Near-surface conductivity structures of quaternary volcanic maars in the Western Bohemian Massif: 3D imaging using the Radio-Magnetotelluric method
Authors: Willkommen Gregor , Klanica Radek , Kováčiková Světlana , Mrlina Jan , Platz Anna , Weckmann Ute 
Published: March 2020

&lt;p&gt;As part of the Bohemian Massif, the Cheb Basin is one of the most active areas of the European Cenozoic Rift System. Separated from the ENE-WSW striking Eger Rift to the west by the morphological prominent Mari&amp;#225;nsk&amp;#233; L&amp;#225;zne Fault Zone (MLF), the basin shows presently no active volcanism at the surface. Nonetheless it is characterized by degassing of mantle derived CO&lt;sub&gt;2&lt;/sub&gt; in mofettes and mineral springs and by repeated occurrences of swarm earthquakes along the Poc&amp;#225;tky-Plesn&amp;#225; Zone (PPZ) and MLF near Nov&amp;#253; Kostel. All these activities are vivid signs of ongoing magmatic processes in the lithospheric mantle. Over the last 15 years four potential maar diatreme structures were discovered and join the two known scoria cones Komorn&amp;#237; hurka and Zelezn&amp;#225; hurka in the western part of the Cheb Basin. Unlike scoria cones there are no prominent morphological indications for maar diatreme structures, why only modern approaches in remote sensing and systematic gravimetrical surveys led to the discovery of the M&amp;#253;tina Maar in 2007 (Mrlina et. al., 2007), the Neualbenreuth Maar in 2017 (Rohrm&amp;#252;ller et. al., 2017) and recently the two potentials Ztracen&amp;#253; rybn&amp;#237;k maars close to Lib&amp;#225; (Hosek et. al., 2019; Mrlina et. al. 2019). All these quaternary volcanic structures are located very close along the Tachov Fault Zone (TFZ), one of the major NNW-SSE striking fault zones of the Bohemian Massif. Maar volcanoes were formed when rising magma interacts explosively with groundwater. Advancing explosions left a cone-shaped diatreme that has been filled with post-eruptive sediments which could conduce as a climate archive for the last 300.000 years in central Europe. An interdisciplinary Project &quot;Drilling the Eger Rift&quot; within the International Continental Scientific Drilling Program (ICDP) targets the interactions between fluids, deep biosphere, CO&lt;sub&gt;2&lt;/sub&gt; degassing and earthquake activity to shed light on the tectonic structure and related geodynamic processes. As a part of this project, Radio-Magnetotelluric (RMT) measurements were applied to image the near-surface electrical conductivity structure of these maar volcanoes. From May 2018 on, we conducted field experiments encompassing six 500 m RMT profiles across the Neualbenreuth maar, three 700 m profiles across M&amp;#253;tina Maar and finally eight 400 - 1200 m long profiles over both Ztracen&amp;#253; rybn&amp;#237;k maars. Compared with geo-electric resistivity tomography (ERT), our RMT measurements are more sensitive to conductors such as fluids or metallic compounds and were done with an areal coverage for 3D inversion and interpretation. With advanced and statistically robust data processing techniques typically applied to MT data resulted in impedance tensors in a period range of 10 kHz to 250 kHz. This RMT data sets are then modelled using inversion. The resulting 3D electrical conductivity models across the maar diatreme structures show distinct contrasts between the resistive rocks of the diatreme and the rather conductive post-eruptive sediments. The inversion results will be compared and discussed, in particular regarding a position for a potential core drilling in one of the maar structures.&amp;#160;&lt;/p&gt;