Seong Kon Lee

MT2DInvMatlab-A program in MATLAB and FORTRAN for two-dimensional magnetotelluric inversion

MT2DInvMatlab is an open-source MATLAB^(R) software package for two-dimensional (2D) inversion of magnetotelluric (MT) data; it is written in mixed languages of MATLAB and FORTRAN. MT2DInvMatlab uses the finite element method (FEM) to compute 2D MT model responses, and smoothness-constrained least-squares inversion with a spatially variable regularization parameter algorithm to stabilize the inversion process and provide a high-resolution optimal earth model. It is also able to include terrain effects in inversion by incorporating topography into a forward model. This program runs under the MATLAB environment so that users can utilize the existing general interface of MATLAB, while some specific functions are written in FORTRAN 90 to speed up computation and reuse pre-existing FORTRAN code in the MATLAB environment with minimal modification. This program has been tested using synthetic models, including one with variable topography, and on field data. The results were assessed by comparing inverse models obtained with MT2DInvMatlab and with a non-linear conjugate gradient (NLCG) algorithm. In both tests the new inversion software reconstructs the subsurface resistivity structure very closely and provides an improvement in both resolution and stability.

Investigating 2-D MT inversion codes using real field data

There are currently a significant number of two-dimensional (2-D) and three-dimensional (3-D) inversion codes available for magnetotelluric (MT) data. Through various 2-D inversion algorithms suggested so far, the classical Occams inversion, the data space Occams inversion, the nonlinear conjugate gradient (NLCG) method, and the Gauss–Newton (GN) method are fundamental driving methods to find optimum earth models, and OCCAM, DASOCC, NLCG, and MT2DInvMatlab are possible candidates one can find in the public domain that implement these algorithms for 2-D MT inversions, respectively. In this study, we investigate the pros and cons (strength and weakness) of these codes to help one use them efficiently in practical works and, as an introductory guide, further develop (sophisticate or extend) them, especially for the 3-D case. To achieve this goal, we applied each one of the four aforementioned codes on a profile of real MT field dataset. Then, further investigations have been done by performing several inversion tests to see how each code can find the appropriate model to reconstruct the subsurface resistivity structure. Numerical experiments show that the two parameters, regularization and target misfit, in addition to the main criteria of inversion (such as the forward and the sensitivities calculation method, and the type of inversion algorithm), are very important to produce the expected model in inversion. The regularization parameter that acts to trade off between model norm and data misfit can affect the inversion process in terms of both the computational efficiency and the accuracy of the obtained model. Also, lack of insufficient precision to choose the target misfit can lead the inversion to produce and reach an incorrect model.

Magnetotelluric measurements along a reflection seismic profile: reprocessing and reinterpretation of MT data for crustal-scale electric resistivity structure in central Victoria, Australia

Magnetotelluric(MT) soundings were done in Central Victoria, Australia, to investigate the electrical conductivity structure of the Bendigo and Melbourne structural zones. We reprocess and reinterpret the MT data, which were acquired along the same path as the seismic transect in 2006 by GeoScience Victoria (GSV). Deep crustal faulting in this region is considered to be responsible for significant mineralization and magmatic processes. The resulting two dimensional (2D) MT conductivity model is consistent with the seismic interpretation in that it clearly shows known boundaries as well, as additionally reveals four clearly isolated resistors, which could be hardly imaged by reflection seismic. With the result previously published, these resistors can be interpreted as granite or granodiorite intruded in the Devonian, or alteration associated with fluid migration. The 2-D electric conductivity model also agrees with seismic interpretations, which shows that internal faults in the Bendigo Zone are thrust fault systems with listric geometries that are connected in the mid-crust, and clarifies that the Bendigo Zone and Melbourne Zone experienced different tectonic evolution during the Benambran Orogeny in the Silurian. The model also supports the conjecture that the Melbourne Zone formed as a foreland basin to the uplifted Bendigo Zone.

Case Study on a Low-enthalpy Geothermal Exploration in Pohang Area, Korea

ABSTRACT Korea Institute of Geoscience and Mineral Resources (KIGAM) launched a new project to develop the low-enthalpy geothermal water in the area showing high geothermal anomaly, north of Pohang city, for large-scale space heating from Korea Research Council of Public Science & Technology(KORP) funding. Surface geologic and geophysical surveys including Landsat TM image analysis, gravity, magnetic, magnetotelluric (MT) and controlled-source audio-frequency MT (CSAMT) and self-potential (SP) methods have been conducted and the possible fracture zone was found that would serve as deeply connected geothermal water conduit. By the end of 2003, two test wells of 1 km depth were drilled, various kinds of borehole survey along with additional MT measurements and sample analysis will follow and then the detailed subsurface condition is to be characterized. Next step would be drilling the production well of 2 km depth and all further steps remain to be determined depending upon the results of the test well studies.

Experiment and response analysis of marine monostatic EM instrument toward mapping seafloor sulfide massive deposit

We performed numerical and physical experiment with a marine monostatic electromagnetic (EM) survey system, which is developed primarily for exploration of seafloor mineral deposits. The system is a multi-frequency EM system housed to endure high pressure in deep sea up to 2000 m. To keep stable altitude of the system from the seafloor in rough seafloor topography condition, the system is connected rigidly to ROV (Remotely Operated Vehicle). We performed a test with the new loop EM system by keeping altitude of ROV 2 m above from the seafloor at a depth of 300 m sea water. Experimental results showed that electromagnetic anomaly was clearly identified at the location of metallic object, and the pattern of the EM responses matches well with the theoretical responses from a 3-D integral equation EM modeling code. With the off-shore test and the numerical analysis, we confirmed that the method of operation of the EM survey system by ROV is readily feasible, and the system are expected to detect actual seafloor sulfide massive deposits in the highly conductive seafloor environment.