| Title | Imaging Faults Within Geothermal Systems from 3-D MT Inversion: a Comparison Between Smooth and Blocky 3-D Approaches |
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| Authors | Flora SOLON, Sophie HAUTOT, Pascal TARITS |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | 3D inversion, magnetotelluric imaging, geothermal systems, faults |
| Abstract | Because geothermal systems produce strong variations in underground electrical conductivity, electromagnetic methods such as magnetotellurics (MT) have become commonly used in geothermal reservoir exploration. Over the last years, several imaging methods for MT data have emerged. These methods can use different mathematical approaches (finite differences, finite elements or integral equations) to provide an estimate of the 3D resistivity distribution within the subsurface. Here we use field data sets to study the capacity of two end members 3D MT imaging methods to characterize some key features of geothermal systems. The two methods we applied have been developed to obtain solutions using different ways of parameterizing the subsurface, regularization and inversion strategies. The main difference relies in the relationship between constraints and unknowns, or simply, data and parameters. They have been characterized as over-determined inverse problem (ODIP) in which the number of data is larger than the number of parameters, and under-determined inverse problem (UDIP) when the number of parameters is larger than the number of data. We demonstrate that the resulting images obtained by both methods can describe the main structures of the geothermal system such as the clay cap, heat source and faults system. Differences lay in the resolution power of the different techniques. UDIP models tend to generate shallow features (bubble like) and to produce smooth distributions at depth. The UDIP inversion method gives good results for areas where the subsurface resistivity changes in a gradual manner. This is related to the large number of parameters used and the mesh size, which needs to be small. In contrast ODIP models present stronger and continuous conductors and resistors. The model features tend to be blocky but the ODIP inversion method gives significantly better results where there are sharp boundaries. It is able to recover the main strikes of the geothermal fault system. The horizontal resolution of both methods can be limited due to site spacing however the obtained results from each of the approaches present stable solutions, fit to the data and general agreement. |