Geoscience Reference
In-Depth Information
1. Heterogeneity of the composition and the physical
status, caused by the lithological changeability of
the rocks in the massif, by the depositional char-
acter, by the folding and fracturing in various
localities, including big faults, intrusions, frag-
mentation zones, weathering processes, and water
influx.
2. Heterogeneity of the stress condition, caused by
the natural stresses in the rock massifs.
3. Large-scale heterogeneity—varies by quality and
quantity features of the rocks in studying different
volumes from them.
The mineral composition of sedimentary rocks,
particularly limestones and dolomites, plays a rela-
tively insignificant role for the anisotropy at meso-
and macro-level. The folded structures and the faults
impact substantially the physical properties of the
rocks, and the resulting rocks anisotropy depends to a
great extent on the particular type of deformations and
the position of the studied site relatively to these
structures. The question about the ''scale effect'' is
also a complex phenomenon and the conformity with
it is an obligatory condition for realistic interpreta-
tions. For this reason, in geophysical practice, the
term ''apparent anisotropy'' is used, instead of the
term ''anisotropy'' applied mostly for the mineral
characteristics (Sheriff
1984
).
Two factors are extremely significant at all levels
of the studies—the fracturing and the tectonic stres-
ses. As the fracturing is predominantly a result of the
impact of the consecutive acts during the geological
time of tectonic stresses in the rocks, in fact every
study of rock anisotropy in situ could be also pre-
sented as a study of the history of tectonic stresses for
a particular site. The following working hypotheses
should be kept in mind, while the results and analyses
presented
rupture joint system. There is a reason to suggest
that after the first-joint system arises, every con-
secutive by time tectonic stress field will act on
already disrupted medium, and will have a weaker
deformation effect. The extensional fractures have
a definite advantage in anisotropy forming, and
their usage at paleotectonic stresses reconstruction
is grounded from other authors as well (for
example, Letouzey 1986; Caputo and Caputo
1988
). Practically, these extensional fractures are
presenting the preferential underground routes of
the water. The knowledge of the anisotropy in
karst areas can be used for determining the dom-
inant direction of the underground karst galleries.
4. In clayey rocks, marls, non-lithified sediments as
sands and their varieties, a well-expressed frac-
turing is not observed, or is missing at all. How-
ever, in such kind of materials a compaction along
the direction of maximum stress (at regime of
compression) or decompaction along the direction
of minimum pressure (at regime of extension) is
obtained. In both cases, the initial isotropy of the
rock material is disturbed.
One of the approaches for anisotropy determina-
tion in rocks is to investigate the physical parameters,
such as the electrical resistivity. Our experience has
provided a number of successful implementation of
this type of studies especially on karst terrains.
Some important basic principles are necessary to
be presented here for understanding the relationship
of the electrical anisotropy measurements and the
anisotropic characteristics of the rocks. Theoretical
and practical information can be found in many
handbooks in different languages. We can suggest the
textbook of Sharma (
1997
) presenting at compre-
hensible way the modern basis of the geoelectrical
survey, including theoretical principles, field proce-
dures and interpretation techniques.
The commonly used method for electrical mea-
surements is to drive an electrical current through the
ground and to measure the resulting potential differ-
ences on the surface. Electrically better or poorer
conducting layers deflect the current and distort the
normal potential. Thus, when measuring subsurface
variations in electrical resistivity it us possible to
detect the anomalous conditions and inhomogeneities
within the ground. Different schema of field mea-
surements and related specific methods of interpre-
tations
are
obtained
from
sedimentary
terrains
predominantly:
1. The fracturing of sedimentary rocks is due to the
tectonic
stress
fields,
acting
after
the
rocks
lithification.
2. All changes in the direction of principal normal
stress axes in space have been reflected in the rock
massifs through formation of new joint systems.
Our experience has shown that normally more than
three sequences of tectonic impacts are difficult to
be detected.
3. The anisotropy in the brittle rock varieties (lime-
stones, dolomites, sandstones) is due mainly to the
give
the
possibility
to
recognize
the
