Effect of Cracks on Elastic Properties of Low Porosity Rocks

Effect of Cracks on Elastic Properties of Low Porosity Rocks by Terrence Patrick Todd Submitted to the Department of Earth and Planetary Sciences on May 1, 1973, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Microcracks in terrestrial and lunar rocks significantly alter elastic, thermal, and mechanical properties. The degree to which microcracks alter physical properties of low porosity rocks depends on the state of stress in the rock (hydrostatic pressure, non-uniform stress, pore pressure), the magnitude of various crack parameters (directional distribution, volume, aspect ratio, connectibility, etc.), the per cent saturation, and the physical properties -of-pore fluids. Various techniques have been employed to study, both qualitatively and quantitatively, the effect of cracks on the elastic properties of low porosity rocks. Elastic wave velocities and static compressibility, measured as functions of direction, indicate the presence of anisotropy, determine symmetry patterns of non-uniform crack or mineral distributions, and separate mineral from crack anisotropy. Elastic properties measured under hydrostatic confining pressure illustrate crack closure effects. Crack counting from thin section, or photomicrographs of thin sections, give crack parameters and directional information about cracks. Staining cracks with dyes facilitates the observation of cracks in thin section. Some crack parameters (e.g. the distribution function of crack aspect ratios) are calculated directly from theoretical expressions which present the particular parameter in terms of elastic property values. Other crack parameters (e.g. crack porosity) are measured directly. Acoustic emission techniques give direct information about crack closure, crack formation, phase transitions, twinning, and other stress release mechanisms in rocks. Different values of crack parameters are produced by different cracking mechanisms. Cracks produced by natural igneous processes or by thermal cycling are low aspect ratio cracks present along grain boundaries and within individual grains. Cracks produced by shock processes are higher aspect ratio cracks. In particular, lunar rocks have cracks characteristic of those produced by shock processes. Thesis Supervisor: Gene Simmons Title: Professor of Geophysics