Detailed Seismic Refraction Surveys at Mt Bulga

Abstract

The Mt Bulga ore body, which is located near Orange in south-eastern Australia, consists of a narrow (5 ? 10 m) syngenetic fine grained banded massive pyrite-galena-sphalerite- chalcopyrite in steeply dipping unfolded Silurian altered metasediments. Several major shear zones are associated with the ore body. A 2D seismic refraction profile was recorded with a station separation of 2.5 m, across a small ridge, which also marks the approximate location of the massive sulphide ore body. The data were processed with wave eikonal traveltime (WET) tomography using a starting model generated with the 1D tau-p inversion algorithm and with the GRM SSM. The 1D tau-p refraction tomogram showed that the ore body has a higher seismic velocity than the adjacent regions, whereas the GRM SSM showed a low seismic velocity in the ore body. A density model, which was generated with the head wave amplitudes and the refractor velocities, showed a marked increase over the ore body, suggesting that the lower seismic velocity in the ore body is more likely. These results demonstrate that seismic refraction profiles can provide useful information on depths of weathering and density contrasts for joint inversion with either airborne or ground gravity data. These results also demonstrate that the application of seismic reflection methods to the imagining of sulphide ore bodies will require detailed mapping of the near-surface layers, in order compute useful statics corrections. The rapid changes in depths, as well as the large lateral variations in the seismic velocities in the unweathered material suggest that wave equation re-datuming, rather than simple time shifts would be required.