Geoscience Reference
In-Depth Information
Fig. 2.13
Model of field
measurement of the electrical
resistivity of layered rocks
Two extreme case exist—when the array is parallel
to the layering (or to the extension of the inhomogene-
ities), i.e., b = 0, and when the array is perpendicular to
the layering (b = p/2). For the first case the maximum
apparent resistivity will be measured, because:
q
a
ð
0
Þ¼
practical cases, the longest axis of the electrical
anisotropy in horizontal plane is coinciding with the
dominant direction of the tensile fractures, normally
controlling
the
general
orientation
of
the
karst
galleries.
As example, a study in karst area in Bulgaria is
presented here.
p
q
l
:
q
t
¼
q
max
:
ð
2
:
2
:
16
Þ
For the second case the apparent resistivity is
lower
2.2.3.1 Rumiantsevo Region (Central North
Bulgaria)
The investigation aimed to study the space position of
3 m high karst cavity crossed by an exploration well at
a depth of 19.6 m below the surface. The studied area is
located near the village of Rumiantsevo (North Bul-
garia). This is a potential area for a future mining of
limestones from Kailaka Formation. It is built of white,
massif, and organic limestones with various biological
inclusions. The age is considered to be Upper Maes-
trichtian. Limestones are easily karstified near the
fractured zones and faults. The region of nearby Kar-
lukovo village is known with its numerous surface
negative karst forms and many caves (Fig.
2.14
).
Above-mentioned modification of Vertical Elec-
trical Sounding (VES) method was used to applying
an azimuth scheme on the site where Well No.10
crossed an underground cave. An important data on
the electric anisotropy of the rock at different depths
and prevailing direction of the fractures could be
obtained by using this scheme of measurements. VES
1 is coinciding with the wellhead. Knowing the real
depth of the layers and the crossed cavity, the survey
at this site was as a ''parametric'' VES, because the
information from the well was used for calibration of
the rock resistivity (Fig.
2.15
). When fixing the
known depths, it was possible to evaluate the real
pseudo-resistivities of the cross-section. Nevertheless,
the
p
1
þ
k
2
1
q
l
q
t
q
a
p
=
2
ð
Þ ¼
q
sin
2
a
q
max
¼
q
1
þ
k
2
1
:
ð
2
:
2
:
17
Þ
sin
2
a
Here k is the coefficient of macro-anisotropy,
because it characterizes the integral anisotropy of the
studied rock volume and it is k [ 1:
r
:
q
t
q
l
k
¼
ð
2
:
2
:
18
Þ
It is evident, that q
a
ð
0
Þ
[ q
a
ð
p
=
2
Þ
. The measured
apparent resistivity parallel to the elongation of the
inhomogeneities is higher then perpendicularly to
them. So, this is the ''paradox of the anisotropy,''
because
in
reality
the
transversal
resistivity q
t
is
higher than the parallel resistivity q
l
.
In practice, using arrays with different azimuths
around a given point and with different distance
between the electrodes A and B, it is possible to obtain
the electrical anisotropy ellipse for different depths
(Azimuthal Vertical Electrical Sounding). For differ-
ent depths (different stratigraphic layers), the ellipse
of the electrical anisotropy can be obtained, which is
closely related to the rock bedding and fracturing
through the ''paradox of anisotropy.'' In most of the
automatic
interpretation
of
the
curve
by
the
