Rayleigh wave sensitivity to partial water saturation in highly altered volcanic hydrothermal systems

The characterization of volcanic hydrothermal systems (VHS) is fundamental for the early detection of precursors to phreatic or magmatic eruptions and for understanding hazards related to slope instabilities. Since these phenomena can be related to pore-fluid dynamics within the volcanic edifice, monitoring the spatial distribution of the different fluid phases (water, air, vapor) is of great importance. Ambient noise seismic interferometry has been employed for this task, correlating temporal changes in seismic velocities with variations in the water table depth in volcanic areas. However, this technique usually considers that above this depth, the pore space within the rock is totally occupied by a gaseous phase. This, in turn, implies that the body wave velocities and the density of the unsaturated zone are constant values, which is expected to impact on the determination of the water table depth. In this work, we assess the influence of partial saturation in the unsaturated zone of a VHS on ambient seismic noise, by employing a comprehensive rock physics model based on a saturation profile given by the Van Genuchten model. We focus on the sensitivity of Rayleigh waves, which are usually considered to be the most important contribution to the ambient seismic noise.

 

We consider an altered andesite layer overlying a half-space consisting of a relatively unaltered andesite representing the volcanic basement, which is representative of the VHS of La Soufrière de Guadeloupe volcano (Eastern Caribbean, France). We base our rock physics model on Gassmann’s equations to compute body wave velocities as a function of fluid saturation. We compute Rayleigh wave phase velocities for different positions of the water table and analyze their relative difference with respect to a reference scenario that corresponds to the mean value of the water table depth in this region. Our results suggest that the existence of a partial water saturation distribution could affect the Rayleigh wave velocities, and that this effect depends on the range of frequencies considered and the degree of hydrothermal alteration of the medium. For highly altered andesite, characterized by a higher porosity and a lower rock-mass stiffness, the relative variation in velocity obtained with a partial saturation distribution can be up to twice or down to half of the variation for the scenario corresponding to a constant saturation, depending on the frequency range considered. The depth sensitivity kernels of the Rayleigh wave phase velocities exhibit significant variations within regions with variable water content for frequencies between 4 and 6 Hz, which indicates that these seismic waves are able to distinguish the presence of a partial saturation distribution. These results indicate that relative velocity differences derived from ambient noise interferometry provide the possibility of inferring spatial distributions of water content and, therefore, density variations within VHS. This technique therefore emerges as a useful tool to inform on volcanic hazards related to hydrothermal activity, such as erratic explosions and partial flank collapses.