Ultrasonic and NMR-based estimation of the microstructure at ice-rock interface

Abstract

The mechanical properties of frozen rock mass are primarily determined by that of ice and ice-rock interface, the latter is further controlled by the microstructure at ice-rock interface. To investigate the temperature-dependent variations in the microstructure at ice-rock interface, the concept of ice-rock interface cohesive zone (ICZ) was firstly defined. Ultrasonic and NMR (nuclear magnetic resonance) tests were conducted on the frozen intact sandstone, pure ice and ice-rock binary samples at varying thawing temperatures. Observations of the microstructure at the ICZ were carried out through 1D-MRI (one-dimensional magnetic resonance imaging) experiments. The results indicate that: (1) by comparing the ultrasonic results of frozen intact sandstone, pure ice and ice-rock binary samples, it is found that the ICZ notably alters the propagation of ultrasonic waves through the samples. (2) During thawing process, the ultrasonic parameters and the total NMR signal amplitude in ice-rock binary samples exhibit a two-stage variation trend, with an inflection point temperature of −2 °C. (3) The NMR-estimated ICZ thickness displays a two-stage variation trend with thawing temperature, with inflection point temperature both at −2 °C as well. (4) The primary factor contributing to the higher attenuation rate of ultrasonic parameters in the ice-rock binary sample is the continuous increase of ICZ thickness at the ice-rock interface with thawing temperature. Through the ICZ thickness estimation model, it reveals that the experimental and estimated values of the ICZ thickness maintain consistency in the overall trend. Both sets of values demonstrate a two-stage variation with different temperatures and the inflection point temperature of −2 °C.