The Cryosphere Discuss., 6, 793-819, 2012
www.the-cryosphere-discuss.net/6/793/2012/
doi:10.5194/tcd-6-793-2012
© Author(s) 2012. This work is distributed
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This discussion paper has been under review for the journal The Cryosphere (TC). Please refer to the corresponding final paper in TC.
P-wave velocity changes in freezing hard low-porosity rocks: a laboratory-based time-average model
D. Draebing and M. Krautblatter
Department of Geography, Bonn, Germany

Abstract. P-wave refraction seismics is a key method in permafrost research but its applicability to low-porosity rocks, that constitute alpine rock walls, has been denied in prior studies. These 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 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 decimeter-large low-porosity (<6 %) metamorphic, magmatic and sedimentary permafrost rock samples with a natural texture (>100 micro-fissures) from 25 °C to –15 °C in 0.3 °C increments close to the freezing point. P-wave velocity increases by 7–78 % when freezing parallel to cleavage/bedding and matrix velocity increases from 5–59 % 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 2-phase equation implementing changes in matrix velocity dependent on lithology and demonstrate the physical basis for refraction seismics in low-porosity bedrock.

Citation: Draebing, D. and Krautblatter, M.: P-wave velocity changes in freezing hard low-porosity rocks: a laboratory-based time-average model, The Cryosphere Discuss., 6, 793-819, doi:10.5194/tcd-6-793-2012, 2012.
 
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