Indirect Electromagnetic Geothermometer: Methodology and Case Study

Abstract

Studies were carried out to determine the feasibility of indirect temperature estimation in the Earth’s interior from electromagnetic data collected at the surface. Basing on the neuronet analysis of magnetotelluric (MT) and temperature data measured at the Bishkek geodynamical testing ground in northern Tien Shan, optimal methodologies for calibration and application of indirect electromagnetic geothermometers were developed. It is shown that temperature estimation by means of the EM geothermometer calibrated by 6-8 temperature logs results in 12% average relative error (instead of the 30% error achieved using only temperature logs). Availability of prior geological information about the region under study makes it possible to reject inappropriate site locations. This may, in turn, decrease average error by at least 3%.
The results of the electromagnetic temperature extrapolation at depth indicate that the extrapolation accuracy essentially depends on the ratio of the well depth to the extrapolation depth. In particular, when extrapolating to a depth twice as large as the well depth, the relative error is 5-6%, and if the extrapolation depth is three times as large, the error is around 20%. This makes it possible to significantly increase the depth of indirect temperature estimation in the Earth’s interior (in particular, for geothermal exploration) based on available temperature logs.