Detection and localization of acoustic signals from fluid ascent channels near CO2 degassing sites in the Cheb basin (West-Bohemia)

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The West Bohemia/Vogtland region is a place of presently ongoing geodynamic processes which result in the occurrence of repeated earthquake swarms, mantle-derived fluid exhalations, mofettes, among others. The PIER-ICDP (Probing of intra-continental magmatic activity: drilling the Eger Rift) initiative aims at establishing a scientific drilling program in that region to create an observatory at depth for the long-term study of earthquake swarms and related phenomena. Most of the seismicity is released at the Novy Kostel epicentral area at depths below 6 km, the main fluid escapement centers at the surface, however, are located in the Cheb basin, some 10 km to the south. Fluid migration paths in between are unknown. Gas flux at the surface is partly concentrated in wet or dry mofettes, but another part is more diffusive. Hypotheses are that fluids preferably migrate along existing faults in the crystalline basement and more diffusive flow takes place in the sedimentary cover of the Cheb basin. It is well known from volcanic and geothermal areas that fluid flow acts as permanent source of seismic vibrations. Detecting and locating these sources can therefore help to delineate more precisely the conduit paths of upwelling fluids and to pin down preferred places for drill holes as monitoring sites. We have successfully tested the Matched Field Processing (MFP) method to localize continuous seismic noise sources in the mofette fields of South Hartousov and Dolní Cástkov in the Cheb basin with the help of small seismic array deployments, and obtained source images within a subsurface volume of several 10 m edge length. Now, we will extend the data acquisition to more mofette sites, and will do repeated measurements at one site to search for time variations in the degassing strength and location of fluid pathways. We will also increase the aperture of the arrays to obtain mages of larger subsurface volumes. The data acquisition and analysis will be complemented by enhancements of the MFP method, and by numerical modelling to synthetically study the resolution power and uncertainties of the imaging process. The detailed imaging of fluid ascent paths down to several 100m depth, together with their correlation with seismic and conductivity structures as well as lithology will be of great benefit for the selection of drill sites for establishing a long-term monitoring laboratory within the fram of ICDP.