Feasibility of virtual source reflection seismology using interferometry for mineral exploration: A test study in the Lalor Lake volcanogenic massive sulphide mining area, Manitoba, Canada

Abstract

A B S T R A C TApproximately 300 hours of ambient noise data were recorded on a grid of receivers covering an area of 4 km 2 over the Lalor Mine, Canada, to test the capability of seismic interferometry to image ore deposits in the crystalline rock environment. Underground mining activities create the main source of ambient noise in the area. Alongside the ambient noise survey, a larger three-dimensional active-source seismic survey was also acquired and used to evaluate the interferometry results. Power spectral density calculations show random ambient noise with a frequency range of 2 Hz-35 Hz. A beamforming analysis identified body waves arriving from the west-northwest (pointing towards the mine) and surface waves propagating from the northeast. The calculated virtual shot gathers retrieved by cross-correlating ambient noise at all receivers were processed following both two-dimensional and three-dimensional approaches using a sequence similar to the one applied to the activesource three-dimensional data. The dip-moveout stacked section reveals a number of events similar to those observed on the processed active seismic sections. In particular, the passive seismic interferometry method is capable to partly image shallowly dipping reflections but did not produce convincing images of steeply dipping reflections. Dip-moveout stacked sections obtained with different cross-correlation time windows indicate that the strength and number of reflections generally increase with longer noise records. However, a few reflections at depth show reduced coherency with longer noise time windows. The passive seismic interferometry results over the Lalor mining area are encouraging, but image quality of the passive survey is lower than the acquired active three-dimensional survey at the area. Future ambient noise surveys with longer offsets, shorter receiver spacing, and wider azimuth distribution are needed in crystalline rock environment to address the potential of the method for mineral exploration.

Passive seismic reflection interferometry: A case study from the Aquistore CO2storage site, Saskatchewan, Canada

“…Filtering of noise panels (Ruigrok et al, 2011) and beam-forming analysis (Draganov et al, 2013) are two methods that have been used. Standard multichannel CMP reflection processing of virtual shot gathers also helps enhance reflections and reduce surface-wave energy (Cheraghi et al, 2015).…”

“…We used the algorithm applied by Cheraghi et al (2015). A frequency range of 3-10 Hz was analyzed for velocities in the range of 1-5 km∕s.…”

“…Precorrelation processing applied to the data was based on that described by Draganov et al (2013) and Cheraghi et al (2015). It included band-pass filtering (2-35 Hz) and trace-to-trace amplitude balancing (energy normalization).…”

“…Crosscorrelations are calculated for each receiver pair along L1 and L2 (Figure 1), and then causal and acausal parts of the correlation results are summed to generate a correlated noise panel. Summing the causal and acausal crosscorrelation results takes into account the fact that some part of Green's function might be retrieved at positive times and other parts at negative times (Draganov et al, 2013;Cheraghi et al, 2015). Finally, all crosscorrelated noise panels are summed together to produce the final virtual shot gather for that specific virtual shot location.…”

“…Ambient-noise interferometry has the potential of providing a less expensive alternative technique, but it must first be successfully demonstrated that it can provide reliable monitoring results at a specific field site with a proposed geometry. Seismic interferometry has been tested with some success for seismic reflection imaging in mineral and oil exploration applications (Draganov et al, 2007(Draganov et al, , 2009(Draganov et al, , 2013Nakata et al, 2011;Xu et al, 2012;Cheraghi et al, 2015), but to date, feasibility studies for CO 2 storage monitoring with passive seismic interferometry are limited (Xu et al, 2012;Boullenger et al, 2015;Gassenmeier et al, 2015).…”