The role of petrophysics in interpreting geophysical images: the effect of rock chemistry and texture on seismic velocities

The diversity and resolution of current geophysical models in the Alps make it possible to interpret changes in rock properties in terms of deformation or metamorphic reactions.However, there are only a small number of rocks for which the seismic properties at different PT are known, and eclogitisation is the only metamorphic reaction whose effect on the seismic velocities of rocks is known. Yet there are other metamorphic reactions that can modify seismic velocities, particularly those involving micas and amphiboles. For example, by studying the effect of rock chemistry on seismic velocities using field analogues from the European lower crust, we show that amphibole-to-granulite transformation is a strong alternative to eclogitisation in the European lower crust.On the other hand, by modifying velocity contrasts according to wave propagation directions, rock anisotropy is likely to produce or erase some conversions in receiver function models, depending on the seismic events considered. In order to constrain interpretations, a better understanding of the evolution of rock anisotropy as a function of P and T is therefore required, as well as the way this property is transferred to the higher scale, i.e. seismic. Within a sample, the bedding, the shape of the anisotropic minerals and the presence of cracks (shape, orientation and filling) are factors that greatly influence the anisotropy of rocks. By comparing the results of direct measurements on a macroscopic scale with those calculated from crystallographic measurements on a thin-section scale, we show that bedding increases anisotropy on a larger scale and discuss the effects of the shape of crystals or cracks.