Seismo-electrics, electro-seismics, and seismo-magnetics for earth sciences

International audienceThe seismo-electromagnetic method (SEM) is used for non-invasive subsurface exploration. It shows interesting results for detecting fluids such as water, ice, oil, gas, CO_2 , and also to better characterise the subsurface in terms of porosity, permeability, and fractures. However, a limitation of this method is the low level of the induced 5 signals. We first describe SEM's theoretical background, and the role of some key parameters. We then detail recent studies on SEM, through theoretical and numerical developments, and through field and laboratory observations, to show that this method can bring advantages compared to classical geophysical methods

Gravitational memory of natural wormholes

A traversable wormhole solution of general scalar-tensor field equations is presented. We have shown, after a numerical analysis for the behavior of the scalar field of Brans-Dicke theory, that the solution is completely singularity--free. Furthermore, the analysis of more general scalar field dependent coupling constants indicates that the gravitational memory phenomenon may play an important role in the fate of natural wormholes.Comment: 14 pages revtex, 1 ps figur

Brane Worlds, String Cosmology, and AdS/CFT

Using the thin-shell formalism we discuss the motion of domain walls in de Sitter and anti-de Sitter (AdS) time-dependent bulks. This motion results in a dynamics for the brane scale factor. We show that in the case of a clean brane the scale factor describes both singular and non-singular universes, with phases of contraction and expansion. These phases can be understood as quotients of AdS spacetime by a discrete symmetry group. We discuss this effect in some detail, and suggest how the AdS/CFT correspondence could be applied to obtain a non perturbative description of brane-world string cosmology

A review on electrokinetically induced seismo-electrics, electro-seismics, and seismo-magnetics for Earth sciences

The seismo-electromagnetic method (SEM) can be used for non-invasive subsurface exploration. It shows interesting results for detecting fluids such as water, oil, gas, CO2, or ice, and also help to better characterise the subsurface in terms of porosity, permeability, and fractures. However, the challenge of this method is the low level of the induced signals. We first describe SEM's theoretical background, and the role of some key parameters. We then detail recent studies on SEM, through theoretical and numerical developments, and through field and laboratory observations, to show that this method can bring advantages compared to classical geophysical methods

An analytical solution to assess the SH seismoelectric response of the vadose zone

International audienceWe derive an analytical solution of the seismoelectric conversions generated in the vadose zone, when this region is crossed by a pure shear horizontal (SH) wave. Seismoelectric conversions are induced by electrokinetic effects linked to relative motions between fluid and porous media. The considered model assumes a 1D soil constituted by a single layer on top of a half-space in contact at the water table, and a shearing force located at the earth's surface as the wave source. The water table is an interface expected to induce a seismoelectric interfacial response (IR). The top layer represents a porous rock in which porous space is partially saturated by water and air, while the half-space is completely saturated with water, representing the saturated zone. The analytical expressions for the coseismic fields and the interface responses, both electric and magnetic, are derived by solving Pride's equations with proper boundary conditions. An approximate analytical expression of the solution is also obtained, which is very simple and applicable in a fairly broad set of situations. Hypothetical scenarios are proposed to study and analyse the dependence of the electromagnetic fields on various parameters of the medium. An analysis of the approximate solution is also made together with a comparison to the exact solution. The main result of the present analysis is that the amplitude of the interface response generated at the water table is found to be proportional to the jump in the electric current density, which in turn depends on the saturation contrast, poro-mechanical and electrical properties of the medium and on the amplitude of the solid displacement produced by the source. This result is in agreement with the one numerically obtained by the authors, which has been published in a recent work. We also predict the existence of an interface response located at the surface, and that the electric interface response is several orders of magnitude bigger than the electric coseismic field, whereas it is the opposite using compressional waves as shown by theoretical and experimental results. This fact should encourage the performance of field and laboratory tests to check the viability of SHTE seismoelectrics as a near surface prospecting/monitoring tool

Dependence of shear wave seismoelectrics on soil textures: a numerical study in the vadose zone

International audienceIn this work, we study seismoelectric conversions generated in the vadose zone, when this region is traversed by a pure SH wave. We assume that the soil is a 1-D partially saturated lossy porous medium and we use the van Genuchten's constitutive model to describe the water saturation profile. Correspondingly, we extend Pride's formulation to deal with partially saturated media. In order to evaluate the influence of different soil textures we perform a numerical analysis considering, among other relevant properties, the electrokinetic coupling, coseismic responses and interface responses (IRs). We propose new analytical transfer functions for the electric and magnetic field as a function of the water saturation, modifying those of Bordes et al. and Garambois & Dietrich, respectively. Further, we introduce two substantially different saturation-dependent functions into the electrokinetic (EK) coupling linking the poroelastic and the electromagnetic wave equations. The numerical results show that the electric field IRs markedly depend on the soil texture and the chosen EK coupling model, and are several orders of magnitude stronger than the electric field coseismic ones. We also found that the IRs of the water table for the silty and clayey soils are stronger than those for the sandy soils, assuming a non-monotonous saturation dependence of the EK coupling, which takes into account the charged air–water interface. These IRs have been interpreted as the result of the jump in the viscous electric current density at the water table. The amplitude of the IR is obtained using a plane SH wave, neglecting both the spherical spreading and the restriction of its origin to the first Fresnel zone, effects that could lower the predicted values. However, we made an estimation of the expected electric field IR amplitudes detectable in the field by means of the analytical transfer functions, accounting for spherical spreading of the SH seismic waves. This prediction yields a value of 15 µV m −1 , which is compatible with reported values

A hybridized mixed finite element domain decomposed method for two dimensional magnetotelluric modelling, Earth Planets Space

A numerical algorithm to solve the 2D forward problem in magnetotellurics is presented. The method solves Maxwell's equations as a first order system of partial differential equations employing an iterative hybridized mixed domain decomposed finite element procedure. Absorbing boundary conditions are used on the artificial boundaries, diminishing undesired reflection effects and allowing the use of substantially smaller computational domains. Although the algorithm presented can be implemented on both serial and parallel computers, its capabilities are fully utilized on the latters. Results obtained on an IBM SP/2 parallel supercomputer of Purdue University are shown. Also the accuracy of the numerical method is verified by comparison with both numerical and analytical solutions provided by well known methods

Un método de máxima entropía para el análisis de una mezcla de tierras raras

Se propone un método basado en el Principio de Máxima Entropía para analizar mezclas de Tierras Raras en base a medidas magnéticas. El algoritmo está especialmente diseñado para tener en cuenta el error de los datos y la restricción de la positividad de la solución buscada. SUMMARY A method based upon the Maximum Entropy Principle is here developed in order to analyze Rare Earth mixtures with the help of magnetic measurements. The proposed algorithm is so devised as to take into account both experimental errors and the requirement of posivity of the solution.Peer Reviewe

Un método de máxima entropía para el análisis de una mezcla de tierras raras

A method based upon the Maximum Entropy Principle is here developed in order to analyze Rare Earth mixtures with the help of magnetic measurements. The proposed algorithm is so devised as to take into account both experimental errors and the requirement of posivity of the solution

Probabilistic 3-D time-lapse inversion of magnetotelluric data: application to an enhanced geothermal system

Surface-based monitoring of mass transfer caused by injections and extractions in deep boreholes is crucial to maximize oil, gas and geothermal production. Inductive electromagnetic methods, such as magnetotellurics, are appealing for these applications due to their large penetration depths and sensitivity to changes in fluid conductivity and fracture connectivity. In this work, we propose a 3-D Markov chain Monte Carlo inversion of time-lapse magnetotelluric data to image mass transfer following a saline fluid injection. The inversion estimates the posterior probability density function of the resulting plume, and thereby quantifies model uncertainty. To decrease computation times, we base the parametrization on a reduced Legendre moment decomposition of the plume. A synthetic test shows that our methodology is effective when the electrical resistivity structure prior to the injection is well known. The centre of mass and spread of the plume are well retrieved. We then apply our inversion strategy to an injection experiment in an enhanced geothermal system at Paralana, South Australia, and compare it to a 3-D deterministic time-lapse inversion. The latter retrieves resistivity changes that are more shallow than the actual injection interval, whereas the probabilistic inversion retrieves plumes that are located at the correct depths and oriented in a preferential northâ\u80\u93south direction. To explain the time-lapse data, the inversion requires unrealistically large resistivity changes with respect to the base model. We suggest that this is partly explained by unaccounted subsurface heterogeneities in the base model from which time-lapse changes are inferred