Stable 3D Inversion of MT Data and its Application to Geothermal Exploration

Abstract

A stable inversion technique has been developed for threedimensional (3D) interpretation of magnetotelluric (MT) data. The inversion method was based on the Gauss-Newton (linearised least-squares) method with smoothness regularisation. Static shifts were also treated as unknown parameters in the inversion. The forward modelling was done by using the staggered-grid finite difference method. The Akaike Bayesian information criterion (ABIC) was applied to the search for the optimum trade-off parameters while the minimising the data misfit, model roughness, and static shifts. The inversion method was then applied to a large volume MT dataset obtained in the Ogiri geothermal area, southwestern Japan. The inversion was performed stably, resulting in a good fit between the observed and computed apparent resistivities and phases. The 3D resistivity structure recovered was generally similar to that obtained by a two-dimensional (2D) inversion in previous work, although the deeper portion of the 3D model seemed to be more realistic than that of the 2D one. The 3D model was also in a good agreement with the geological interpretation of the geothermal reservoirs. These results indicated the necessity for 3D interpretation in geothermal exploration and in other applications in complicated geological environments.

Three‐dimensional resistivity structure and magma plumbing system of the Kirishima Volcanoes as inferred from broadband magnetotelluric data

“…Hydrothermally altered minerals (especially smectite) show a significantly low resistivity, and this has been considered an important cause of the low resistivity within active volcanoes [e.g., Jones and Dumas, 1993;Ogawa et al, 1998;Nurhasan et al, 2006;Coppo et al, 2008;Aizawa et al, 2009a;Yamaya et al, 2009;Garcia and Jones, 2010;Kanda et al, 2010;Siniscalchi et al, 2012]. By taking into account bore data at Ogiri geothermal field [Uchida and Sasaki, 2006], the widespread, shallow (a few kilometers deep) conductors are interpreted as a zone of hydrothermal alteration whose base corresponds to the 200°C isotherm. Conversely, the alteration in the subvertical conductor is difficult to evaluate because smectite breaks down at temperatures above 200°C.…”

La Palma island (Spain) geothermal system revealed by 3D magnetotelluric data inversion

“…Such a composition in the water can be related to groundwater circulation: due to the dikes' geometry the circulating meteoric water follows the main structural N-S rift-zone of Cumbre Vieja, from the recharge area towards the south of the island, experiencing heating and contamination due to the presence of volcanic gases such as CO 2 , SO 2 and H 2 S. The use of MT to map the internal structure of volcanic/geothermal areas is well documented, both for the assessment of the geothermal potential and the connected hydrothermal circulation, and for structural investigation and magmatic reservoir characterization 1,18 . In particular, a 3D resistivity model of the subsurface of a volcanic-geothermal area surveyed by MT can reveal the presence and the shape of such structures [19][20][21][22][23][24][25][26][27][28][29][30] . Volcanic geothermal areas are usually characterized by a low resistivity clay-cap layer associated with the presence of smectite (< 10 Ω⋅m) and illite-smectite (up to a few tens of Ω⋅m) overlying a resistive zone where the reservoir is located, and such a sharp contrast can be easily detected by the application of MT 1 .…”

Three-dimensional geologic mapping to assess geothermal potential: examples from Nevada and Oregon

“…Where available, magnetotelluric data and other tools that measure relative electrical resistivity/conductivity can be incorporated into the geologic interpretation. High conductivity < 10 Ω-m) in 2D or 3D MT inversions can be caused by circulating fluids and intense smectite clay alteration, indicating the presence of the 'clay cap, ' a diagnostic feature of some geothermal reservoirs (Uchida and Sasaki 2006;Cumming and Mackie 2007;Newman et al 2008;Peacock et al 2012;Munoz 2014). High conductivities and abundant clays are also common in many basin sedimentary sequences and are not related to geothermal processes.…”