Modelling the sensitivity of magnetotelluric monitoring data to geothermal fluids at depth in Northern Alsace

Abstract

Magnetotelluric (MT) is classically used in geophysical exploration for imaging electrical conductivity structures and is recently developing as a monitoring technique. In geothermic during fluid injections and/or EGS stimulation experiments, MT is used in addition to microseismic observations and can provide critical information to geothermal fluid flows because the electrical conductivity is related with temperature, porosity, water content and minerals of rocks. Some experiments have shown that such MT signals might difficult to be observed because they are at periods of 1-10 s, within the so-called MT deadband. We consider the sensitivity of MT monitoring by forward modelling. We use ModEM open source code to build a 3-dimensional model of the northern Alsace, which includes topography and simple resistive layers inspired from geology. Modelling allows us to simulate different changes that could be caused by brine and/or acid injection within fractures at depth and show subsequent MT monitoring parameters with differences of the impedance response function (apparent resistivity and phase), phase tensor, and possibly magnetic transfer function (tipper vectors). From these models, it seems that MT monitoring in a sedimentary environment at 20 W.m could be sensitive to an increase of onductivity in a fault area at geothermal depths of 2-3 km if the size of the disturbed domain reaches about 10x0.3x2 km3.