D7.5: Report on joint site experiments at Las Cruses

Brief description :

Executive Summary: The passive seismic survey of the La Cruces mine site was initiated during discussion between partners of the PACIFIC and INFACT H2020 projects in December 2018. The initial design for the deployment covered a large area, about 7 x 4 km extending to the north and south of the mine but this was reduced to a smaller tighter 2 x 1 km array in February 2019. A collapse of the northside of the open pit then eliminated the possibility of placing nodes to the west of the pit and this resulted in an even smaller array. Data treatment proved to be very difficult for several reasons. The array was smaller than originally planned, but more importantly a significant proportion of the nodes, about 30%, were placed in the pit. The large differences in elevation between adjacent nodes and the differences in orientation of pit walls and terraces introduced unanticipated difficulties in processing the seismic data. 

We used 33 days of passive seismic records to retrieve the fundamental mode of Rayleigh waves propagating in the subsurface. We mostly used man-made ambient noise generated in the vicinity of the mine in the period band [0.3 - 1.5] s. Strong anthropogenic noise in the middle of the array forced us to use one-bit normalization and very intense pre-processing to retrieve usable cross-correlation signals. We were able to pick individual group and phase velocity dispersion curves from correlations computed between the majority of sensor pairs for stations separated by less than 2 km. We retained about 15% of all possible dispersion curves after a thorough quality check based on expert visual inspection. The aperture of the array and the frequency content of the noise allowed us to invert a velocity model down to 500 m depth. Long offsets are mainly discarded inducing a poor coverage of the central part of the pit. A high velocity anomaly beneath the northern part of the site where topography is not a problem and where the array is denser can be resolved and could correspond to the massive sulphide ore body at depth. The depth of cover in the north-eastern part of the study area is well represented by the iso-velocity surface of 750 m/s.

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