Frédéric Perrier

Radon emanation and electric potential variations associated with transient deformation near reservoir lakes

Two of the most often cited earthquake precursors are radon emanation and electric potential variations, but these few reported examples have generally been deemed questionable. If a mechanism relating crustal deformation to radon emanation or electrical signals does indeed exist, it is thought to involve fluids. Some preliminary insight has been gained into these processes from the study of natural systems under controlled mechanical and hydrological conditions. 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.

Streaming potential measurements. 1. Properties of the electrical double layer from crushed rock samples

The ξ potential has been inferred from streaming potential measurements with crushed rock samples as a function of pH and electrolyte concentration for various salts. The value obtained for crushed Fontainebleau sandstone at pH = 5.7 and a KCl solution with a resistivity of 400 Ω m is −40 ± 5 mV, where the error is dominated by sample to sample variations. The sensitivity of the ξ potential to the electrolyte resistivity for KCl is given experimentally by ρ_f^(0.23±0.014) where ρ_f is the electrolyte resistivity. The point of zero charge (pzc) is observed for pH = 2.5 ± 0.1, and the ξ potential is positive for pH pzc. For pH > 5 the variations of the ξ potential with pH can be approximated by ξ(pH)/ξ(5.7) = 1 + (0.068 ± 0.004)(pH - 5.7) for ρ_f = 100 Ω m. The ξ potential has been observed to be sensitive to the valence of the ions and is approximately reduced by the charge of the cation, unless specific adsorption takes place like in the case of Al^3+. The experimental results are well accounted for by a three-layer numerical model of the electrical double layer, and the parameters of this model can be evaluated from the experimental data. The sensitivity of the ξ potential to the rock minerals has also been studied. The ξ potential obtained for granitic rocks is comparable to that obtained for Fontainebleau sandstone but can be reduced by a factor of 2–4 for sandstones containing significant fractions of carbonates or clay. To take into account the effect of the chemical composition of the electrolyte, a chemical efficiency is defined as the ratio of the ξ potential to the ξ potential measured for KCl. This chemical efficiency is measured to be ∼80% for typical groundwater but can be as low as 40% for a water with a high dissolved carbonate content. The set of empirical laws derived from our measurements can be used to assess the magnitude of the streaming potentials expected in natural geophysical systems.

Electrical structure of the Himalaya of central Nepal: High conductivity around the mid-crustal ramp along the MHT

Twelve broadband magnetotelluric (MT) soundings were performed across the Himalaya of Central Nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics. The MT impedance tensors were obtained for frequencies between 0.001 and 500 Hz. The 2‐D section, derived from joint inversion of TE‐ and TM mode after RRI and Groom/Bailey decomposition, shows high conductivity in the foreland basin (∼30 Ω.m) that contrasts with the resistive Indian basement (>300 Ω.m) and Lesser Himalaya (>1000 Ω.m). In addition, our MT sounding reveals a major conductive feature beneath the front of the Higher Himalaya, also characterized by intense microseismic activity, and the position of a mid‐crustal ramp along the major active thrust fault (MHT). This high conductivity zone probably reflects metamorphic fluids, released during underthrusting of the Indian basement and pervading well connected microcracks induced by interseismic stress build‐up, or distributed brittle deformation around the ramp.

Streaming potential measurements 2. Relationship between electrical and hydraulic flow patterns from rock samples during deformation

Streaming potential and resistivity measurements have been performed on Fontainebleau sandstone and Villejust quartzite samples in a triaxial device during compaction, uniaxial compression, and rupture. Measurements on individual samples do not show any clear intrinsic dependence of the streaming potential coefficient with permeability. An apparent dependence of the streaming potential coefficient with permeability is, however, observed during deformation. The effect of surface conductivity is taken into account and is small compared with the observed changes in the streaming potential coefficient. The observed dependence is therefore interpreted in terms of a difference in the evolution of the electrical and hydraulic connectivity patterns during deformation. This effect causes the streaming potential coefficient, and consequently the inferred ξ potential, to be reduced by a geometrical factor R_G representing the electrical efficiency of the hydraulic network. Estimates of the R_G factor varying between 0.2 and 0.8 for electrolyte resistivity larger than 100 Ωm are obtained by comparing the values of the ξ potential inferred from intact rock samples with the values obtained from crushed rock samples, where the geometrical effects are assumed to be negligible. The reduction of the streaming potential coefficient observed during compaction or uniaxial compression suggests that the tortuosity of the hydraulic network increases faster than the tortuosity of the electrical network. Before rupture, an increase in the streaming potential coefficient associated with the onset of dilatancy was observed for three samples of Fontainebleau sandstone and one sample of Villejust quartzite. The changes in streaming potential coefficient prior to failure range from 30% to 50%. During one experiment, an increase in the concentration of sulfate ions was also observed before failure. These experiments suggest that observable streaming potential and geochemical variations could occur before earthquakes.

Water in the mantle: Results from electrical conductivity beneath the French Alps

[1]xa0A deep magnetotelluric sounding in the French Alps provided a vertical electrical conductivity profile between ∼200–1000 km. Two prominent features are observed. First, the conductivity in the depth range 400–800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm. Second, the data do not require the conductivity to change throughout the transition zone (410–660 km). In this part of the mantle, a temperature of 350–450 C less than normal explains the magnetotelluric conductivity profile. At 200–400 km, our model favors a cold mantle with 1000–1500 ppm of water dissolved in olivine. If correct, this model suggests that the subducted slab is dehydrated before reaching the transition zone.

Radon-222 signatures of natural ventilation regimes in an underground quarry

Radon-222 activity concentration has been monitored since 1999 in an underground limestone quarry located in Vincennes, near Paris, France. It is homogeneous in summer, with an average value of 1700 Bq m(-3), and varies from 730 to 1450 Bq m(-3) in winter, indicating natural ventilation with a rate ranging from 0.5 to 2.4 x 10(-6) s(-1) (0.04-0.22 day(-1)). This hypothesis is supported by measurements in the vertical access pit where, in winter, a turbulent air current produces a stable radon profile, smoothly decreasing from 700 Bq m(-3) at 20 m depth to 300 Bq m(-3) at surface. In summer, a thermal stratification is maintained in the pit, but the radon-222 concentration jumps repeatedly between 100 and 2000 Bq m(-3). These jumps are due to atmospheric pressure pumping, which induces ventilation in the quarry at a rate of about 0.1 x 10(-6) s(-1) (0.009 day(-1)). Radon-222 monitoring thus provides a dynamical characterisation of ventilation regimes, which is important for the assessment of the long-term evolution of underground systems.

Characterization of Electrical Daily Variations Induced by Capillary Flow in the Non-saturated Zone

Abstract—Electrical daily variations without an associated magnetic field were observed for a period of six weeks on independent parallel dipoles during the one-year electrode inter comparison experiment that took place in Garchy, France, from April 1995 to April 1996. These daily variations appeared suddenly during summer when the soil was dry on the surface. Their amplitude ranged from 0.1 to 1.5 mV for 50-m long dipoles, and characteristic relative phases. The amplitude of the variations was found to be related to soil temperature. These daily variations are accounted for in a model where streaming potentials are produced by coherent capillary flow in the non-saturated zone. Such daily variations are important to control in long-period magnetotelluric sounding and may be of practical use for agricultural applications. As their amplitude strongly depends on time-varying soil conditions, they may also lead to erroneous interpretations as earthquake precursors.

Electric potential variations associated with yearly lake level variations

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.

Fluid flow near reservoir lakes inferred from the spatial and temporal analysis of the electric potential

Electric self-potential (SP) variations have been monitored continuously from 1995 to 1998 at 14 points on a ridge separating the Roselend and La Gittaz reservoir lakes in the French Alps. The lakes have level variations of at least 50 m over yearly cycles. Seasonal variations of SP associated with lake-level variations are observed on five points of the array. For three points located on the banks of the lakes, a positive correlation to the lake-level variations is observed with a maximal amplitude of about 180 mV, corresponding to an average response of 2.4 mV per meter of water. For two points located on the bottom of each lake, the correlation is negative, with a maximal magnitude of about −50 mV, corresponding to an average response of −1.1 mV per meter of water. Two independent temporary electrical arrays located on the banks of each lake confirm these measurements and allow a better spatial characterization of the sources associated with the observed SP variations. In particular, near the Roselend lake, the electrical response to lake-level variations is increasing for decreasing altitude. The measured SP variations are proposed to result from the electrokinetic coupling associated with a vertical groundwater flow connecting a constant pore pressure source to the bottom of the lakes. Numerical modeling indicates that the spatial variation of the response and the nonlinear response observed at one point can be explained by leakage currents in the conductive lake water. The values of the streaming potential coefficient (SPC), measured in the laboratory with crushed rock samples from the site, range from 14 to 50 mV/0.1 MPa for an electrolyte resistivity of 40 Ω m and are compatible, to first order, with the magnitude of the observed seasonal SP variations. A detailed quantitative electrokinetic modeling is currently limited mainly by the poor knowledge on the contribution of electrical leakage currents and the local variability of the SPC. This experiment indicates that spatial and temporal variations of the electric potential are promising tools to characterize and monitor shallow groundwater flow and provide practical data for the investigation of groundwater flow associated with volcanic or tectonic activity.

Groundwater electromagnetic imaging in complex geological and topographical regions: A case study of a tectonic boundary in the French Alps

Very‐low‐frequency (VLF), audiomagnetotelluric (AMT), and water geochemistry surveys were performed on the Sur‐Fretes 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‐Fretes 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 m...