Seismic experiments at Macraes Flat, Otago, New Zealand

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SR_2005-032-pdf
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Leslie, A.; Woodward, D.J.; Cox, S.C.; Gorman, A. 2005 Seismic experiments at Macraes Flat, Otago, New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2005/32 35 p.

Abstract: The use of seismic reflection studies in hard rock environments is a new and potentially important tool in the mining industry. The shift from surface to underground mining at Oceana Gold’s Macraes Project in eastern Otago has generated a desire to obtain grater information about buried rock structure. In particular, the Hyde-Macraes Shear Zone (HMSZ) – which is the primary host of gold mineralisation at Macraes – and the surrounding rockmass have potential to be imaged by seismic methods. This report details the acquisition and analysis of a seismic dataset in 2004-2005 that built on earlier results from seismic data acquired in 1993. Prior to the new dataset acquisition, parameters were evaluated during a test survey to ensure best possible imaging of structures down to about 800 m depths. A range of sources and geophones were tested. Best results were obtained using 400g Pentex boosters as a source, placed at 2.5 m depth with good tamping, and geophones were with a dominant frequency response of 40 Hz. An asymmetric geophone spread was chosen with near offsets of 15 m, far offsets of 370 m in the up-dip direction, and 130 m in the down-dip direction. Anisotrophy tests recorded significant azimuthal dependencies in the near surface velocities, but due to difficulties in interpretation of results and the constant orientation E-W survey lines, no velocity anisotrophy corrections were applied. Data were recorded on a Seistronix RAS-24 seismograph system linked to a laptop then processed with GLOBE Claritas software using a processing flow that included dip move-out and coherency filtering. A 2.6 km seismic reflection survey composed of two parallel lines, offset by 200 m and overlapping by 400 m, was carried out at the Frasers locality in late 2004. The lines were oriented parallel to the HMSZ dip-direction, crossing from the confirmed underground gold resource at Frasers (Panel 2) to a lesser known region of the Eastern Lodes. Numerous drill-holes and the Frasers mine exposures define the geology beneath Line 1, allowing for reliable comparisons between seismic and geological datasets, and a well-constrained interpretation. Further east, beneath Line 2, the geology is less well known. Bands of well-defined reflections up to 90 m wide and more subtle, discontinuous reflections were ''mapped'' throughout the seismic sections. Dipping discontinuities and truncations of reflections are also observed. An upper seismic unit, containing mostly east-dipping reflections, is distinct from a lower unit with variable east- and west-dipping reflections. Reflections are derived from changes in the physical properties of adjacent rock units and often cannot be directly related to specific geological bodies due to the scale of the bodies being similar to, or smaller than, the scale of the seismic waves passing through them. However, in the Otago Schist, patterns of reflectivity often do correspond to such geological features as (1) bodies of different schist litho logy, (2) changes in the orientation of schistosity at folds, or (3) large quartz veins. Reflections appear to mimic the orientation of schist fabric in the Hangingwall and Footwall of the HMSZ, and the upper and lower seismic units correlate with the general structure of the HMSZ. The transition zone from upper to lower seismic units dips gently east but its position is subjective and internal details are variable. Structures within the HMSZ could not be positioned definitively without prior knowledge, which limits the application of these methods elsewhere in the Otago Schist. However, where the geology is well constrained by mining or drilling, stronger mineralisation is shown to be associated with well-defined continuous reflections at the top of the transition zone, and westerly-dipping reflections and data trends within it. This relationship provides an empirical model that can be used locally for predictive exploration and drill targeting. (auth)