To evaluate and upscale the feasibility of using 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 approximately 650 m beneath the ground surface, above the world-class Lombador volcanogenic massive sulfide 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 global positioning system (GPS) time transmitter enabling accurate GPS synchronization of cabled and nodal seismic recorders, 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. We have evaluated the challenges and innovations necessary for active-source tunnel seismic acquisition, characterized by high levels of vibrational noise from the mining activities. In addition, we evaluated the LSM vibrator's signal and overall seismic-data quality in this hard rock mining environment. Our processing results from the tunnel data and 3D reflection imaging of the Lombador deposit below the exploration tunnels were checked for consistency through constant-velocity 3D ray-tracing traveltime forward modeling. For imaging purposes, 3D Kirchhoff prestack depth and poststack 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. ... Read more

Seismic imaging characteristics of a prototype electrically driven, linear synchronous motor-based, vertical-force seismic vibrator ("e-vib") were evaluated at a site in the Netherlands. The system weighs 1.65 t and excites seismic signals with a peak force of 6.7 kN. Data were recorded along two collocated geophone-based nodal and landstreamer microelectromechanical system - MEMS-based sensor 2D seismic profiles. To obtain a broad bandwidth data set, the e-vib operated with a 1-200 Hz linear sweep. Shot gathers of the merged nodal-landstreamer data set indicated good-quality seismic data of a broadband nature. The processed merged data set demonstrates high-resolution reflections of the stratigraphic members from approximately 200 m to 2 km, with visible reflections as deep as 2.5-2.9 km. As a reference, we also processed a legacy 3D microspread data set acquired at the same site with a magnitude stronger (14.1 t, 67.5 kN) hydraulic vibrator. Comparison of our nodal-landstreamer seismic section versus 2D slices extracted from the processed microspread seismic volume suggested similar signal penetration depth and the same key marker horizons seen in both. Analysis of the reaction mass and base-plate accelerometer signals recorded with the e-vib source operating on grass and on asphalt surfaces indicates that the e-vib has low total harmonic distortion. The results obtained indicate that, although relatively small, the e-vib is capable of generating high-quality broadband seismic data. ... Read more