Bernd Milkereit

Development of 3-D seismic exploration technology for deep nickel‐copper deposits—A case history from the Sudbury basin, Canada

Following extensive petrophysical studies and presite surveys, the Trill area of the Sudbury basin was selected for conducting the first 3-D seismic survey for mineral exploration in North America. The 3-D seismic experiment confirms that in a geological setting such as the Sudbury Igneous Complex, massive sulfide bodies cause a characteristic seismic scattering response. This provides an excellent basis for the direct detection of massive sulfides by seismic methods. The feasibility study suggests that high-resolution seismic methods offer a large detection radius in the order of hundreds to thousands of meters, together with accurate depth estimates.

Physical properties and seismic imaging of massive sulfides

Laboratory studies show that the acoustic impedances of massive sulfides can be predicted from the physical properties (Vp, density) and modal abundances of common sulfide minerals using simple mixing relations. Most sulfides have significantly higher impedances than silicate rocks, implying that seismic reflection techniques can be used directly for base metals exploration, provided the deposits meet the geometric constraints required for detection. To test this concept, a series of 1-, 2-, and 3-D seismic experiments were conducted to image known ore bodies in central and eastern Canada. In one recent test, conducted at the Halfmile Lake copper‐nickel deposit in the Bathurst camp, laboratory measurements on representative samples of ore and country rock demonstrated that the ores should make strong reflectors at the site, while velocity and density logging confirmed that these reflectors should persist at formation scales. These predictions have been confirmed by the detection of strong reflections from...

Seismic reflections from subvertical diabase dikes in an Archean terrane

A 20 km seismic reflection profile in the Superior province of northwestern Ontario imaged polydeformed Archean terrane, including the Manitouwadge greenstone belt and the adjacent Wawa-Quetico subprovince boundary. In addition to reflectivity from Archean lithostructural elements, we recognize discrete reflections that correlate with undeformed subvertical Proterozoic dikes. Moderately to steeply inclined linear events in the stacked section are related to Matachewan dikes that intersect the seismic profile at 50°–75°. The reflections project to the surface at points corresponding to dike-profile intersections and have apparent dips in the seismic section consistent with the intersection angles in map view. Subhorizontal, linear to curvilinear reflections originate from Marathon dikes that are subparallel to the seismic line. On the basis of forward modeling studies, the events occur at two-way traveltimes for horizontally propagating waves. We conclude that a number of dipping and subhorizontal events represent refracted waves propagated along horizontal ray paths near the surface and reflected from subvertical dikes. In many other seismic profiles in crystalline crust, reflections originating from subvertical dikes have likely been overlooked or misinterpreted, either because the presence of dikes was not recognized, or because their seismic response was not evaluated.

Integrated borehole geophysical and 3‐D seismic study of a deep massive sulfide deposit, Sudbury Basin, Canada

The Sudbury Basin in Canada was selected for conducting one the world’s first 3-D seismic survey for deep base-metal exploration. The 3-D seismic experiment confirms that in a geological setting such as the Sudbury North Range, massive sulphide bodies cause a characteristic seismic scattering response. This provides an excellent basis for the direct detection of deep massive sulphides in the crystalline crust by seismic methods.

Borehole Seismic Imaging of Near-vertical Structures: A Case History

Conventional seismic methods are generally not effective for imaging geological structures with very steep dips, a common situation in some mining camps. Deep boreholes provide an alternative acquisition datum for seismic imaging in this type of setting. At the giant Kidd Creek CuZn deposit, the Archean volcanic stratigraphy has been folded and overturned so that most units are nearly vertical. Borehole logging and laboratory measurements conducted here show that certain stratigraphic contacts, especially those where massive-sulphide deposits and felsic host rocks are juxtaposed, are characterized by large impedance contrasts and are thus good candidates as seismic reflectors. To investigate this possiblity, a series of borehole-seismic profiling experiments were conducted in an area of known sulphide mineralization. Several source configurations were used, yielding both vertical and horizontal seismic images of the volcanic stratigraphy. In addition to observing reflections that correlate with stratigraphic contacts, prominent seismic anomalies were detected and have been correlated with a known massive-sulphide deposit.