Title: Multidimensional inversion of MT data from Krýsuvík high-temperature geothermal field, SW-Iceland, and study of how 1D and 2D inversion can reproduce a given 2D/3D resistivity structure using synthetic MT data

Abstract

Electromagnetic (EM) methods are frequently used in the exploration of geothermal resources for
determining the spatial distribution of electrical conductivity. Of the various EM methods,
magnetotelluric (MT) method was found to be the most effective in defining a conductive reservoir at
a depth exceeding 1 km overlain by a larger and more conductive clay cap.
The two main objectives of this study are: firstly to explore how 1D and 2D inversion can reproduce a
given 2D and 3D resistivity structure using synthetic MT data and secondly to investigate the
subsurface resistivity distribution of Krýsuvík high-temperature geothermal field in SW-Iceland, using
multidimensional inversion of MT data. The 2D and 3D synthetic models considered in the study were
better reproduced by 1D inversion of the determinant of impedance tensor and 2D inversion of TM
mode data than by 2D inversion of TE mode and combined TE and TM mode data. 1D and 2D
inversion of MT data along two profiles and 3D inversion of 58 MT sites were performed in Krýsuvík
area. In the 1D inversion of MT data, transient electromagnetic (TEM) data from the same location as
the MT soundings are jointly inverted in order to correct the static shift in the MT data. The 2D and
3D inversion of MT data were performed on previously shift corrected MT data. The full impedance
tensor elements were used in the 3D inversion of MT data. The Atlantic Ocean was included as a fixed
feature in the 2D and 3D inversion to account for the effect of the highly conductive ocean on MT
measurements. The 1D and 2D inversion of MT data from Krýsuvík high-temperature geothermal
field revealed three main resistivity structures down to a depth of 10 km: a high-resistivity surface
layer underlain by conductive layer followed by high resistivity. The 3D inversion confirmed this
main result. In addition, the 3D inversion model showed a low-resistivity zone trending ENE-WEW at
a depth of about 1.35 km which has the same direction as transform faults inferred from seismicity.
The nature of this low resistivity which is overlain by high resistivity is not well known. The 3D
inversion, moreover, revealed a deep conductive body embedded at a depth of about 2 km and
reaching a depth of 5 km within the high resistivity. It is located in the central part of Krýsuvík area
between the two hyaloclastites ridges and is about 10 km2 in horizontal dimension. The deep
conductive body is presumably associated with the heat source of the geothermal system.