Modélisation de méthodes électromagnétiques basses fréquences dans le domaine de la géophysique - Application au suivi d'un réservoir en production

International audienceModélisation de méthodes électromagnétiques basses fréquences dans le domaine de la géophysique - Application au suivi d'un réservoir en productio

Approximate three-dimensional resistivity modelling using Fourier analysis of layer resistivity in shallow soil studies

International audienceThe approximate forward modelling method using Fourier analysis has been used in 2-D applications for several decades. It involves decomposition of the terrain parameters, either the resistivity or the layer thickness, into a Fourier series expansion to simplify the problem to that of a 1-D situation. In this study, the Fourier analysis is applied to 3-D forward modelling for the purposes of shallow DC resistivity imaging with pole-pole array. Our work is to assess advantages and drawbacks of the simplified approach by comparing to exact 3-D solutions, method of moments (MoM) and surface integrals and to the Born approximation applied to MoM. While the Fourier analysis method offers very short calculation times, it shows a significant, albeit systematic, reduction of the anomaly amplitudes; and its ability to delineate anomaly sources is lower than the other methods. Nevertheless, its rapidity makes it an interesting first approach in the modelling of DC resistivity results

4D CSEM feasibility study: a land example

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1D single-site and laterally constrained inversion of multifrequency and multicomponent ground-based electromagnetic induction data—Application to the investigation of a near-surface clayey overburden

International audienceElectromagnetic induction (EMI) methods are widely used to determine the distribution of the electrical conductivity and are well adapted to the delimitation of aquifers and clayey layers because the electromagnetic field is strongly perturbed by conductive media. The multicomponent EMI device that was used allowed the three components of the secondary magnetic field (the radial Hr, the tangential Hϕ, and the vertical Hz) to be measured at 10 frequencies ranging from 110 to 56 kHz in one single sounding with offsets ranging from 20 to 400 m. In a continuing endeavor to improve the reliability with which the thickness and conductivity are inverted, we focused our research on the use of components other than the vertical magnetic field Hz. Because a separate sensitivity analysis of Hr and Hz suggests that Hr is more sensitive to variations in the thickness of a near-surface conductive layer, we developed an inversion tool able to make single-sounding and laterally constrained 1D interpretation of both components jointly, associated with an adapted random search algorithm for single-sounding processing for which almost no a priori information is available. Considering the complementarity of Hr and Hz components, inversion tests of clean and noisy synthetic data showed an improvement in the definition of the thickness of a near-surface conductive layer. This inversion code was applied to the karst site of the basin of Fontaine-Sous-Préaux, near Rouen (northwest of France). Comparison with an electrical resistivity tomography tends to confirm the reliability of the interpretation from the EMI data with the developed inversion tool

4D CSEM feasibility study: a land example

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A comparison of helicopter-borne electromagnetic systems for hydrogeologic studies

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Application of the Small-Loop TDEM Method to the Quantification of Both Electrical and Magnetic Parameters of the Subsurface (Numerical Approach)

International audienceThe inductive electromagnetic geophysical method in the temporal domain (TDEM) allows, in certain conditions, the measurement of a signal integrating information from several physical independent parameters (electrical resistivity, magnetic viscosity, polarization). Being able to separate and quantify those physical and independent contributions is a real issue. It is particularly important when one of those contributions overwhelms the others which can happen depending on the geological context and the specific setup used. The non-conventional use of this method with small transmission and reception loops (a few meters wide at most for the transmitter loop) increases the sensitivity to the magnetic viscosity in addition to the usual electrical resistivity. Through numerical modeling and field tests, we are designing a setup geometry that would allow us to discriminate between the influences of these parameters on the signal. The most promising option would be to make a measurement in the central configuration, which is very sensitive to the magnetic viscosity, and another one with an offset between the two loops as the sensitivity to the magnetic properties decreases with the distance separating the transmission and reception loops. This use of small loops also increases the sensitivity to the noise from the system itself. The interactions between the reception and transmission loops and the measurement device create a distortion on the measured signal. This distortion depends on the setup geometry and on the electrical characteristics of the ground. Current work concerns the study of equivalent electrical circuits to model the mutual characteristic and complex impedance between both the transmitter and receiver coils of the TDEM setup. The accurate evaluation of the mutual impedance over a wide band of frequency is necessary to deconvoluate the instrumental response (including all electronic and coils parts) from the part of the measured transient signal coming from the subsurface

Application of the Small-Loop TDEM Method to the Quantification of Both Electrical and Magnetic Parameters of the Subsurface (Numerical Approach)

International audienceThe inductive electromagnetic geophysical method in the temporal domain (TDEM) allows, in certain conditions, the measurement of a signal integrating information from several physical independent parameters (electrical resistivity, magnetic viscosity, polarization). Being able to separate and quantify those physical and independent contributions is a real issue. It is particularly important when one of those contributions overwhelms the others which can happen depending on the geological context and the specific setup used. The non-conventional use of this method with small transmission and reception loops (a few meters wide at most for the transmitter loop) increases the sensitivity to the magnetic viscosity in addition to the usual electrical resistivity. Through numerical modeling and field tests, we are designing a setup geometry that would allow us to discriminate between the influences of these parameters on the signal. The most promising option would be to make a measurement in the central configuration, which is very sensitive to the magnetic viscosity, and another one with an offset between the two loops as the sensitivity to the magnetic properties decreases with the distance separating the transmission and reception loops. This use of small loops also increases the sensitivity to the noise from the system itself. The interactions between the reception and transmission loops and the measurement device create a distortion on the measured signal. This distortion depends on the setup geometry and on the electrical characteristics of the ground. Current work concerns the study of equivalent electrical circuits to model the mutual characteristic and complex impedance between both the transmitter and receiver coils of the TDEM setup. The accurate evaluation of the mutual impedance over a wide band of frequency is necessary to deconvoluate the instrumental response (including all electronic and coils parts) from the part of the measured transient signal coming from the subsurface