How can earth-tides and earthquakes measure permeability?

Good constraints on hydrogeological properties are an important first step in any quantitative model of groundwater flow. Field estimation of permeability is difficult as it varies over orders of magnitude in natural systems and is highly scale-dependent. This work aims to study the scale dependence of hydraulic properties in fractured rocks by utilizing a combination of tidal, barometric and seismic response analyses on pressure head time-series from the Santa Susana Field Laboratory in Southern California. We benefit from the access to a selection of wells on this inactive industrial site that we deploy 40 pressure transducers into, in order to measure water level for over a year. The first step was to compare tidal response results to two conventional, long-term and large scale pumping tests performed at the same site. We obtained consistent values between the methods for a range of site-specific permeability values. The second step consisted in inverting permeability and specific storage from tidal response, barometric forcing and seismic response. The latter was used to compute hydro-seismic transfer functions after 30 local and tele-seismic events. The three techniques sample different effective volumes and therefore may help to investigate the scale-dependent structure near fault zones.

Figure. (Haut) Variations de pression (h) enregistrées sur le site du Santa Susana Field Laboratory après le séisme de magnitude Mw=6.6 du 25 avril 2015 au Népal. (Bas) Spectrogramme correspondant.