Impact of Thermally Induced Cracks on Elastic Modulus Dispersion and Attenuation in Fluid-Saturated Granite

Abstract

Toward better understanding seismic wave propagation in hard rocks, we investigated the impact of thermally induced cracks on the frequency-dependent elastic properties of granitic crystalline rocks. Thin-section microscopy revealed an increase in microcrack density and aperture with increasing treatment temperatures of the samples. Using the low-frequency forced-oscillation technique we probed the frequency-dependent Young's modulus and extensional-mode attenuation of the samples under dry and fluid-saturated conditions. Water and glycerin were used as pore fluids. The measurements under fluid-saturated conditions showed significant modulus dispersion and attenuation. Dispersion of Young's modulus, from low (0.2 Hz) to high (10⁵ Hz) apparent frequencies, reached up to 60%, and was accompanied by bell-shaped attenuation curves with Q_E⁻¹ as high as 0.16. With increasing treatment temperature, the peak attenuation shifted to higher frequencies. We attribute such frequency-dependent behavior of the Young's modulus and attenuation to microscopic (pore-scale) fluid flow between interconnected compliant cracks. The experimental results are consistent with predictions from the Crack-Pores Effective Medium (CPEM) model, indicating that the interplay between crack geometry and fluid dynamics governs the elastic response. This study highlights the necessity of accounting for squirt flow mechanisms when interpreting seismic field data and laboratory measurements of elastic properties in cracked crystalline rocks. Incorporating these effects into seismic modeling can significantly improve the accuracy of rock property estimations under subsurface conditions.