Temporal variation of seismic surface-wave properties and impact on exploration and monitoring of geothermal resources

Ambient seismic noise surface waves are routinely used to explore subsurface velocities. In contrast to active seismic exploration, seismic noise methods are cost-efficient, sensitive to shear velocities that can indicate fluid content, and can be repeated to highlight temporal variations.

Three-component ambient noise beamforming (B3AM) uses three-component sensor arrays to determine dominant wave types and their properties in small time windows at individual frequencies. Thus, wavefield composition, wavenumber, propagation direction, and Rayleigh wave ellipticities can be stacked for all times or analysed consecutively. To interpret temporal variations in these properties, the variations of noise sources need to be isolated from variations in the subsurface. We aim to understand the sensitivity of seismic surface waves to subsurface changes by analysing an existing ten-month long dataset in the Lower Rhine Embayment, Germany, where during the recording time no significant subsurface changes can be anticipated.

Analysing the continuous timeseries with B3Am and stacking properties for a rolling five days, we found two prominent noise source directions active at different times during the year. Observing the surface wave properties over time jointly and separated by apparent noise sources revealed different scales of property variations. Small-scale sharp variations of a few days and long-term property changes over a few months were observed. Finally, dispersion curves show significant differences when observing different times of year but not when comparing different noise sources. This enables us to interpret the variations of surface-wave properties in the context of the local geological context and global noise source variations.