Antony Butcher*1, Anna Stork1, J-Michael Kendall1, James Verdon1
1School of Earth Sciences, University of Bristol, UK. (firstname.lastname@example.org).
Seismic methods provide important insitu measurements of the rock mass strength, which can be assessed through variations in seismic velocities or the detection of picoseismic events relating to fracture development. The frequency range in which seismic instruments operate dictates numerous key elements of a survey (e.g.detectability limits and resolution), and for engineering applications it is often advantageous to operate in kHz frequency range. Here we present two datasets from ground excavations at Hinkley Point C, which were acquired using a piezoelectric seismic system with a very high 1MHz sampling rate.
Velocities are directly relatable to engineering parameters (e.g. UCS & shear strength), but as they are bulk measurements their resolution is dependent on the frequency of the recorded wavefield. Increasing the frequency content through the use of piezoelectric instruments results in an improvement in resolution, and we demonstrate this through the use of crosshole survey dataset. We also show that due to the highly repeatable nature of the piezoelectric source, these instruments allow for accurate estimates of attenuation using spectral ratio methods.
Fracture development generates picoseismicity due to disturbance of a rock mass through mechanical excavation or stress release. These very small seismic events exhibit high frequency content (1-100kHz), which occur within the sensitivity range of these piezoelectric sensors. We present a dataset of pico-seismic events recorded within a rock mass after completion of slope excavations, and show the appropriateness of this system for these types of monitoring applications.