Three-dimensional inversion of static-shifted magnetotelluric data

Abstract

A practical method for inverting static-shifted magnetotelluric (MT) data to produce a 3-D resistivity model is presented. Static-shift parameters are incorporated into an iterative, linearized inversion method, with a constraint added on the assumption that static shifts are due to a zero-mean, Gaussian process. A staggered finite-difference scheme is used to evaluate both the forward problem and the 'pseudo-forward' problem needed to construct the full sensitivity matrix. The linear system of equations is efficiently solved by alternating the incomplete Cholesky biconjugate gradient (ICBCG) solver with the static divergence correction procedure. Even with this efficiency in the forward modeling, generating the full sensitivity matrix at every iteration is still impractical on modern PCs. To reduce the computer time to a reasonable level, an efficient procedure for updating the sensitivities is implemented: (1) in the first few iterations, the sensitivities for the starting homogeneous half-space are used, (2) the full sensitivity matrix is computed only once (e.g. at the third iteration), and (3) for the subsequent iterations it is updated using Broyden's algorithm. The synthetic and real data examples show that the method is robust in the presence of static shifts and can be used for 3-D problems of realistic size.

The advantages of complementing MT profiles in 3-D environments with geomagnetic transfer function and interstation horizontal magnetic transfer function data: results from a synthetic case study

“…Following this idea, several publications recommend inverting all four components of the MT impedance tensor (Z), for example Siripunvaraporn et al (2005b), Tietze & Ritter (2013) and Kiyan et al (2014), instead of the approach of inverting only the off-diagonal components (Z o ) (e.g. Sasaki 2004;Sasaki & Meju 2006). However, the presence of anthropogenic noise can strongly reduce the quality of the Z data, in particular the quality of the diagonal components, making it sometimes worthless to use all components of the Z tensor for the inversion process because of scatter and/or extremely large errors in the diagonal components of Z.…”

Imaging Multi-Dimensional Electrical Resistivity Structure as a Tool in Developing Enhanced Geothermal Systems (EGS)

“…To solve the inverse problem for both subsurface conductivity model parameters m and static shift s, and to constrain the solution, the following objective function is defined (Sasaki, 2004): …”

“…The third term imposes an adherence constraint to an a-priori model, m apr . The static shifts are assumed Gaussian distributed with a mean of zero, with the size constrained by the term 2 s , The objective function is minimized using a direct GaussNewton scheme, operating on the following system of equations (Sasaki, 2004):…”

“…Three-dimensional interpretation historically has required large computing resources and long run times (Newman et al, 2005a). The inversion algorithm of Sasaki (2004) solves simultaneously for static shift and 3-D subsurface conductivity distribution parameters on a personal computer and can handle moderate-sized data sets.…”

3D Magnetotellurics for Mineral Exploration beneath Cover

“…A number of 3D inversion codes are available (Mackie et al 1994, Newman and Alumbaugh, 2000, Sasaki, 2004, Siripunvaraporn et al 2005a, and many model studies have been published (e.g. Siripunvaraporn et al 2005b, Han et al 2009).…”