Locating mofettes using seismic noise records from small dense arrays and Matched Field Processing Analysis in the NW Bohemia/Vogtland Region, Czech Republic

Abstract:

The NW Bohemia/Vogtland region is characterized by currently ongoing geodynamic processes within the intracontinental lithospheric mantle. Among others, this activity results in the occurrence of mid-crustal earthquake swarms as well as CO₂ degassing zones, called mofettes. These two natural phenomena are related to each other since it is considered that fluid flow and fluid-induced effective stress trigger earthquake swarms. At the Earth’s surface they appear spatially separated, but their connection could be explained by the existence of pathways within the crust that allow efficient and permanent fluid transport. However, neither structure nor position of such pathways has so far been imaged.

With this background we applied the Matched Field Processing (MFP) analysis within the NW Bohemia/Vogtland region to locate mofettes and investigate their characteristics. Considering the CO₂ degassing process as high frequency noise source, we chose two different test sites: the Dolní Částkov Borehole, which is an artificial mofette that we used to validate the method’s functionality, and the South Hartoušov Mofette, a natural CO₂ degassing field. On both sites, we measured seismic noise in continuous mode over several hours (7-9 hours) with a sampling frequency of 250 samples per second in multiple campaigns using an array of approximately 30 randomly distributed stations. Each array covered an area of about 60 x 60 m² and consisted of vertical geophones (4.5 Hz) connected to Reftek Texans.

For the MFP computation the phase velocity of the study area is required, which we obtained from active seismic experiments with hammer blows as source. For Dolní Částkov the phase velocity varies between 200-420 m/s; and in South Hartoušov between 100-280 m/s, both in a frequency interval of 10-60 Hz.

With the MFP analysis at the artificial mofette in Dolní Částkov we could relocate the noise source successfully and hence, the method’s functionality was confirmed. In the South Hartoušov mofette field, we detected one dominant noise source, probably a fluid pathway, as well as small MFP maxima at the surface that can be related to dry mofettes.