Investigation of non-linear behavior of hard rock using relative seismic velocity changes - a case study at the GERES array in Germany

Variations in strain/stress and fluid content can change seismic velocities in the subsurface. Monitoring velocity changes, e.g., using ambient seismic noise, may thus constrain these variations as well as the material elastic properties and their non-linear behaviour. We can test this capability by inspecting velocity changes from known effects, such as tides, temperature or atmospheric pressure affecting the upper crust. Here we present a workflow to use ambient seismic noise to derive the non-linear behaviour of hard rocks during the influence of tides, temperature and atmospheric pressure. We study one year of data from the GERES array in south Germany, which provides data to the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO). The seismological GERES array consists of 25 high-quality stations located in concrete vaults with depths between 3 and 5 meters. The aperture is 4 km. We estimate hourly Green’s function by cross-correlating ambient seismic noise recorded at pairs of stations. A Wiener filter increases the signal-to-noise ratio and stabilizes the hourly calculation of relative seismic velocity change in the 1-4 Hz frequency band. We compare different techniques to measure seismic velocity changes with high precision in time, frequency, and wavelet domain. The results indicate short and long term variations of the seismic velocities. This study aims to compare this non-linear behaviour of hard rocks with other geological settings used in earlier investigations.