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The variables for the inversion are log resistivity and chargeability, and the recovered models are shown in Figure 2. In Figure 3 below, these models are displayed with geological information obtained from drilling. The resistivity inversion has been useful in delineating the resistive limestone overburden on the right hand side (the bluer zones). Resistivity is not correlated with mineralization. This conclusion is supported by measurements on core taken from boreholes which found resistivities of the ore zone to be in the range of 100-300 ohm-m, and the host siltstone and shales to be in the range of 60-80 ohm-m. The measured resistivity of the limestone was in excess of 1000 ohm-m.
The IP inversion has delineated the horizontal extent of and depth to the ore body. It also indicates a major fault between x=27,000 metres and x=27,500 metres, which dislocates the ore sequence. The chargeable body on the inverted section is somewhat thicker than drill hole results. This occurs for two reasons. Our objective function constructs smooth models, and hence discrete boundaries will appear as gradational changes in the images. Also, downhole IP and petrophysical data indicate that while Century ore is strongly polarisable, the adjacent units, particularly the footwall sediments, are weakly chargeable. This adds to the geophysical response and thickens the region of polarization. Overall, the IP image provides important information about mineralization and structure.
Figure 3. The same inverted sections as Figure 2, with overlaid overburden boundary (white line), faults (black lines), and the extent of the ore body (dashed lines). |
 home © UBC-GIF
January 9, 2007
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