10.1 - Shallow Seismic Refraction Survey of the Woodlawn Orebody

Abstract

A seismic refraction survey over the Woodlawn orebody clearly determined the subcrop extent of the massive sulphides as a zone of low seismic velocity set within the much higher velocity host rocks. The seismic method is a useful ancillary method for delineating shallow, massive lead-zinc sulphide deposits. The travel-time curves have unusual and distinctive features generally showing negative or infinite apparent velocities on the opposite side of the orebody to the location of the shotpoints. These are more marked for shots at larger offset distances from the recording geophone spread, and there is some migration of the commencement of the negative or infinite apparent velocity segments in the direction of the shotpoints for the more distant shots. This effect is produced by the recording of arrivals of waves which have travelled through the wall rocks of the orebody after being diffracted from points below the top of the orebody. These diffracted waves are then refracted from the wall rocks through the orebody back to the surface. Neither the very low in situ seismic velocities observed within the massive sulphides of the Woodlawn orebody, nor these unusual features of the travel time curves across the orebody were observed over the pyritic and electrically conductive Black Shale. The seismic refraction method would, therefore, also seem to be a useful, additional geophysical screening method in distinguishing between shallow, massive sulphide orebodies such as Woodlawn, and uneconomic, graphitic conductive zones.