Image-D7.04: Resistivity Techniques. Electromagnetic Soundings In The Litomerice Geothermal Area (Czech Republic)
Creators: name: Wawrzyniak, Pierre
Publisher: name: Zenodo
Year: 2016
Subjects: geothermal energy
exploration
resistivity
czech republic
litomerice geothermal area
electromagnetic soundings

description: Within the frame of the FP7 European Project IMAGE, a special emphasis was put on improving and testing exploration techniques (WP7) in peri-urban environments, especially geophysical methods like seismic, gravimetry, magnetics and electromagnetics. In this report, we will focus on electromagnetic techniques for resistivity mapping. 

Magnetotelluric (MT) method is widely used for exploration in geothermal areas. The challenge is that such a passive method is severely affected by electromagnetic (EM) noise induced by anthropogenic activities such as cities and industries. In particular, MT exploration is almost not feasible in countries where trains are powered by DC-current (such as in Czech Republic and some other regions in Europe). As an alternative, we proposed to deploy active source EM techniques (Controlled Source EM) using surface and borehole high power emission to improve the EM signal to noise ratios and allow mapping resistivity variations at the depth of the geothermal targets. 

To demonstrate the feasibility of active and passive electromagnetic measurements for geothermal exploration, a geophysical survey has been performed in the area of Litoměřice (Czech Republic) where a geothermal exploration borehole (2 km deep) has been drilled in the Bohemian massif. Measurements have been performed from 6 to 11th September 2014 by BRGM and Geomedia staff, with the logistical help of Litoměřice city. The survey aims at measuring electrical resistivity variations at the depth of the alleged geothermal reservoir around ~2 km. For that purpose, a magnetotelluric (MT) profile has been performed at the same as a Controlled Source EM (CSEM) survey. To improve the MT signal quality, robust MT signal processing has been developed and applied using synchronous recordings performed locally (30 km) and at far (&gt; 500 km) remote stations. To maximize the CSEM signal to noise ratio, a Long Electrode Mise-à-la-Masse (LEMAM) setup has been successfully deployed. 

Analysis of the MT data shows that despite the use of a remote reference, MT data are corrupted at low frequency (&lt;5Hz) by a permanent anthropic noise source. At higher frequencies, comparison with CSEM and borehole resistivity data shows that reliable resistivity measurements have been obtained. On the other hand, processing of the CSEM data shows the impact of industrial noise on CSEM data is minimal and only manifests itself at high frequencies (&gt;32Hz). Modelling and inversion of such data shows that we can retrieve realistic resistivity measurements up to a depth of a few kilometers i.e. deep enough to image the geothermal reservoir of interest. 

As a way forward in the next Work Package (WP8.2 application of exploration techniques) and as soon as the level of noise of an EM survey is believed to be an issue (e.g. due to the proximity of a city, train, high power line etc), we strongly recommend to compliment passive MT measurements with active CSEM measurements to image resistivity variations at reservoir depth. This can be done with single or double LEMAM setups but also with conventional surface only setups – a feasibility study is required to define on a case by case basis the appropriate approach.
description: FP7