Towards rockfall forecasting through observing deformations and listening to microseismic emissions

Reliable forecasting of rockfall is a challenging task, mainly because of the lack of clearly noticeable forerunners as well as due to the geological and geo-mechanical complexity of the rock movements involved. Conventional investigation devices still present some drawbacks, since most measurements are generally carried out at isolated locations as well as on the surface only. Novel remote-sensing monitoring instruments, such as Terrestrial Laser Scanning (TLS) and Ground-Based Interferometric Synthetic Aperture Radars (GB-InSAR), are capable of inspecting an unstable slope with a high spatial and temporal frequency. But they still rely on measurements of the failure surface, from which displacement or velocity are measured. On the contrary, acoustic emission/microseismic monitoring may provide a deeper insight of stress and strain conditions within the subsurface rock mass. In fact, the capability to detect microseismic events originating within an unstable rock mass is a key element in locating growing cracks and, as a consequence, in understanding the slide kinematics and triggering mechanisms of future collapses. Thus, a monitoring approach based on the combination of classical methodologies, remote sensing techniques and microseismic investigations would be a promising research field. In the present paper we discuss the technologies and we illustrate some experiments conducted in the framework of a project whose final goal is the installation of an integrated monitoring and alerting system on a rockface nearby Lecco (Italy). In particular, we present a review of performances and applications of remote sensing devices and some results concerning a terrestrial laser scanner preliminary campaign. Then, we report findings regarding amplitude, frequency content and rate of signals recorded during an in situ test carried out to evaluate the performance of three different microseismic transducers.