Controlled-illumination reflection imaging of massive sulphide deposits utilizing a tunnel-to-surface acquisition setup at the Neves–Corvo mining site, Portugal

SUMMARY Seismic data acquisition can innovatively be implemented on the surface and within underground infrastructure to illuminate subsurface targets. In the seismic data processing and imaging phases, prior subsurface information, such as approximate interface dipping angles, can enhance reflection imaging in a target-oriented manner. We leverage a unique field data set from an unconventional seismic acquisition setup to image a volcanogenic massive sulphide (VMS) deposit at the Neves-Corvo mining site in southern Portugal. The setup involved seismic sources positioned in a tunnel at a depth of approximately 650 m, from which the wavefields were recorded by surface receivers deployed along a 2-D line directly above the tunnel. The data were marred by strong noise and limited acquisition aperture due to the tunnel length, resulting in significant smearing artifacts in images generated from conventional migration techniques, which impeded a detailed delineation of the deposit. By utilizing directional information from illumination vectors, derived from the gradients of source-side and receiver-side traveltime fields, we implemented a controlled-illumination strategy within the Kirchhoff pre-stack depth migration workflow. This approach resulted in enhanced imaging of the targeted Lombador VMS deposit. The improved image revealed a subtle discontinuity in the Lombador reflector, indicating a possible fault, which is also present in the area. The reflection imaging results highlight the advantages of employing underground infrastructure, such as tunnels, for seismic applications in supporting detailed in-mine exploration and drilling programmes for resource estimations.