P-wave velocity changes in freezing hard low-porosity rocks: a laboratory-based time-average model

Abstract

Abstract. P-wave refraction seismics is a key method in permafrost research but its applicability to low-porosity rocks, which constitute alpine rock walls, has been denied in prior studies. These studies explain p-wave velocity changes in freezing rocks exclusively due to changing velocities of pore infill, i.e. water, air and ice. In existing models, no significant velocity increase is expected for low-porosity bedrock. We postulate, that mixing laws apply for high-porosity rocks, but freezing in confined space in low-porosity bedrock also alters physical rock matrix properties. In the laboratory, we measured p-wave velocities of 22 decimetre-large lowporosity (< 10 %) metamorphic, magmatic and sedimentary rock samples from permafrost sites with a natural texture (> 100 micro-fissures) from 25 • C to −15 • C in 0.3 • C increments close to the freezing point. When freezing, pwave velocity increases by 11-166 % perpendicular to cleavage/bedding and equivalent to a matrix velocity increase from 11-200 % coincident to an anisotropy decrease in most samples. The expansion of rigid bedrock upon freezing is restricted and ice pressure will increase matrix velocity and decrease anisotropy while changing velocities of the pore infill are insignificant. Here, we present a modified Timur's twophase-equation implementing changes in matrix velocity dependent on lithology and demonstrate the general applicability of refraction seismics to differentiate frozen and unfrozen low-porosity bedrock.

The Efficacy of Frost Weathering Processes in Alpine Rockwalls

“…The closing behavior contradicts to field observations of several authors (Collins & Stock, 2016;. In the (i) top-down-tests, Phase-2 starts at RT between -1.63 and -1.45°C due to freezing point depression, which is in accordance with previous observations by Draebing and Krautblatter (2012) and results from solutes, pressure, and crack geometry (Lock, 2005). The crack opening during Phase-2 results from freezing of the crack water infill and concurrent volumetric expansion.…”

“…Elastic properties are derived from laboratory tests (Draebing & Krautblatter, 2012) and complement modeled CD ice to calculate ice pressures σ ice (Jia et al, 2017) and ice stress intensity factors K ice (Jia et al, 2017;Tharp, 1987). A thermal expansion coefficient α is derived from linear regression of CD and RT during a preliminary testing period and is used to simulate thermally induced rock deformation (CD th ).…”

Thermal and Mechanical Responses Resulting From Spatial and Temporal Snow Cover Variability in Permafrost Rock Slopes, Steintaelli, Swiss Alps

“…The physical properties of the paragneiss bedrock are based on values given by Cermák and Rybach (): rock density was set to 2600 kg m ‐3 , thermal conductivity 2.5 W m ‐1 K ‐1 and specific heat capacity 800 J kg ‐1 K ‐1 . The porosity of the rock (2.40 ± 0.12%) was measured in the laboratory (Draebing and Krautblatter, ) and the solid content was assumed to be 97%, accounting for pores and fractured space containing ice or water.…”

Thermo‐cryogenic controls of fracture kinematics in permafrost rockwalls

“…Thermal expansion and contraction occur at both positive and negative temperatures (Figures b, d, and f) and thermo‐mechanical CD model (Figures a, c, and e) is in good agreement with the measured CD during snow‐free periods when insulation by snow cover is absent. The derivation of the thermal expansion coefficient from field measurements of CD results in fluctuations of α due to high anisotropy of the slaty paragneiss bedrock at the Steintaelli [ Draebing and Krautblatter , ] and moisture effects [ Aldred et al , ; Eppes et al , ]. The thermo‐mechanical rockwall expansion with coincident crack closing during warming and reversed behavior during cooling was previously described in laboratory tests on microcracks by Cooper and Simmons [].…”