Seismic imaging and modelling of the Hyde-Macraes Shear Zone, Otago, New Zealand

Leslie, Anna.

The use of seismic reflection studies in hard rock environments has over the past ten years become an important tool in the mining industry. This study has reprocessed, modelled and interpreted data acquired in 1993. This required the use of new processing techniques that have been developed over the years subsequent to the initial ~~ed. Finite-differen~lling obtained a cross-section over an area where the geology was known, ~mining in the area. This allowed the characterisation of the physical characteristics of the rocks in order to obtain the seismic signature that was observed from the data. This included the use of fabrics within the schist, which were modelled as having different dips and velocities in order to recreate the anisotropic effects of the subsurface. This allowed the identification of the characteristics that are attributed to structures such as faults, pelite bodies, the "' ~ footwall and yfangingwall structures, and dipping bodies, and assisted in the identification of these structures in other areas of the seismic sections where the subsurface geology was not as well constrained. The design of a new survey to image the· structure at depths of 400 - 800 m was undertaken, using a variety of sources, geophones and depth of source. The most efficient way of imaging the structure at these depths was shown to be a 400 g booster as a source at a depth of 2.5 m with good tamping and stemming. The 40 Hz geophones 'were most efficient at recording the signals required at a spacing of 5 m. Near offsets of 15 m and far offsets of380 m were deemed to be the most efficient way to cover the entire survey without changing the parameters of the layout. Anisotropy tests were included during the test period in order to investigate the in-situ attributes that may affect the velocity in the near surface. It was found that there was an effect, and this accounted for approximately 20% of the velocity change at 90° angles, which is about 6% more than published lab velocity tests at this metamorphic grade within the Otago Schist. This could be due to the depth of the schist in the laboratory tests and also the dip of the schist at this position. The prevalence of fractures at deeper levels in the schist (100- 500 m), observed in the mine may provide more of an indication for azimuthal anisotropy, which may be at a different angle to the schistosity that was 1 Preface measured in these tests. ~e l:~~ anisotropy will characterise the stress regime that was in place when they ~. - .., The application of these known parameters and seismic characteristics of the schist was useful in the design and acquisition of a new seismic survey, and will assist in the design of future surveys within the complex Otago Schist.

x, 114 leaves, [10] leaves of plates (fold.) : ill., maps ; 30 cm.

2005Leslie