GEOMETRIC CORRECTIONS IN ATTENUATION MEASUREMENTS1     

THEODOROS KLIMENTOS 《Geophysical Prospecting》1991,39(2):193-218

Seismic wave attenuation in porous rocks consists of intrinsic or anelastic attenuation (the lost energy is converted into heat due to interaction between the waves and the rocks) and the extrinsic or geometric attenuation (the energy is lost due to beam spreading, transmission loss and scattering). The first is of great importance because it can give additional information on the petrophysical properties of rocks (permeability, degree of saturation, type of saturant, etc.). The most difficult problem in attenuation measurements is estimating or eliminating extrinsic attenuation, so that the intrinsic attenuation can be obtained. To date, in laboratory attenuation measurements using wave propagation, several methods have been used. The difficulties vary with the method. The coupling effect and the geometric divergence or beam spreading are the major problems. Papadakis’ diffraction corrections have been used extensively by Winkler and Plona in their modified pulse-echo high-pressure attenuation measurements. These corrections are computed for homogeneous liquid media and their failure to fit data for solid material implies that these corrections must be used with caution, especially for high Q values. Three new methods for laboratory ultrasonic attenuation measurements are presented. The first is the ‘ultrasonic lens’ method for attenuation measurements at atmospheric pressure, in which an ultrasonic lens placed between transmitter and sample transforms the initially oblique incident beam into normal incidence so that the geometric divergence is eliminated. The second method is the ‘panoramic receiver’, in which the beam spreading can be eliminated by integrating the ultrasonic energy over a large area. The third method is called 'self-spectral ratio’ and is applicable for all pressure conditions. Attenuation is estimated by comparing two signals recorded on the same rock but with two slightly different thicknesses under the same pressure conditions. Hence the extrinsic attenuation for both thicknesses is approximately the same. A comparison between the self-spectral ratio method and that of Winkler and Plona demonstrates a very good agreement for a broad band of frequencies. Hence the Winkler-Plona technique and Papadakis’ diffraction corrections can be accepted as reliable in any future work.  相似文献   

Mineral chemistry of lava flows from Linga area of the Eastern Deccan Volcanic Province,India     

SOHINI?GANGULYEmail author  JYOTISANKAR?RAY  CHRISTIAN?KOEBERL  THEODOROS?NTAFLOS  MOUSUMI?BANERJEE 《Journal of Earth System Science》2012,121(1):91-108

Several basaltic lava flows have been identified in the study area in and around Linga, in the Eastern Deccan Volcanic Province (EDVP) on the basis of distinctly developed structural zones defined by primary volcanic structures such as columnar joints and vesicles. These basaltic lava flows are spatially distributed in four different sectors, viz., (i) Bargona–Gadarwara (BG) sector (ii) Shikarpur–Linga (SL) sector (iii) Arjunvari–Survir Hill (AS) sector and (iv) Kukrachiman–Morand Hill (KM) sector. A three-tier classification scheme has been adopted for the characterization and classification of individual lava flows. Each lava flow consists of a Lower Colonnade Zone (LCZ) overlain by the Entablature Zone (EZ) and Upper Colonnade Zone (UCZ). The LCZ and UCZ grade into a distinct/indistinct Lower Vesicular Zone (LVZ) and Upper Vesicular Zone (UVZ), respectively. The LCZ and UCZ of the flows are characterized by columnar joints while the EZ is marked by multi-directional hackly jointing. The geometry of different joint patterns corresponds to different styles of cooling during solidification of lava flows. Detailed petrographic studies of the investigated lava flows reveal inequigranular phenocrystal basalts characterized by development of phenocrystal phases including plagioclase, clinopyroxene and olivine, whereas groundmass composition is marked by tiny plagioclase, clinopyroxene, opaque mineral and glass. Electron microprobe analyses indicate that the olivine has a wide range ∼Fo₂₂ to Fo₆₆ revealing a wide spectrum of compositional variation. Pyroxene compositions are distinctly designated as Quad pyroxenes. Phenocrystal pyroxenes are mostly diopsidic, while the groundmass pyroxenes mainly correspond to augite with a minor pigeonite component. Pyroxene phenocrysts are characterized by a prominent Ti-enrichment. Phenocrystal plagioclase grains are calcic (An_52.7–An_72.9), whereas groundmass plagioclase are relatively sodic (An_39.2–An_61.6). Groundmass opaque minerals are characteristically found to be Ti–magnetite/ilmenite/pyrophanite. Pyroxene thermometry reveals a temperature span of 850°C to 1280°C for the studied lavas while olivine–clinopyroxene thermometry yields a temperature range from 1040°–1160°C. The variation of temperature for the lava flows is ascribed to their normal cooling history after eruption.  相似文献