Multidimensional inversion of MT data from Alid geothermal area, Eritrea : comparison with geological structures and identification of a geothermal reservoir

Abstract

Electrical resistivity methods have been the most powerful tool in geothermal exploration for decades. Of these, MT and TEM are important methods in delineating geothermal resources. The main objective of this study is to understand the nature (dimensionality) of the MT data from Alid Geothermal area, Eritrea and apply multidimensional inversion to the data and compare the result with geological structures and the identification of a geothermal reservoir. In this project, 47 MT soundings from Alid geothermal area were used for 1D and 3D inversion as well as 2D along four profiles, for inversion. MT data have traditionally been inverted in 1D and 2D for several decades but with the advancement of modern computing techniques, 3D modelling and inversion is now becoming a more general method. 3D MT inversion has been used in various fields such as geothermal studies. For the purpose of static shift correction, in the 1D inversion, the MT data were jointly inverted with transient electromagnetic (TEM) data from the same location or close by. In the 2D inversion, the apparent resistivity for both the xy and yx modes were previously shift corrected prior to inversion. Whereas for the 3D inversion, shift correction was previously applied to the impedance tensor.

According to the dimensionality analysis, the dimensional indicators of most of the impedance tensors of Alid MT data indicate that the resistivity structure is highly three dimensional or at least of a strong non-1D nature for long periods, greater than 1 s. The resistivity distribution from the models of 3D inversion is comparable to the model results of 1D TEM/MT joint inversion at shallow depths down to about 1 km below the surface. At deeper levels the resulting models from 1D and 3D inversion are different. Four main resistivity structures are observed in most parts of the survey area mainly from 1D and 3D inversion: A shallow thin high resistivity layer followed by low resistivity. Below there is a high resistivity layer underlain by a deep laying conductor. Lithological contacts or lineaments are identified by the three inversion approaches. A sharp resistivity contact or fault line with an orientation of NE-SW is observed at the depth intervals of approximately 0.5-2 km. This fault line is clearly observed particularly from the modelling result of 3D inversion where the southern side of the fault line is downthrown.