Anisotropy Of Rock Samples Research Articles

Estimation of elastic anisotropy from three-component ultrasonic measurements using laser Doppler interferometry

Ultrasonic measurements using laser Doppler interferometry (LDI) have been reported to provide robust estimates of elastic anisotropy of rock samples. In this approach, an ultrasonic wave is emitted by a piezo-electric source and detected by the LDI, which can be configured to measure three components of the particle velocity in a very small area (~1 mm2) of the sample. Repeating these measurements for a dense array of points on the sample’s surface gives a distribution of traveltimes and polarisation fields on the surface. Anisotropy is then obtained by inverting these fields using analytical expressions or numerical algorithms for computing phase and group velocities. The existing implementation of this approach involves the inversion of direct compressional (P) and shear (S) wave arrivals only. A previous study showed that this approach produces stable results if only a small range of source–receiver offsets is included in the inversion. This limitation resulted in a relatively large uncertainty of the result. This uncertainty can be reduced by inverting the entire traveltime field. To this end, we numerically simulate the wavefield in the sample. Analysis of the computed wavefield reveals the presence of P- and S-waves as well as a critically refracted converted PS-wave. Hence, the inversion of the entire traveltime field must include these three waves. We implement this inversion using global minimisation of the traveltime misfit function, coupled with numerical computation of ray velocities. Application of this algorithm to laboratory LDI measurements on a transversely isotropic phenolic sample provides stable anisotropy estimates consistent with previous studies.

Changes in mean magnetic susceptibility and its anisotropy of rock samples as a result of alternating field demagnetization

Measurements of low-field magnetic susceptibility ( K) and its anisotropy (AMS) on different rock types during stepwise alternating field (AF) demagnetization in increasing fields revealed not only significant changes of the AMS principal susceptibilities, but also an increase of the mean magnetic susceptibility ( K m). Studied collections of loess/paleosol samples from different sections in Belgium, Bulgaria, China, Siberia and Tadjikistan and diorites, granites and gneisses from Antarctica show systematic K m-increase, between 2 and 27% as compared to the initial values, after AF-demagnetization up to 100 or 200 mT maximum amplitude. The relationships between magnetic susceptibility increase and magnetic hysteresis parameters and their ratios, indicate that the K m-increase is due to changes in magnetic domain configuration of the initial natural remanent magnetization (NRM) state of the remanence carriers during AF-treatment. The obtained linear relationship between K-increase and the degree of anisotropy P′ for strongly anisotropic gneiss samples suggests that magnetostatic interactions also play a role in the observed AF-effect on K m.