Research

• 1-D streaming potential modelling by finite element method. We modeled the Richards eq. and the Poisson eq.. We showed that the streaming potential measurement can be performed by a measurement of the difference of electrical potential and of the difference of the pressure, along the sand column during the drainage of the sand.  We showed that it is necessary to make the hypothesis of a non-monotoneous behavior of the streaming potential coefficient as a function of the water-saturation, to explain the observations.  [Allègre et al., GJI, 189, 285-295, 2012 pdf].

• We observed continuous measurements of streaming potentials induced by fluid flow in unsaturated sand. We developed an original experimental setup allowing the continuous measurements of the streaming potentials, the pressures, and the water-saturations. We observed a behavior of the streaming potential coefficient as a function of the water-saturation not explained by the current models [Allègre et al., GJI, 182, 1248-1266 2010 pdf]. This study was a collaboration with LHyGeS. This study was following previous observations on unsaturated sand  [Guichet et al., JGR 108, 2141, 2003 pdf].

•  We observed seismo-electric conversions in the field, as a function of water-saturation, allowing us to propose a transfer fonction between the electric field and the acceleration as a function of the water-saturation [Strahser et al., GJI, 187,1378-1392, 2011 pdf]

• We proved the existence seismo-magnetic conversions, predicted by the theory since 1994, but not observed up to now. We developed an experimental setup with a seismic source remote-controlled, to induce seismic wave propagation in a saturated sand column. We measured the magnetic part of the seismo-electromagnetic conversions. We showed that the seismo-electric field is coupled to the P_wave propagation and extension waves (velocity of 1300 m/s) and that the seismo-magnetic field is coupled to the S_wave propagation (velocity of 800 m/s), as demonstrated in Pride's theory (1994), but not evidenced up to now. This study was performed in collaboration with université de Pau and université de Grenoble [Bordesetal., GJI 174, 489-504, 2008 pdf and GRL33, L01302, 2006 pdf], and developed in the Low Noise Underground Laboratory (LSBB-Laboratoire Souterrain à Bas Bruit).

•We measured, through laboratory experiment, the behavior of the streaming potential when calcite was precipited in sand. We deduced the evolution of the zeta potential as a function of pH (4to 12) and showed that its sign changed. We compared these observations to the results of a triple-layer model (TLM) to analyse our measurements [Guichet et al. GJI 166, 445-460, 2006 pdf]

• We showed, by anisotropic measurements of electrical conductivity on carbonates, that the electrical conductivity increases continuously when the samples are deformed under uniaxial condition as soon as the sample is not fully water-saturated but about 85%, by a power law 10^0.002P of the axial pressure P (in MPa) [Jouniaux et al. GJI 167, 1017-1026, 2006 pdf].

• In the field, the streaming potentials are difficult to detect. Results obtained in Mayet de Montagne, related to fluid injections at depth, are shown below [Pinettes et al. Pageoph 159, 2629-2657,2002 pdf].

•We modeled the electric and magnetic field induced by water-flow in the Nankai prism to know if it is possible to use such measurements to detect fluid flow variations within the prism, possibly linked to seismic cycle [Jouniaux et al., JGR, 104, 29293-29309, 1999 pdf] (collaboration with LETI from CEA in Grenoble ; program franco-japonais Kaiko-Tokai). We showed that signals of 3 mV and 3 nT could be induced by 20 % change in the fluid flow.

• Then we tried to quantify the anisotropy of permeability. These measurements are needed for the study of the propagation of the décollement. We wanted to quantify the permeability under low effective pressure because it is thought that high overpressures are present within the prism. We measured values of permeability of 2-6 x 10-19 m2 [ Bourlange et al., P.ODP Sci. Results, 190/196, 1–16, 2004 pdf; www-odp.tamu.edu publications/190196SR/215/215.htm]. These values were used to deduce a diffusion time of the pore pressure - in situ, and to conclude that an overpressure could be maintained only during transitory period that could be period of propagation of the décollement [Bourlange et al., 2003]. Then measurements of the anisotropy of the electrical conductivity on samples from Nankai (Leg 190) were performed by P. Henry, who proposed to use these observations to quantify the deformation in these sediments [Henry et al., JGR, 108(B9) 2407, 2003 pdf].

• This study was performed after measurements of the streaming potential, permeability, and electrical conductivity during the deformation of a sample from the prism of Nankai (Leg 131), that showed an increase of both the permeability and the streaming potential during the deformation  [Jouniaux et al., GRL, 21, 149-152, 1994 pdf].