Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 365

Figure 13.10 Measurement of discontinuity spacing on a drill core (Wittke 1990)
13.5.2 Observation of Borehole Walls
Optical and acoustic borehole logging methods enable us to gain information on ori-
entation, spacing and aperture of discontinuities, joint fillings and fault zones by ob-
serving the wall of a borehole. These methods serve as a valuable complement to drill
cores, particularly in cases where the latter are disturbed and provide only poor results.
The simplest method is to use an optical system consisting of a camera and a ro-
tatable periscope that is inserted into the borehole and enables the borehole wall
to be observed from different directions. With modern borehole camera devices
oriented and colored pictures of the borehole wall can be obtained. However, this
procedure is rather time-consuming and cannot be carried out with high accuracy
(Felfer 2006). In addition, the maximum exploration depth for this method is lim-
ited to approximately 30 m (GIF 2004).
A more elaborate method is the use of a borehole scanner - a so-called “optical
televiewer” - which monitors the borehole wall with a digital camera via a truncated
cone mirror. The truncated core images give, joined together, a deformed reproduc-
tion of the borehole wall. Arithmetic rectification allows us to produce an orientated
developed view of the borehole wall. In addition, the image of an artificial rock core
representing the surface of the borehole wall and referred to as a “pseudo-core” can
be calculated with special software. Figure 13.11 shows schematically a device using
an optical televiewer called an “Ettlinger total image borehole system” (ETIBS).
Using this method, orientation, spacing and aperture of structural elements such as
discontinuities can be evaluated (GIF 2004).