Comparison of 2D pre-stack and 3D post-stack migrations from the Matagami mining camp, Québec

Brian H. Hoffe, Andrew J. Burton, Laurence R. Lines

Exploration for new massive sulphide deposits in mature mining camps relyalmost exclusively on deep drilling to test prospective stratigraphic contacts since conventional geophysical methods are generally limited to the first few hundred metres of the subsurface (Adam et al., 1997). Thus probing for new mineral depositsto greater and greater depths increases exploration costs dramatically. As a consequence, exploration companies are required to research, develop and apply alternate geophysical prospecting techniques in their search for these deeper mineral prospects (Adam et al., 1997). In response to this need, Noranda Mining and Exploration Ltd. turned to seismic reflection methods as a possible alternate prospecting technique for these deeper deposits through the founding of its Seismic Technology Research Project, which was initiated in 1990.

In 1995 and 1996, Noranda Mining and Exploration Limited carried out both 2D and 3D seismic reflection surveys at their Matagami mining camp situated in northwestern Quebec. The primary objective of these surveys was to resolve the seismic characteristics of the newly discovered Bell Allard massive sulphide deposit in relation to the host rocks. An important aside to the 2D reflection survey was, due to the availability of 120 unused recording channels during the acquisition Noranda deployed 40 3C geophones at 20 metre interval to acquire multi-component seismic data from the Matagami area. Although this data is not examined in this report, the CREWES Project is hoping to process this data in the near future to test whether multi-component data is better suited to seismic exploration in mining environments.

The objective of this study was to better image the volcanic sequence stratigraphy that hosts the Bell Allard volcanogenic massive sulphide ore body which has been estimated at 6 million tonnes. In imaging the Bell Allard deposit, we hoped to compare 2D and 3D migration methods using fast frequency domain codes. Since all seismic velocities in these rocks were generally similar and large (6,000 - 6,500 m/s), we were able to use constant velocity frequency domain migration codes. There is a bad news - good news scenario associated with seismic data acquired for mineral exploration. The bad news is that the seismic reflectors tend to be discontinuous with low signal to noise. The good news is that seismic velocities are often high and constant so constant velocity migration algorithms can be implemented which have much shorter execution times.