The concentration of radon-222 has been monitored since 1995 in the atmosphere of a 2 m transverse dimension, 128 m long, dead-end horizontal tunnel located in the French Alps, at an altitude of 1600 m. Most of the time, the radon concentration is stable, with an average value ranging from 200 Bq m−3 near the entrance to about 1000 Bq m−3 in the most confined section, with an equilibrium factor between radon and its short-lived decay products varying from 0.61 to 0.78. However, radon bursts are repeatedly observed, with amplitudes reaching up to 36×103 Bq m−3 and durations varying from one to several weeks, with similar spatial variations along the tunnel as the background concentration. These spatial variations are qualitatively interpreted in terms of natural ventilation. Comparing the radon background concentration with the measured radon exhalation flux at the wall yields an estimate of 8±2×10−6 s−1 (0.03±0.007 h−1) for the ventilation rate. The hypothesis that the bursts could be due to transient changes in ventilation can be ruled out. Thus, the bursts are the results of transient increased radon exhalation at the walls, that could be due to meteorological effects or possibly combined hydrological and mechanical forcing associated with the water level variations of the nearby Roselend reservoir lake. Such studies are of interest for radiation protection in poorly ventilated underground settings, and, ultimately, for a better understanding of radon exhalation associated with tectonic or volcanic processes.

Very‐low‐frequency (VLF), audiomagnetotelluric (AMT), and water geochemistry surveys were performed on the Sur‐Frêtes Ridge in the French Alps to evaluate the groundwater circulation system within the ridge. At this site, temporal variations of the electric field have been observed in association with water‐level variations of neighboring artificial lakes. The Sur‐Frêtes Ridge is 1 km wide and trends east–west. Water samples were collected at 52 points distributed throughout the area. VLF soundings were carried out along three parallel east–west profiles, and 7 AMT soundings were carried out along an east–west profile on the top of the ridge. This site is characterized by a rugged topography of the ridge where geological and topographic trends are almost perpendicular, making the structure fully three dimensional. We constructed a 2‐D resistivity model of the ridge from 2‐D and 3‐D analyses of the VLF and AMT data, associating geology and topography models. When combined with the water chemistry data, a model is proposed for groundwater percolation below the ridge across the geological contacts. This study demonstrates that electromagnetic imaging in a highly heterogeneous context can be combined with water chemistry to map groundwater circulation at the kilometer scale. The approach is relevant for hydrogeological and environmental applications.

Electric potential variations were recorded in the vicinity of the Dhor Barahi periodic spring located southeast of Pokhara, western Nepal. This spring flows for a few minutes, with a repetition rate of about 30 min. During water flow, positive electric potential pulses with an amplitude varying from 0.17 to 0.36 mV are observed on the ground surface, and negative pulses with an amplitude of 8.4 mV in the water channel. These observations provide a striking illustration of the electrokinetic effect. Such electric potential measurements may be used in the future to monitor hydraulic systems and may also improve our understanding of electrical effects associated with natural systems. ((C) Academie des sciences/Elsevier, Paris.)

Two of the most often cited earthquake precursors are radon emanation and electric potential variations1,2,3,4,5,6, but these few reported examples have generally been deemed questionable7,8,9,10,11. If a mechanism relating crustal deformation to radon emanation or electrical signals does indeed exist, it is thought to involve fluids12,13,14,15,16,17,18,19. Some preliminary insight has been gained into these processes from the study of natural systems under controlled mechanical and hydrological conditions20. Here we report electric potential variations, radon emanation and deformation measurements recorded since 1995 in the French Alps in the vicinity of two artificial lakes which have strong seasonal variations in water level of more than 50 metres. We observe that electric potential variations and radon emanations are repeatedly associated with transient deformation events induced by variations in lake levels. These events are characterized by a change in ground tilt which deviates from the expected elastic response, and are associated with periods of accelerating strain, which suggests that accelerated loading can enhance fluid transport properties. Qualitatively, this behaviour can be accounted for by a model in which straining induces fluid overpressure and dynamic flow in cracks. These observations mayshed light on the sensitivity of rock transport properties to deformation.

Electric potential variations have been recorded from November 1995 to February 1996 and continuously since October 1996 at 14 measurement points on a one km wide ridge separating two lakes in the French Alps. The levels of the lakes vary by several tens of meters on a yearly cycle, inducing stress variations and fluid percolation. At one point, unambiguous variations as large as 120 mV are observed over a year, linearly correlated with the levels of the lakes with a magnitude of 2 mV per meter of water level change. This particular measurement point lies at the edge of a SP anomaly, which supports the presence of a localized zone of ground water flow forced by the lake level, suggesting an electrokinetic mechanism. The observed correlation implies a ζ‐potential of the order of ‐8 mV for a 60 Ωm electrolyte, in agreement with laboratory measurements.