In this paper, a new version of the field-mill sensor\ud
structure for atmospheric electric fieldmeasurements is presented.\ud
Both the hardware components (i.e., the mechanical structure,\ud
the electronic front end, and the acquisition and control systems)\ud
and the data processing software are designed in order to reduce\ud
power consumption and enhance the instrument metrological performance\ud
in terms of accuracy, sensitivity, and frequency band
On a heterogeneous karstic site in the Larzac plateau (France), we performed cross-correlations of ambient seismic noise recorded at two broadband seismometers to obtain daily seismic velocity changes. Rayleigh velocity changes at the 6-to 8-Hz frequency band show variations of ±0.2% over 1 yr. Assuming a simple velocity profile, changes are expected to come from depths of tens of meters. Therefore velocity changes at 6 to 8 Hz were interpreted as induced by water saturation changes. A slow infiltration rate would explain the delay of several months between the rainy season (November) and the minimum velocity (June). Superconducting gravimeter, evapotranspiration, and magnetic resonance sounding (MRS) measurements were then combined with seismic data in one-dimensional physical simulations. Velocity changes clearly constrain hydrological parameters, like saturated hydraulic conductivity, even if the BiotGassmann theory does not explain all of the amplitude observed. Nevertheless, this nondestructive method demonstrates great potential in hydrological model calibration. It overcomes the lack of depth resolution of gravimetry and the lack of temporal resolution of MRS. The combination of ambient seismic noise with gravimetry and MRS could fill the instrumental gap currently existing in hydrology for the study of deep and/or complex critical zones.
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