Seismic methods are an affordable and effective way of studying the subsurface for mineral exploration. With the goal of testing new technologies for mineral exploration in highly challenging mining areas, in early 2019 an innovative seismic survey was conducted at the Neves-Corvo mine, south Portugal. This study focuses on the data and results from the surface array data, while an accompanying article deals with the underground seismic data. The surface seismic survey consisted of two perpendicular 2D profiles positioned above the known world-class tier-1 Lombador deposit. Simultaneously a survey inside the active underground mine took place, being unique as it included the testing of a prototype system that enabled accurate GPS-time (micro-second accuracy) synchronization inside the mine tunnels, approximately 650 m below the surface profiles. Due to the active mining operations the surface data are noisy. To handle this a carefully tailored processing algorithm was developed and applied to enhance reflections in the data, interpreted to originate from lithological contacts and the Lombador deposit. The results and interpretations from 2D processing were validated taking advantage of the known deposit geometry using 3D exploding reflector modeling and pseudo-3D cross-dip analysis. These analyses suggest that there is an out-of-plane signature of the Lombador deposit on the surface data. Additionally, source points activated in the exploration tunnels and simultaneously recorded on the surface profiles allowed to create a 2D velocity model that was used for migration and time-to-depth conversion, providing a reliable 2D seismic section of the subsurface under the surface profiles. This study demonstrates that limited surface coverage 2D surveys and a velocity model derived from the tunnel-to-surface seismic recordings, allow for imaging of key subsurface geological structures and to delineate mineral deposits of economic interest.

Seismic methods are becoming an established choice for deep mineral exploration after being extensively tested and employed for the past two decades. To investigate whether the early European mineral‐exploration datasets had potential for seismic imaging that was overlooked, we recovered a low‐fold legacy seismic dataset from the Neves–Corvo mine site in the Iberian Pyrite Belt in southern Portugal. This dataset comprises six 4–6 km long profiles acquired in 1996 for deep targeting. Using today's industry‐scale processing algorithms, the world‐class, ca. 150 Mt, Lombador massive sulphide and other smaller deposits were better imaged. Additionally, we also reveal a number of shallow but steeply dipping reflections that were absent in the original processing results. This study highlights that legacy seismic data are valuable and should be revisited regularly to take advantage of new processing algorithms and the experiences gained from processing such data in hard‐rock environments elsewhere. Remembering that an initial processing job in hard rock should always aim to first obtain an overall image of the subsurface and make reflections visible, and then subsequent goals of the workflow could be set to, for example understanding relative amplitude ratios. The imaging of the known mineralization implies that this survey could likely have been among one of the pioneer studies in the world that demonstrated the capability of directly imaging massive sulphide deposits using the seismic method.

To evaluate and upscale the feasibility of utilizing exploration tunnels in an operating mine for active-source seismic imaging, a seismic experiment was conducted at the Neves-Corvo mine, in southern Portugal. Four seismic profiles were deployed in exploration drifts ca. 650 m beneath the ground surface, above the world-class Lombador volcanogenic massive sulfide (VMS) deposit. In addition to the tunnel profiles, two perpendicular surface seismic profiles were deployed above the exploration tunnels. The survey was possible due to a newly developed prototype GPS-time transmitter enabling accurate GPS synchronization of cabled and nodal seismic recorders, both below and on the surface. Another innovative acquisition aspect was a 1.65 t broadband, linear synchronous motor (LSM) driven - electric seismic vibrator (e-vib) used as the seismic source along two of the exploration tunnels. Challenges and innovations necessary for active-source tunnel seismic acquisition, characterized by high levels of vibrational noise from the mining activities, are discussed. Additionally, the LSM vibrator’s signal and overall seismic data quality in this hard rock mining environment are evaluated. Processing results from the tunnel data and 3D reflection imaging of the Lombador deposit below the exploration tunnels are shown and the results checked for consistency through constant-velocity 3D ray-tracing traveltime forward modeling. For imaging purposes, 3D Kirchhoff pre-stack depth and post-stack time migration algorithms were used, with both successfully imaging the targeted deposit. The results obtained show that active-source seismic imaging using subsurface mining infrastructure of operational mines is possible. However, it requires innovative exploration strategies, a broadband seismic source, an accurate GPS-time system capable of transmitting GPS-time hundreds of meters below the surface and careful processing. The results obtained open up possibilities for similar studies in different mining or tunneling projects.

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