Seismic imaging of volcanogenic massive sulphide deposits : From legacy to innovative surveys

Sammanfattning: The volcanogenic massive sulphide deposits at the Neves-Corvo mine area, part of the Iberian Pyrite Belt in southern Portugal, were studied using seismic methods.  Three distinct reflection seismic datasets were used in this study: a 2D legacy data from 1996, an innovative 2019 surface and in-mine acquisition, and a 2011 3D seismic survey. The seismic data were processed with the main objective of improving the seismic signature of the volcanogenic massive sulphide deposits in the area by the use of modern and innovative seismic methods.The legacy dataset, acquired in 1996, was reprocess using today’s processing software. The Lombador massive sulphide was better imaged and a number of never-before-seen shallow but steeply dipping reflections were detected. In the 2019 seismic survey, 2D seismic profiles were acquired on the surface above the Lombador deposit, and four seismic profiles were deployed inside mine exploration tunnels at 650 m depth. Arguably, the first time a seismic acquisition survey of this size has been carried-out inside an active underground mine. The processing of the surface profiles was complemented by additional methods that indicate how the out-of-plane contribution of the Lombador deposit may not be detectable when employing 2D data only, showing the imaging potential of a small-scale survey implemented with innovative acquisition technologies. The 3D seismic data processing showed sensitivity to parameter selection for imaging. Processing results using pre-stack dip move-out and post-stack migration methods show moderate to steeply dipping reflections. Which can be correlated with known lithological contacts, some are interpreted to be originated from the Semblana and Lombador deposits. Despite the mixed signal-to-noise ratio the seismic volume reveals improved 3D seismic images of both shallow and deep structures, allowing to account for the deposit’s lateral extension beyond the capabilities of 2D seismic imaging alone. It was possible to distinguish strong diffraction patterns, interpreted as originating from faults and edges of the Lombador deposit, illustrating the usefulness of diffraction patterns for better interpretation of geological features in hard-rock environments. All of the aforesaid allows to conclude that an exploration programme solely based on 2D seismic data would have showed false-positive results in depth. This encourages the employment of 3D seismic methods, instead of 2D, in mineral exploration for accurate detection and targeting of VMS deposits.