Detection and localization of acoustic signals from fluid ascent channels near CO2 degassing sites in the Cheb basin (West-Bohemia)
Geophysics
Final Report Abstract
The NW Bohemia/Vogtland region (Czech Republic) is an intracontinental and geodynamically active area, which is characterized by the occurence of different natural phenomena like earthquake swarms at midcrustal depths and CO2 exhalations at the surface. One of the major goals of the ICDP Eger Rift initiative is to identify the pathways that allow efficient and permanent fluid transport within the crust and to study the mechanisms of fluid transport as well as fluid-rock interactions. At the Earth's surface, the fluid exhalations become visible, e.g. around the village Hartoušov, and appear as mofettes: little sinks characterized by degassing of highly concentrated CO2. Here we aimed to detect supposed feeding channels beneath these mofettes, to image their structure and depth range, as well as to monitor their spatio-temporal activity. This is done by passive seismic experiments where we use array methods to locate the sources of incoherent noise signals that are generated by fluid flow activity. Multiple dense temporary arrays consisting of 25 to 100 randomly distributed stations (70 to 600m diameter) were installed in the Hartoušov Mofette Field. They recorded continuous seismic data during several hours in 2016. With Matched Field Processing (MFP), two distinct fluid channels could be located beneath the test site and traced down to a common source area in up to 2000m depth. The locations could be linked to particular mofettes at the surface, thus confirming the MFP analysis. Additionally, the spatial and temporal variability of fluid flow activity was monitored over several hours. Due to the small array extent compared to the target depth, MFP resolution at greater depth decreases. Also the acoustic approximation inherent in the method and the assumption of a homogeneous velocity model limit the resolution However, Finite Difference Modelling and inversion of the synthetic data sets for similar setups supports the validity of our results. This work serves as a base for the future execution of the ICDP Eger Rift project, especially for the observation of spatio-temporal development of fluid flow and fluid ascent channels with stationary arrays.
Publications
- Locating mofettes using seismic noise records from small dense arrays and matched field processing analysis in the NW Bohemia/Vogtland Region, Czech Republic. Near Surface Geophysics, 14:1-9, 2016
H. Flores-Estrella, J. Umlauft, A. Schmidt and M. Korn
(See online at https://doi.org/10.3997/1873-0604.2016024) - Drilling into an active mofette: pilot-hole study of the impact of CO2-rich mantle-derived fluids on the geobio interaction in the western Eger Rift (Czech Republic). Scientific Drilling, 23:13-27, 2017
R. Bussert, H. Kämpf, C. Flechsig, K. Hesse, T. Nickschick, Q. Liu, J. Umlauft, T. Vylita, D. Wagner, T. Wonik, H. Flores-Estrella and M. Alawi
(See online at https://doi.org/10.5194/sd-23-13-2017) - 3D Noise Source Location using Matched Field Processing. PhD thesis, Faculty of Physics and Geosciences, Leipzig University, July 2018
Umlauft, J.
