Downhole seismic imaging of a massive sulfide orebody with mode‐converted waves, Halfmile lake, New Brunswick, Canada

Abstract

Multioffset, multiazimuth downhole seismic data were acquired at Halfmile lake, New Brunswick, to image known massive sulfide lenses and to investigate the potential existence of a steeply dipping mineralized zone connecting them. The massive sulfide lenses, which have significantly higher elastic impedances than host rocks, produce strong scattering. The downhole seismic data show prominent scattered (P‐P and S‐S) and mode‐converted (P‐S and S‐P) waves originating from the deposit. Such complex scattering from massive sulfide ore was not observed previously in vertical seismic profiling data. Migration of the scattered and mode‐converted waves from several shot points imaged different parts of the deepest lens. The scattered S‐waves and mode‐converted waves provide additional imaging capabilities that should be considered when selecting downhole seismic methods for mining exploration.

Seismic methods in mineral exploration and mine planning: A general overview of past and present case histories and a look into the future

“…Downhole seismic surveys such as side scan, VSP and mine seismic profiling (MSP), and in-mine seismic surveys are best suited for imaging steeply dipping to subvertical structures (e.g., Price, 1974;Cosma, 1983;Wong et al, 1983Wong et al, , 1984Gustavsson et al, 1984;Galperin, 1985;Peterson et al, 1985;Spathis et al, 1985;Harman et al, 1987;Mutyorata et al, 1987;Duncan et al, 1989;Juhlin et al, 1991;Sinadinovski et al, 1995;Frappa and Moinier, 1993;Cao and Greenhalgh, 1997;Greenhalgh and Bierbaum, 1998;Urosevic and Evans, 2000;Greenhalgh et al, 2000Greenhalgh et al, , 2003Wong, 2000;Adam et al, 2003;Cosma et al, , 2007Perron et al, 2003;Bellefleur et al, 2004aBellefleur et al, , 2004bXu and Greenhalgh, 2010). They have typically higher resolution than surface seismic data, which make them attractive for delineating fracture and fault zones for mine planning or as a complement to surface seismic surveys.…”

“…Part of this work was carried out by the Downhole Seismic Imaging (DSI) consortium created in the late 1990s by Falconbridge, Inco, Noranda, Quantec Geoscience, and the Geological Survey of Canada to assess the usefulness of downhole seismic methods for exploration of massive sulfide deposits in steeply dipping stratigraphy. DSI data acquired over the Halfmile Lake deposit (Bellefleur et al, 2004b) identify a complex scattering response from a deep sulfide lens (1.2 km) that included P-P, P-S, S-P, and S-S waves, demonstrating that mode-converted and shear waves also are valuable signals to record for mineral exploration. At the ore scale, crosshole seismic surveys were used successfully to delineate mineralization and estimate grades.…”

“…The use of 3C downhole sensors has two immediate benefits: The travel distance of the reflected wave is reduced, hence, there is less attenuation of the higher frequencies; and S-waves, which travel at about 60% of the speed of the P-wave velocity and thus have an inherently shorter wavelength, can be used. Pretorius et al (2011) report that S-wave VSP data give as much as an 80% improvement in resolution over surface seismic surveys (also see Bellefleur et al, 2004b). However, downhole seismic methods usually are implemented at the development stages or for mine planning.…”

“…Adopting an approach of cost-benefit evaluation of 3D seismic used in hydrocarbon exploration, Pretorius et al (1997) conducted the first successful application of 3D seismic for mine planning and development. The first successful 3D seismic delineation of a blind massive sulfide deposit was reported from the western part of the Bathurst mining camp, in the Halfmile Lake area, Canada, at a depth of about 1.2 km (Matthews, 2002;Bellefleur et al, 2004b;Malehmir and Bellefleur, 2009). These surveys were well integrated with petrophysical measurements carried out in boreholes and in the laboratory to constrain the seismic interpretations.…”

Improving Seismic Resolution Through Array Processing Techniques

“…5.3 essentially introduces a weighting of the traces based on the traveltimes. Other or additional weighting factors are feasible and the controlled-source seismic community applies e.g., weighting factors based on the incidence angle of the energy (Takahashi 1995;Bellefleur et al 2004;Luth et al 2005). Kito et al (2007a) extended this principle to use earthquake sources to study the Earth's deep interior.…”