Optimization of a land-based non-invasive passive seismic approach for the exploration of deep-seated critical raw materials in the North of Spain

A sustainable supply of critical and strategic raw materials, like copper, cobalt, lithium, or fluorite, amongst others, is a critical prerequisite for the decarbonisation of the economy and the successful implementation of the green transition. However, Europe currently lacks sufficient knowledge, exploration activity, and domestic supply of these commodities. To overcome these limitations, the DEXPLORE project aims at developing surface to subsurface sustainable cost-effective geological and geophysical techniques for mineral exploration using three pilot zones in Spain and Estonia. Within the framework of this project, we present an innovative non-invasive passive seismic exploration approach and a field test conducted to optimize acquisition and processing parameters. The main objective was to achieve sufficient resolution at prospect depths of 500-1000 m, enabling the identification of ore-associated geological structures in one of the pilot zones, corresponding to the Villabona Fluorite deposit in Asturias (northern Iberian Peninsula, Spain).   

The passive seismic methodology relies on the recording and processing of ambient seismic noise acquired by seismic nodes. We designed a preliminary configuration and workflow based on an extensive review of the passive seismic method to run a five-day small-scale field test in the Minersa Pilot Zone, located in the central part of Asturias (N Spain). In this area, the currently active Villabona Mine produces fluorite hosted by Mesozoic sediments affected by extensional faults on an epigenetic Mississippi-Valley-type ore deposit. The fieldwork encompassed the deployment of 38 seismic nodes along a profile with a total length of 3300 meters, with a sensor spacing of 90 meters. Five days of continuous passive data were acquired. Processing methods included the Extended Spatial Autocorrelation (ESPAC) methodology and the Ambient Noise Interferometry (ANI) procedure. The inversion of 31 dispersion curves enabled the construction of a 2-D S-wave velocity model extending to a maximum depth of 700 m. The model shows two velocity sectors separated by a low velocity corridor and identifies velocity anomalies that correspond with structural variations and major fault systems. These results validate the proposed ambient seismic noise workflow for imaging geological and structural features to depths of approximately 700 meters. Additionally, this study demonstrates that the ESPAC processing method enhances survey efficiency and flexibility, particularly when using irregular array configurations. The ESPAC method provided the most reliable results for developing an S-wave velocity model, with lateral resolution dependent on the number and spacing of seismic nodes. Future works include the development of additional passive seismic profiles in the Villabona Pilot Zone, together with planned tests in two additional pilot areas in Spain and Estonia. The main aim is to further validate and apply the passive seismic methodology across diverse geological settings characterized by variable ore deposit distribution and structural configurations.