Passive seismic imaging of the orthomagmatic ore deposits using regional earthquake interferometry: a case study of the Akanvaara V-Cr-PGE deposit in Northern Finland

This paper presents a method for studying the structure of the subsurface in hard rock terrains by interferometry of the Pn-wave coda of the regional seismic events. Using theoretical background and application examples of the teleseismic coda wave interferometry and the teleseismic receiver function methods, we developed data processing and interpretation schemes that convert the Pn coda, recorded in local high resolution passive seismic experiments, into body wave part of the empirical Green’s tensor (EGT), i.e. an impulse response of the medium to a virtual seismic source placed either on the ground surface or underground. Based on the general theoretical considerations, we assume that the different components of the EGT evaluated from the Pn coda include body wave arrivals which are reflected and converted on the structural boundaries having different elastic properties. The algorithm was tested with real data recorded within high-resolution passive seismic experiments with 771 three-component seismic instruments in the “Volfram-Chrome-Platinum Group Elements” (V-Cr-PGE) orthomagmatic deposit of the Akanvaara ultramafic intrusion in Northern Finland. From the evaluated EGT it was possible to identify numerous signatures of converted and reflected waves. To interpret these arrivals, we conducted a numerical simulation of a plane wave interaction with a synthetic model of the deposit and found that the arrivals correlate not only with the known structural features of the deposit, but with unknown ones as well. Our study shows that interferometry of the regional Pn-wave coda can be an effective tool for mineral exploration both in greenfield and brownfield cases.