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the RQD of its extremities with an inverse distance squared (IDS) as used in
the kriging techniques (Davis, 1973; Barnes, 1980). This procedure was
repeated over and over again, with the help of a computer, until all possible
combinations (PARBNOS) for the paired-hole intersections of a given
sampling were exhausted:

PARBNOS = [ {Sum(i) F(i)*N(i)}^2 - Sum(i) F(i)*{N(i)}^2 ] / 2

where F = frequency of a same number of weighted RQD values, and N =
maximum number of intervals for a given borehole or particular order.

Thence, the multitude of correlational PARBNOS obtained were represented
on a stereogram.


	*Stereograms*

In a stereographic projection it is possible to represent a plane as a
cyclographic trace of a great circle, and a line as a point. Two types of nets
are commonly used for structural or geomechanical studies: the Schmidt
equal area net and the Wulff equal angle net. However, many other types of
stereograms are yet possible as the orthographic net, the Kavraiskii net
(Duncan, 1981), and others where their order is n > 1 (convenient for
discontinuity attitudes of low dip), and which we can reproduce using the
following equation for their relative radius "r{θ(n)}".

--------------------------------------------------------------------------------
r{θ(n)} = [D/2]*[cos θ/2-sin θ/2]/[{cos θ/2+sin θ/2}^n]
--------------------------------------------------------------------------------

where D = diameter of the stereogram, θ = plunge (or dip),
orthographic (O) n=-1, Schmidt (S) n=0, Kavraiskii (K) n = approx.
0.30035, Wulff (W) n=1.

Thus, despite the large variety of choices in stereograms, we resorted to
the Schmidt net and the stereonet, and obtained some clearly interesting
results.


	*Stereotomograms and histomograms*

At this point, we had to deal with a large number of intersections or
RQD data pairs. The way to transpose the information on a support like the
stereogram was quite similar to making a complex pattern of embroidery
or a technique known by artists as the pointillism. In this art technique,
instead of mixing pure colors on his palette before brushing them on a
canvas, the artist will make little spots of pure juxtaposed colors on his
picture, in order to obtain the same result when seen from a distance.

In our study two kinds of tools were needed. Firstly, a stereotomogram
which consists essentially of a stereogram, but a stereogram used for


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