isotopically homogeneous with respect to Nd. The range reported here for arc rocks is substantially smaller than found by other workers. However, the Sr isotope ratios vary considerably (ɛ Sr=−24 to −14). Those samples from small volcanic centers north of the main arc (second arc) are characterized by low ɛ Sr. Our data in combination with previous studies suggest that there are slight geochemical differences between discrete sections of the arc. The general uniformity of Nd isotope ratios are thought to be the surface expression of an efficient mixing or homogenization process beneath the arc plate, but which still causes a wide dispersion in Sr isotopic composition. To relate the arc rocks to the broader tectonic setting and to identify possible sources of arc magmas, measurements were done on volcanic and sedimentary rocks from the North Pacific/Bering Sea area. Alkali basalts from the back-arc islands St. George, Nunivak and St. Lawrence and alkali-rich tholeiites from the fore-arc have ɛ Nd=+4 to +9 and are correlated on the ɛ Sr-ɛ Nddiagram parallel to the mantle array but shifted to lower ɛ Sr. These samples are thought to be isotopically representative of the mantle transported to that region. A tholeiitic basalt from the Kamchatka Basin ocean floor (back-arc), however, yielded typical MORB values (ɛ Nd=10, ɛ Sr=−24). Composite sediment samples were made from DSDP cores in the Aleutian Abyssal Plain, Gulf of Alaska and the Alka Basin which represent mixtures of continentally and arc-derived materials. These composites have intermediate Nd isotopic (ɛ Nd= −2 and +2) and high Sr isotopic values (ɛ Sr=+9 and +37). These data show that possible source materials of the Aleutian Arc volcanics are isotopically different from and much more heterogeneous than the arc rocks themselves. On the basis of this study and of literature data, we developed a set of alternative models for volcanic arc magma generation, based on the restricted range in ɛ Nd and the wider range in ɛ Sr for arc rocks. Different isotopic and trace element characteristics found in different arcs or arc sections are explained by varying mixing proportions or concentrations in source materials. The basic observations require rather strict mixing ratios to obtain constant ɛ Nd. The preferred model is one where the melting of subducted oceanic crust is controlled by the amount of trapped sediment with the melting restricted to the upper part of the altered basaltic layer. Homogenization within the upper part of the oceanic crust is brought about by hydrothermal circulation attending dewatering of the slab during subduction and possibly some oxygen exchange of the magmas on ascent.