Joint application of geophysical methods and Direct Push-soil gas surveys for the improved delineation of buried fault zones

This study provides an example of fault structure delineation using both geophysical measurements and soil-gas surveys. Seismic refraction and electrical resistivity tomography investigations were performed in combination with Direct Push (DP) soil gas concentration measurements, with the main objective being the characterization of an assumed permeable fault structure which is covered by sediments that are over 20 m thick. Geophysical methods were used to locate a potential fault zone and to provide an insight into the structural features of the covering sediments. Methods for quantifying the soil-gas concentration were applied to evaluate the permeability of the fault zone. The positioning of gas sampling points was based on results of a geophysical survey undertaken beforehand. Gas sampling was performed using DP-technology to obtain concentration profiles for the inert gas Radon-222 and its carrier gas CO2 along the profile at different depths. Joint interpretation of the spatial distribution of geogenic gases and results from the geophysical survey allowed us to produce a representative model image of the fault structure consisting of two fault branches. Based on this image, it was possible to interpret the observed gas concentration patterns.

► Geophysical information was essential for the positioning of gas sampling points.
► Soil-gas surveys at ground surface can be limited by atmospheric influences.
► Direct Push technology was necessary for the detection of the fault zone.
► Joint interpretation leads to complex characterization of a fault structure.