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

西波希米亚/沃格特兰地区是目前正在进行的地球动力学过程的地方，这些过程导致重复的地震群、地幔衍生的流体喷发、莫夫岩等。PIER-ICDP(探测大陆内岩浆活动：钻探埃格裂谷)倡议旨在在该地区建立一个科学钻探计划，以建立一个对地震群和相关现象进行长期研究的深层观测站。大部分地震活动在深度小于6公里的Novy Kstel震中区释放，但地表主要的流体俘获中心位于切布盆地，向南约10公里。两者之间的流体迁移路径尚不清楚。表面的气体通量部分集中在湿的或干的莫夫石中，但另一部分更具扩散性。假设流体最好沿着结晶基底中的现有断层运移，而在切布盆地的沉积盖层中发生更多的扩散流。众所周知，在火山和地热区，流体流动是地震振动的永久来源。因此，探测和定位这些来源可以帮助更准确地划定上升流体的管道路径，并确定钻孔的首选地点作为监测点。我们成功地试验了匹配场处理(MFP)方法，借助于小型地震台阵部署，在Cheb盆地的南哈图索夫和DolníCástkov的Mofette场中定位了连续地震噪声源，并获得了边缘长度为10m的地下体积内的震源图像。现在，我们将把数据采集扩展到更多的莫菲特地点，并在一个地点进行重复测量，以寻找脱气强度和流体通道位置的时间变化。我们还将增加阵列的孔径，以获得更大的地下体积的图像。数据采集和分析将得到MFP方法的改进，并通过数值模拟来综合研究成像过程的分辨率和不确定性。对深达100米的流体上升路径的详细成像，以及它们与地震和电导结构以及岩性的对比，将对在ICDP框架内建立长期监测实验室的钻探地点的选择大有裨益。

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.