Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 368

Figure 13.13 Video probe with cable winch and monitor (GIF 2004)
While optical borehole logging requires good visibility and can be used in both water
filled and empty boreholes, acoustic borehole logging works only under water. However,
since no visibility is required, it operates also in drilling mud (GIF 2004, Felfer 2006).
With optical logging, a depth of some 800 m can be explored at present, whereas depths
up to 5000 m and more can be examined using acoustic probes. For a deep borehole,
logging borehole diameters of 101 mm are necessary (GIF 2004).
13.5.3 Borehole Direction
Boreholes often deviate significantly from their predetermined direction in rock masses,
exhibiting pronounced bedding-parallel or schistosity-parallel discontinuities. Vertical
boreholes tend to orient themselves perpendicular to these discontinuities when dipping
shallowly (Fig. 13.14, left) and parallel to these discontinuities when dipping steeply
(Fig. 13.14, right). The reason for these deviations is most likely to be strength anisotropy
(Wittke 1990).
Unknown borehole deviations from the planned direction can lead to misinterpreta-
tions with respect to the dip angle of discontinuities and layers (Fig. 13.14). However,
the correct dip angle of those structural elements can be determined if the true borehole
direction is known.
Borehole direction can be checked by measuring borehole inclination using single-shot
and multiple-shot devices consisting of a camera and a combined compass and pendu-
lum which are screwed together and lowered to the measurement level. More precise
measuring devices are inclinometers and deflectometers (Section 17.4.2).