A pseudo-MT formulation for 3D CSEM inversion with a single transmiter

François Bretaudeau (1) , Nicolas Coppo (1) , Pierre Wawrzyniak (1) , Sébastien Penz (1) , Jean-François Girard (2, 1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 EOST - Ecole et Observatoire des Sciences de la Terre

Land EM methods provide useful information in domains such as geothermal energy, CO2 storage or water ressources managements. However, the targets are usually close from urban areas where anthropogenic noise such as high power lines, industries and railways prevents from the use of Magnetotelluric (MT). Controlled-Source EM (CSEM) is thus generaly prefered to MT. However, unlike for marine CSEM surveys used in oil prospection, logistical constrains on land surveys limit to the use of a few transmitters, usually 2 polarizations at a single position. Again due to noise constrains, it is usually prefered to place the transmiter close from the stations, which make impossible the use of CSAMT processing, as far field assumption is not respected. Thus the 3D inverse problem associated to this kind of survey suffers from very high sensitivity heterogeneities and singularities, in particular in the vicinity of the transmiter, that prevents from good convergence of the inversion process. In order to achieve reliable local inversion with such pathologic sensitivities, several crucial aspect of the algorithm must be considered. Firstly, Gauss-Newton inversion must be prefered to a gradient-based method, and a very robust solver such as LSQR is necessary. Then a good parameterization must be chosen. Preconditionning of the system with model reparameterization must also be used to compensate for the sensitivity loss with depth. But we show that even with a Gauss-Newton resolution and efficient preconditionning of both data and model parameter, the inversion fails to converge to an accurate solution with single source position in cases where MT data inversion or CSEM with many transmiters provides sucessful results. In order to mitigate the sensitivity singularities, we propose to reformulate the CSEM inverse problem by recasting the data under the form of a pseudo-MT tensor. The associated sensitivities are a linear combinations of the common CSEM sensitivies for electric and magnetic fields. We show on a synthetic model that this formulation highly reduce the footprint of the tranmitter in the sensitivities even in the near field. It also provides a natural balance between the data and between the whole frequency range. Finaly the approach allows the inversion to converge to a reliable result. This approach has been successefully applied on a 3D land survey in France with very complex geology, where the common CSEM inversion had provided biased results.

Mots clés: Land CSEM; 3D inversion; pseudo-MT tensor; model reparameterization; single transmiter

Domaines: Géophysique [physics.geo-ph]

François Bretaudeau, Nicolas Coppo, Pierre Wawrzyniak, Sébastien Penz, Jean-François Girard. A pseudo-MT formulation for 3D CSEM inversion with a single transmiter. The 23rd Electromagnetic Induction Workshop - EMIW2016, Aug 2016, Chiang Mai, Thailand. p.248. ⟨hal-01328531⟩