2D joint inversion of dc and scalar audio-magnetotelluric data in the evaluation of low enthalpy geothermal fields

• Published in: Journal of geophysics and engineering Vol. 4; no. 1; pp. 53 - 62
• Main Authors: Monteiro Santos, Fernando A, Afonso, António R Andrade, Dupis, André
• Format: Journal Article
• Language: English
• Published: IOP Publishing 27-03-2007
• ISSN: 1742-2140; 1742-2132; 1742-2140
• DOI: 10.1088/1742-2132/4/1/007

Audio-magnetotelluric (AMT) and resistivity (dc) surveys are often used in environmental, hydrological and geothermal evaluation. The separate interpretation of those geophysical data sets assuming two-dimensional models frequently produces ambiguous results. The joint inversion of AMT and dc data is advocated by several authors as an efficient method for reducing the ambiguity inherent to each of those methods. This paper presents results obtained from the two-dimensional joint inversion of dipole-dipole and scalar AMT data acquired in a low enthalpy geothermal field situated in a graben. The joint inverted models show a better definition of shallow and deep structures. The results show that the extension of the benefits using joint inversion depends on the number and spacing of the AMT sites. The models obtained from experimental data display a low resistivity zone ( < 20 W m) in the central part of the graben that was correlated with the geothermal reservoir. The resistivity distribution models were used to estimate the distribution of the porosity in the geothermal reservoir applying two different approaches and considering the clay minerals effect. The results suggest that the maximum porosity of the reservoir is not uniform and might be in the range of 12% to 24%.