Seismic Anisotropy in Salt from Theoretical Study, Modelling, and Field Experiments

Zandong Sun

ABSTRACT

In this thesis, it is first found that there are three types of anisotropic salt: chevron-crystal salt, recrystallized salt and burial-metamorphic salt. Velocity tests of many salt samples under varying pressure and temperature suggest that there is little resulting velocity change. But pure halite shows a different Vp/Vs pattern than mixtures of halite with sylvite and impure salt. Exact expressions for phase velocities in any arbitrary direction are derived for cubic symmetry. Group velocity formulae are also developed in symmetry planes. The maximum difference between the qSV and qSH velocities occurs in a direction halfway between symmetry axes.

Physical modelling experiments were undertaken on different types of salt, including a chevron-salt sphere. Pure chevron-salt and recrystallized-salt are found to exhibit shear-wave splitting. However, impure salts (salt mixed with clay, sands, etc.) show no shear-wave splitting. It is found that chevron-crystal salt exhibits cubic symmetry.

A pillar of potash ore was selected for a high-resolution seismic experiment in the Prairie Evaporite formation about 1 km underground in Saskatchewan. I observed shear-wave splitting for many of the three-component record sets acquired in this experiment and attribute it to the alignment of salt crystals in the rock in response to the stress field prevailing at the time of recrystallization.

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