Abstract
We conducted an extensive study on the elastic properties of Opalinus Clay and the overlying and underlying rock formations, which range in the overall clay mineral content from nearly 0 wt.% to 60 wt.%. Our laboratory experiments focused on seismic and ultrasonic frequencies to determine the extent to which seismic dispersion affects elastic parameters and seismic wave velocities. The results comprise the static stiffness from undrained triaxial cycles (axial - confining stress: 8 - 10 MPa), the dynamic stiffness at seismic frequencies (0.5 - 143 Hz), intrinsic attenuation (0.5 - 20 Hz), compressional velocity measurements (0.5 - 2 Hz), and ultrasonic velocity measurements (250 and 500 kHz). We compared these laboratory results to in-situ sonic logging measurements to assess the role of frequency in measured elastic parameters. The results suggest a notable correlation between clay mineral content and dispersion. Specifically, high clay mineral content leads to increased dispersion, even within the seismic frequency band. The overall dispersion of P-wave velocity in the frequency range from 1 Hz to 500 kHz is up to 16%. This frequency dependency is crucial when establishing a relationship between sonic well log data and static stiffness for geomechanical modelling. The results are discussed with respect to possible dispersion mechanisms, including the role of bound water in clay.