Thermal loading effect on P-wave form and power spectral density in crystalline and non-crystalline rocks

Geomechanical properties of rocks change considerably when subjected to thermal loading. In some cases, the damage induced by thermal loading is advantageous for geotechnical activities such as geothermal reservoir and hydraulic fracturing, and it is non-beneficial for other activities including stability of underground spaces and rock slopes; therefore, this paper deals with parameters such as P-wave form and power spectral density (PSD) as indexes for considering the degree of damage induced by thermal loading in rocks. In this regard, 7 types of rocks in 3 categories including igneous, sedimentary, and metamorphic were selected and heated up to 500 °C at furnace for 5 h; then, the specimens were cooled down immediately, and subsequently, P-wave velocity and P-wave form were measured. The results showed that thermal loading has no significant impact on the frequency component of different rocks; however, PSD parameter decreases dramatically after heating at 500 °C. In addition, the effect of thermal loading at 500 °C on the P-wave form for crystalline and non-crystalline rocks is variable. For crystalline rocks at room temperature, the peak of PSD parameter reduces more than non-crystalline rocks; in other words, in crystalline rocks, the dissipation of energy is greater than non-crystalline rocks. Microscopic observations were also provided in order to investigate the effect of thermal loading on PSD parameter in detail. It can be concluded that the mechanism of intergranular and intragranular microcracks in crystalline rocks and also the mechanism of disintegrated background cement in non-crystalline rocks could be attributed to the reduction in peak of PSD parameter.