Characterizing shallow fault zones by integrating profile, borehole and array measurements of seismic data and distributed acoustic sensing

Abstract

Within the framework of the Intercontinental Scientific Drilling Programme (ICDP) ‘Drilling the Eger Rift’ project, five boreholes were drilled in the Vogtland (Germany) and West Bohemia (Czech Republic) regions. Three of them will be used to install high-frequency three-dimensional (3D) seismic arrays. The pilot 3D array is located 1.5 km south of Landwüst (Vogtland). The borehole, with a depth of 402 m, was equipped with eight geophones and a fibre optic cable behind the casing used for distributed acoustic sensing (DAS) measurements. The borehole is surrounded by a surface array consisting of 12 seismic stations with an aperture of 400 m. During drilling, a highly fractured zone was detected between 90 m and 165 m depth and interpreted as a possible fault zone. To characterize the fault zone, two vertical seismic profiling (VSP) experiments with drop weight sources at the surface were conducted. The aim of the VSP experiments was to estimate a local 3D seismic velocity tomography including the imaging of the steep fault zone. Our 3D tomography indicates P-wave velocities between 1500 m/s and 3000 m/s at shallow depths (0–20 m) and higher P-wave velocities of up to 5000 m/s at greater depths. In addition, the results suggest a NW–SE striking low-velocity zone (LVZ; characterized by (Formula presented.) = 1500–3000 m/s), which crosses the borehole at a depth of about 90–165 m. This LVZ is inferred to be a shallow non-tectonic, steep fault zone with a dip angle of about (Formula presented.). The depth and width of the fault zone are supported by logging data as electrical conductivity, core recovery and changes in lithology. In this study, we present an example to test and verify 3D tomography and imaging approaches of shallow non-tectonic fault zones based on active seismic experiments using simple surface drop weights as sources and borehole chains as well as borehole DAS behind casing as sensors, complemented by seismic stand-alone surface arrays.