3D seismic datasets applied to mineral exploration: revisiting three Canadian case studies
The benefits of working with legacy seismic data are broad and valued in a wide range of geoscience applications and result from the comprehensive volume of industry and public seismic data acquired over several decades. Legacy seismic data exist in many parts of the world including areas that, due to stricter regulatory processes, environmental restrictions, demographic changes, or other factors would be challenging or even impossible to survey today. Examples are numerous but one includes 2D marine seismic data acquired in the 1980s with a large airgun array to provide a deep cross-section through the Appalachians beneath the now sen-sitive Gulf of St Lawrence, eastern Canada (Marillier et al., 1989). Sometimes benefits arise through the use of newly developed tech-niques to extract additional information from legacy seismic data. This idea is obviously not new and many examples can be found in the literature. One example is the use of extended Vibroseis cor-relation developed in 1980s (Okaya and Jarchow, 1989) applied to increase the depth range of marine Vibroseis seismic data acquired in 1971 in two of the Great Lakes of North America (Milkereit et al., 1992). Marine seismic data in the Great Lakes are sparse, but contain information about deep Grenvillian terranes that, in this case, could only be revealed using a technology developed more than a decade after data acquisition. Reflection seismology has been used for mineral exploration for more than 30 years (Reed, 1993). During that period, many case studies demonstrated the utility of the method for imaging deep lithological contacts and structures indirectly associated with mineralization, and for the direct imaging of deposits. Malehmir et al. (2012) provide a comprehensive overview of key accomplishments for various commodities and mining locations in Canada and around the world. In particular, many examples demonstrate the advantages of the 3D seismic method for providing high-resolution images of subsurface structures and deep-seated ore deposits in a variety of geological environ-ments. Several of these legacy 3D seismic data sets have been reprocessed with advanced and novel techniques to provide more reliable and detailed images of the subsurface. Here, we show three early examples of the application of 3D seismic data for mineral exploration in the Bathurst, Sudbury, and Val d’Or mining camps, Canada (Figure 1). Specifically, we demonstrate that reprocessing of 3D datasets from earlier applications of seismic reflection can refine the imaging of known ore deposits and can lead to the identification of new exploration targets. For the case studies presented here, most improvements resulted from processing and imaging strategies built from an understanding of the response of mineralized bodies on seismic data and improving specific steps that prevented their proper imaging on final volumes. The case studies are introduced according to their year of acquisition, starting from the oldest (i.e., the Trill 3D survey in the Sudbury mining camp) to the most recent (e.g., the Louvicourt 3D survey in the Val d’Or mining camp). All 3D seismic surveys are located in areas with limited access and difficult terrain and as a result, were acquired with explosives. Surface conditions varied for each survey area but included steep hills, extensive swamps, and thick glacial sediments. Geology also varied at the three sites. Host rocks are either nearly-transparent-to-seismic waves or are highly heterogeneous and generating seismic scattering potentially masking the response of the mineralized body. In all three areas, however, mineralized bodies scattered or reflected energy in a preferred direction, and were observed only on a limited number of prestack seismic traces. In some cases, such a sparse signature was buried in noise when stacking all traces and required particu-lar attention to be properly imaged.