A new method for obtaining and quantifying the reliability of structural data from axially-oriented drill core using a fabric of known orientation

Abstract

A new method for obtaining structural data from drill cores using a reference fabric (e.g. cleavage) to re-orient the core is presented (minimum-discordance method). Where the plunge and azimuth of the core is known, the most likely orientation of the core is taken to be the position in which the angle between the cleavage (in core) and its assumed regional orientation is at a minimum. In this position the pole to the cleavage (in core) lies in the plane defined by the drill hole axis and the reference orientation of the pole to the cleavage. With the most likely orientation of the core fixed, the orientation of other structural elements may be determined. Statistical analysis (Monte Carlo simulation) is used to assess factors affecting the reliability of the minimum-discordance method. Acceptable results (>70% probability solutions for fabrics of unknown orientation are within 15 degrees of their correct orientation) are obtained where: (i) the standard deviation in the orientation of the reference fabric is less than 15 degrees (1sigma), (ii) the reference fabric is inclined at 20-60 degrees to the core axis, and (iii) either the pole to the plane or the lineation whose orientation is originally unknown is at ~45 degrees to the core axis. Best results are obtained where the reference plane is at ~30 degrees to the core axis. A previously published method that uses only the average strike of the reference fabric to determine the orientation of the core is shown to have additional geometric restrictions that render results unreliable in many situations. Ideally, estimates of the average orientation and variability of the reference fabric are based on field data. However, for surveyed drill holes of widely ranging orientation, the minimum-discordance method can be used to determine the average orientation and minimum degree of scatter for the reference fabric directly from the core. We demonstrate this, and the applicability of our method, with an example from Lewis Ponds, New South Wales.