Anisotropy in a very fine grained sand: A comparison between sand and kaolinite
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Rock mechanics and geophysics are intertwined due to elastic waves being mechanical disturbances propagating through a material, whereas determination of the elastic properties is the alpha and omega in the field of rock physical research. Implementing these variables into geophysical models able to simulate what lies underneath our feet is a powerful analytical tool. Sadly, this is easier said than done as theory and practice are seldom perfectly aligned. In a controlled setting, factors causing anisotropy in terms of ultrasonic P- and S-wave velocities can be studied in the absence and presence of saturation (3.50 weight per cent NaCL), as a function of stress (~1.00 to ~15.0 MPa). Specimens of a fine grained Ottawa sand composed of angular grains were subjected to both uniaxial and triaxial strain conditions facilitated through the use of an oedometer and a triaxial system, respectively. Though both test system are, in theory, suppose to measure the same properties, differences in terms of sample geometries, boundary conditions and stress paths may alter the properties themselves. The focal point of this study was to compare SIL-CO-SIL 90 with sand and kaolinite data derived from other authors in terms of properties, and attempt to attribute discrepancies to differences on a geometrical and mineralogical level. The reasoning behind the choice of sample material was based on this fine-grained sand being in-between sand and kaolinite in terms of grain geometry. This reasoning proved to be valid as findings made in this study show that SIL-CO-SIL 90 shares elastic properties found in both minerals. In terms of strain, the high degree seen in these tests is mainly associated with kaolinites. From a velocity viewpoint, P-waves are identical to its kaolinite counterparts in terms of magnitude, whereas P-waves versus stress is aligned with sand behaviour. Though VSZ is compatible in size with saturated kaolinite data at low levels of stress in the oedometer test, VSZ and VSH are found to be inferior compared to sand and kaolinite in all other cases. Furthermore, P-wave anisotropy was found to be decreasing as more stress was applied under uniaxial strain conditions. P-wave anisotropy can therefore be regarded as stress-induced. Hydrostatic loading shifts the P-wave anisotropy from negative to positive but it is still decreasing as a function of stress. In terms of stress sensitivity, S-wave anisotropy appears to similar to P-wave anisotropy, though showing signs of textural anisotropy at initial stress levels, which are most commonly seen in kaolinites. Saturation alters the premises of grain interaction all together by acting like an isotropic background that downgrades the overall anisotropy and its sensitivity to stress.