Introduction

Exploration of the underground by drilling and the recovery of core samples is an important technique for the geologist, especially in mining, engineering and petroleum projects. Several authors have tackled the geometrical problems which arise (Fisher, 1941; Stein, 1941; Mertie, 1943). In applied projects the identification of fractures is particularly important (Lau, 1983; Kulander, et al., 1990; Sikorsky, 1991). A good overview of many practical aspects of drilling, coring and analysis is provided by Goodman (1976, p. 127–157), and we follow his basic approach.

The information gained from a drilling program depends on the number of holes drilled, the orientation of the holes, the core recovery and the structures seen in the cores. Because drilling is expensive, it is important to extract the maximum information from as few holes as possible.

Inclined holes tend to wander during drilling, especially if long and, at least in part, this is due to rock anisotropy (Brown, et al., 1981). If accurate measurements are needed, down-the-hole survey instruments are lowered along the borehole to measure its plunge and trend at known distances. From this information, the curved path of the hole can then be calculated (Howson & Sides, 1986). Special still or television cameras can also be used.

Our concern here is with the structural information that can be obtained from recovered cores and the basic case involves the attitude of a structural plane.