Analysing the subsurface resistivity structure on two profiles across the Námafjall high-temperature geothermal field, NE-Iceland, through 1D joint inversion of TEM and MT data

Abstract

Resistivity is one of the most variable physical property of materials and has proven to be most useful in the search for geothermal resources. To understand the geothermal significance of the distribution of resistivity, a review of the parameters affecting resistivity in geothermal systems was conducted in this work. The conductive clay products of hydrothermal alteration are the most common cause of low resistivity in the zone above the reservoir. Correlations between alteration type and resistivity can extend further to enable better predictions of the reservoir temperature distribution from surface geophysical measurements. Transient electromagnetics (TEM) are used for measuring shallow structures and Magnetotellurics (MT) are used for probing deeper. The joint inversion of TEM and MT data has proven to be useful in solving the static shift problem resulting from near surface resistivity inhomogeneities. The MT apparent resistivity can be affected especially in volcanic areas and shifted by a multiplicative factor which is frequency independent. The Námafjall high-temperature geothermal field, marked by a lot of geothermal surface manifestations, shows conductive bodies on the two profiles, North-South and West-East, that are perpendicular to one another. The results of the resistivity analysis were compared with temperature data logs from the boreholes drilled in the Námafjall high-temperature geothermal field, as well as with the corresponding mineral alteration. Good agreement was found between the resistivity layering and the alteration mineralogy. Below 5 km depth a conductive zone was found, which could be connected with the heat source.