A Study of Acoustic Velocity and Dielectric Permittivity Anisotropy in Relation to Finite Strain in Deformed Rock
Sweeney, Jerry Joseph
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https://hdl.handle.net/2142/68057
Description
Title
A Study of Acoustic Velocity and Dielectric Permittivity Anisotropy in Relation to Finite Strain in Deformed Rock
Author(s)
Sweeney, Jerry Joseph
Issue Date
1980
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Language
eng
Abstract
The Cambrian Slate Belt of Wales provides an ideal laboratory for comparing variations in total natural strain with variations in other physical parameters. A large fold structure at Penrhyn Quarry, Bethesda, North Wales, U.K. has been mapped and its three dimensional strain determined from natural indicators. The strain has subsequently been calibrated against the degree of preferred orientation of phyllosilicate minerals which is then used independently as a strain measure. Velocity of propagation of acoustic waves and dielectric permittivity were measured for samples from the quarry and then compared with strain determined by preferred orientation for the same samples. Further measurements of acoustic velocity were done on samples from locations along the axis of the Welsh Cambrian Slate Belt and on samples from other regions including the Appalacian fold belt.
Acoustic velocity was measured using the pulse-echo method with a buffer rod. Compressional waves were measured at a frequency of 3 MHz with a precision of (+OR-)0.01 km/s and shear waves were measured at 1 MHz with a precision of (+OR-)0.02 km/s. Slate from Penrhyn Quarry was found to have orthorhombic velocity symmetry with principal directions parallel to the X, Y, and Z tectonic directions. Compressional velocity measured in the principal tectonic directions was found to be very sensitive to variation in strain on a local as well as a regional scale. For variations in principal extensional strain ((epsilon)(,x)) between 0.53 and 1.57, compressional velocities in the X direction range from 6.27 to 6.80 km/s. Similarly, for principal shortening strains ((epsilon)(,z)) between -.51 and -.67 compressional velocities range from 5.02 to 5.74 km/s. Both relationships are linear and the rocks show a similar behavior with respect to shear wave velocity. Calibration of acoustic velocity with strain variation enables velocity anisotropy to be used independently for the accurate analysis of strain on any desired scale.
Changes in compressional and shear mode acoustic velocity were measured as a function of confining pressure and load in a triaxial testing appratus for slate samples with tectonic axes at various orientations with respect to the load in order to observe effects resulting from closure of oriented grain boundary microcracks. The relation between preferred orientation and acoustic velocity anisotropy was found to result from a combination of the mechanical anisotropy of constituent minerals in the rock and the fabric-controlled orientation and aspect ratio of grain boundary microcracks.
The dielectric permittivity, measured on dried evacuated samples using guarded electrodes at frequencies between 0.1 and 100 KHz, shows Debye relaxation with a spread of relaxation frequencies and strong conduction at lower frequencies. Conduction effects are suppressed at liquid nitrogen temperature. The static dielectric constant k(,s), determined from measurements between 0.5 and 100 KHz at room temperature, was anisotropic with principal axes of the permittivity tensor parallel to the principal tectonic axes. For a typical strain, where (epsilon)(,x), = 1.10, (epsilon)(,y) = 0.33, and (epsilon)(,z) = -0.60, the respective values of k(,s) are 9.0, 7.8, and 6.8 respectively. There is, however, no linear relation between strain variation and variation in k(,s) within any principal direction. The low frequency conductivity was found to have a correlation with strain for the X and Z directions and thus is a more promising parameter for use in measuring strain variation.
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