Detecting a salt dome overhang with magnetotellurics: 3D inversion methodology and synthetic model studies

Abstract

Detecting a salt dome overhang is known to be problematic by seismic methods alone. We used magnetotellurics (MT) as a complementary method to seismics to investigate the detectability of a salt dome overhang. A comparison of MT responses for 3D synthetic salt models with and without overhang shows that MT is very sensitive to shallow salt structures and suggests that it should be possible to detect an overhang. To further investigate the resolution capability of MT for a salt dome overhang, we performed a 3D MT inversion study and investigated the impact of model parametrization and regularization. We showed that using the logarithms of the conductivities as model parameters is crucial for inverting data from resistive salt structures because, in this case, commonly used Tikhonov-type stabilizers work more equally for smoothing the resistive and conductive structures. The use of a logarithmic parametrization also accelerated the convergence and produced better inversion results. When the Laplace operator was used as a regularization functional, we still observed that the inversion algorithm allows spatial resistivity gradients. These spatial gradients are reduced if a regularization based on first derivatives in contrast to the Laplace operator is introduced. To demonstrate the favorable performance when logarithmic parametrization and gradient-based regularization are employed, we first inverted a data set simulated for a simple model of two adjacent blocks. Subsequently, we applied the code to a more realistic salt dome overhang detectability study. The results from the detectability study are encouraging and suggest that 3D MT inversion can be applied to decide whether the overhang is present in the shallow salt structure even in the case when only profile data are available. However, to resolve the overhang, a dense MT site coverage above the flanks of the salt dome is required.

Three-dimensional inversion of magnetotelluric impedance tensor data and full distortion matrix

“…Our inversion is based on the 3-D MT inversion code x3Di and details about the method are described in Avdeev & Avdeeva (2009) and Avdeeva et al (2012). It employs a limited-memory quasiNewton optimization method to minimize a Tikhonov-type regularized objective function ϕ.…”

“…, where W is finite-difference approximation of the Gradient operator, which controls model smoothness (for more details, see Avdeeva et al 2012).…”

Magnetotelluric 3-D inversion—a review of two successful workshops on forward and inversion code testing and comparison

“…As the forward solver is the driving engine of an inversion algorithm, the inversion codes are also based on FD, FE and IE methods. For the inversion task of both workshops only FD (Mackie et al 2001;Siripunvaraporn et al 2005;Hautot et al 2007Hautot et al , 2011Siripunvaraporn & Egbert 2009;Egbert & Kelbert 2012) and IE (Avdeev & Avdeeva 2009;Avdeeva et al 2012) codes were used. The 3-D magnetotelluric inverse problem is far from being solved, but these codes using different inversion schemes showed that one can recover the conductivity structure of simple, synthetic test models reasonably well, to greater or less degrees depending on resolution and sensitivity.…”

“…Mackie et al 2001;Hautot et al 2007Hautot et al , 2011Egbert & Kelbert 2012) and Quasi-Newton (QN) methods (e.g. Avdeev & Avdeeva 2009;Avdeeva et al 2012) avoid both the explicit computation and storage of a full sensitivity matrix. Only the gradient vector of the objective functional is needed.…”

Full-Jacobian Gauss-Newton 3D marine magnetotelluric inversion

“…below highly heterogeneous basalt layers or in the presence of complex salt structures. In such situations, resistivity models obtained via MT inversion can for example be used to improve velocity models for seismic processing or in workflows for joint interpretation (Hovertsen et al, 1998;Key et al, 2006;Avdeeva et al, 2012).…”