Frequency-dependent seismic attenuation and velocity dispersion in crystalline rocks: insights from the Grimsel test site

SUMMARY Characterizing the hydraulic and geomechanical behaviour of crystalline rocks is of importance for a wide range of geological and engineering applications. Geophysical methods, in general, and seismic techniques, in particular, are extensively used for these purposes due to their cost-effective and non-invasive nature. In this study, we combine legacy seismic observations to analyse the seismic attenuation and velocity characteristics in macroscopically intact regions of the granodiorite hosting the underground Grimsel test site in the central Swiss Alps across a wide frequency range. By focusing on data from the intact rock volumes we aim to assess the importance of viscoelastic effects in the crystalline host rock. Our results show consistent frequency-dependent characteristics of the seismic velocity and attenuation. We illustrate that it is possible to fit a microcrack-related wave-induced fluid flow (WIFF) model to the data over the entire frequency spectrum under examination extending from the Hertz to the Megahertz range. Utilizing complementary pressure-dependent ultrasonic measurements, we infer microcrack properties that validate the key parameters of the proposed WIFF model. These findings deepen our understanding of dispersion and attenuation mechanisms at the microscopic scale in crystalline environments, which is critical for a coherent analysis and integration of data from different seismic techniques as well as for the identification of dispersion and attenuation effects related to macroscale heterogeneities, such as fractures and faults.