Sensitivity of magnetotelluric data to rhyolite magma encountered by the IDDP-1 well at the Krafla geothermal field, Iceland

Abstract:

Supercritical fluids beneath geothermal reservoirs are attractive for energy production due to their relatively high enthalpy. The Iceland Deep Drilling Project (IDDP) has drilled two wells to reach supercritical fluids beneath two geothermal fields: the IDDP-1 well at Krafla in 2009, and the IDDP-2 well at Reykjanes in 2017. While the IDDP-2 well successfully reached supercritical conditions at 4.5 km depth, the drilling of IDDP-1 ended prematurely at subcritical conditions when rhyolite magma was intersected at 2.1 km depth. Although extensive geophysical surveys have been performed at Krafla, the magma body intersected by IDDP-1 had not been predicted by geophysical methods. Petrological analyses suggest that the rhyolite magma at Krafla formed from re-melting of hydrothermally altered basalt. However, the geometry and distribution of the rhyolite magma remains uncertain.

A new 3-D inversion of the Krafla MT dataset was performed to obtain a high resolution resistivity model of the geothermal field. The resistivity model contains a low resistivity feature beneath the geothermal field which could correspond to partial melt or dehydration of chlorite/epidote alteration products. The top of this conductor occurs near the bottom of IDDP-1, making it unclear whether the MT data are sensitive to rhyolite magma bodies at the bottom of IDDP-1. Therefore, the sensitivity of the MT data to hypothetical rhyolite magma bodies with a range of resistivities and geometries was evaluated.

The sensitivity tests were conducted by calculating the MT data before and after the magma bodies were added to the resistivity model at the bottom of IDDP-1. Two methods were used to quantify the MT data sensitivity: the change in root-mean-square (r.m.s.) misfit and the p-value returned by the Kolmogorov-Smirnov hypothesis test. The sensitivity tests show that the MT data are only sensitive to unrealistically large and/or unrealistically conductive magma bodies at the bottom of IDDP-1.

The sensitivity tests provide an estimate of the size and resistivity of magma bodies that could exist below the Krafla geothermal field. This study also highlights the importance of understanding the sensitivity of MT data to the deeper parts of resistivity models during interpretation.