Abstract

Chile is the third country with the largest number of tailings dams, some of which are the largest in the world. Currently, there are 740 deposits throughout the country, of which 101 are active, 469 are inactive and 170 are abandoned without known owner and information. Although Chile is characterized by adverse geological and climatic conditions, most of their current monitoring use limited technologies, with discrete data capture, a low level of technological innovation and in many cases the parameters monitored are insufficient to ensure physical stability. In the last decade, ambient noise tomography has been used to monitor the time-dependent behavior of volcanoes, underground mining, geothermal fields, active fault zones and characterization of the seismic site response. Recent laboratory experiments have shown that this method is capable of detecting internal erosion in rock dams, one of the main failure mechanisms along with slope instability and overtopping. We run a very short experiment at an inactive tailings deposit, surrounded by high density of population, where we deployed nine short period (4.5 Hz), three component, continuous (200 sps) recording seismic stations, that were in operation during 3 hours. The size of the inactive deposit is 500x400 m2 and the average distance between the stations was about 100 m. Empirical Green’s Functions (EGFs) for selected pairs of stations at different frequency bands were obtained from time-domain cross-correlation of the extended seismic noise records. The EGFs were used to estimate phase velocity dispersion curves and shear wave velocity profiles. The resolution of the method allows solving intermediate depths associated to periods from 0.2 to 1.6 s, a critical period range for this type of infrastructure. The results show average shear wave velocities between 500 to 1,400 m/s for depths larger than 20 m and velocities around 400 m/s for the tailings materials in the central area that are consistent with single-station H/V spectral ratios. This study is a first attempt to show the possibility to monitor the sub-surface condition of a tailings dam using relatively inexpensive microseismic monitoring stations.