Borehole seismic survey for mineral exploration: Case histories from Norman West and Halfmile Lake
Borehole seismic field tests were conducted over two sub-economic massive sulphide deposits located in Norman West, Ontario, and Halfmile Lake, New Brunswick, to assess the usefulness of this technique for the exploration of base metal deposits. The field tests were conducted by the Downhole Seismic Imaging (DSI) consortium, a consortium initiated by the Geological Survey of Canada, Canadian mining and services companies (Falconbridge, Inco, Quantec Geoscience, and Noranda), and universities (University of Western Ontario, University of Alberta, Memorial University, Christian Albrechts University in Kiel, Cambridge University, and University of Helsinki) to demonstrate the utility of downhole seismic methods for exploration of massive sulphide deposits within areas of steeply dipping stratigraphy. Several aspects of borehole seismic methods applied to mineral exploration were studied and investigated by the consortium and its partners. These included physical rock properties of ore and host rocks, numerical modelling of the seismic response from orebodies, field acquisition techniques and survey design, imaging, and interpretation. The intent of this paper is to give an overview on how these aspects were integrated during various phases of downhole seismic surveys at
Norman West and Halfmile Lake.
The Norman West deposit is located within the Sudbury Igneous Complex (SIC), which comprises norite, gabbro, and granophyre cropping out in a roughly elliptical ring. Unfortunately, some of the country rock lithological units in this area are as reflective as the massive sulphide lenses, thus reducing the chances for direct orebody detection with seismic methods. Many seismic reflections correlate with faults and stratigraphic boundaries logged from diamond drillcores. The Norman West deposit, or structures associated with the deposit, produce detectable seismic reflections as far away as 1500 m. However, these reflections are difficult to distinguish from fault reflections and lithological contacts near the base of the Sudbury Igneous Complex. Results indicate that borehole seismic methods can be useful for exploration in the Norman West area, but require a priori information to help the interpretation. They also show that seismic methods are appropriate for mapping promising stratigraphic horizons, such as the sublayer norite, or to define regional faults in mine planning operations.
The Halfmile Lake deposit is located in the Bathurst mining district in northern New Brunswick. The deposit contrasts significantly in impedance (the product of velocity and density) with the almost seismically transparent host rocks, making this area an excellent test site for seismic methods. Strong acoustic impedance anomalies defined from a surface 3D seismic survey previously led to the discovery of this deeply buried sulphide lens. On borehole seismic data, the Halfmile Lake deposit has a clear but relatively complex signature, showing not only P-wave reflections, but also strong reflected S-waves and mode-converted waves (see Figure). Such prominent S-waves and converted waves have not been observed or recognized previously in borehole or surface seismic data acquired over massive sulphide deposits. S-waves and mode-converted waves add complexity to the interpretation but they also provide additional imaging capabilities potentially useful to locate a deposit. The results at Halfmile Lake show that reflected S-waves and converted waves should be considered when planning a seismic survey, as well as in processing, imaging, and interpretation of seismic data to detect massive sulphide deposits.