Acta Polytechnica CTU Proceedings


https://doi.org/10.14311/APP.2023.45.0020
Acta Polytechnica CTU Proceedings 45:20–26, 2023 © 2023 The Author(s). Licensed under a CC-BY 4.0 licence

Published by the Czech Technical University in Prague

GEOPHYSICS FOR UNDERGROUND ENGINEERING

Pavel Bláha∗, Roman Duras, Jan Gebauer

GEOtest, a. s., Šmahova 1244/112, 627 00 Brno, Czech Republic
∗ corresponding author: blaha@email.cz

Abstract. The growth of underground engineering brought a need for the use of geophysical
methods for research of underground constructions rather different in approach compared to that of civil
engineering. These changes relate to surface measurement on the one hand and the development of new
specialised methods used for underground work on the other. The results of a complete documentation
of mining work, the application of the resistivity methods used underground and also the use of 3D
electrical resistivity tomography, are all presented here.

Keywords: Geophysics, underground structure, ERT.

1. Introduction
Using 2D and 3D methods in geophysical surveys is
very enticing, but we must realise that even the latest
mathematical procedures and possibilities of electron-
ics have their limitations. In this paper we want to
point out some problems in geophysical measurement
carried out underground, including interpretation of
2D and 3D geo-electrical measurements. The paper
presents the results of measurement in one mine work-
ing, between mine workings, and also between mine
workings and boreholes.

2. The issue of measurement and
interpretation of 2D and 3D
ERT

For a greater objectivity regarding interpretation of
the results of geophysical measurements, it is always
suitable to process the given measured data in several
ways. The same also applies to the measurement
of electrical resistivity tomography. Figure 1 shows
an example of how to interpret ERT measurement
using the RES2DINV and ZondRes2D programs. The
example manifests simple geological conditions in a
survey for the construction of Motorway D35 at the
municipality of Sedliště, particularly from profile 41–
41’. The area of interest is formed by sedimentary
rocks of the Bohemian Cretaceous Basin typical of
claystone, marlstone, etc., covered by soft Quaternary
sediments typical of loess, loess loam, etc.

The results of interpretations from both the pro-
grams are not identical, but they are similar. It can be
declared without problems that the setup of iterative
parameters in both the programs brings about larger
resistivity changes than by comparing the results of
both the programs mutuall [1]. In both interpreta-
tions it is possible to distinguish a near-surface layer
of loams without any difficulty. The base of loams
is located at a depth of about 7 m according to the
RES2DINV program, and at a depth of 11 m accord-
ing to the ZondRes2D program. Based on the result

of drilling work, the boundary is defined at a depth of
about 12 m, i.e., the interpretation comes out better
from the ZondRes2D program. It is more complicated
with the determination of the surface of Cretaceous
clayey sediments. This geological boundary has not
been determined by direct survey work (it was termi-
nated in the Quaternary), but only by the results of
vertical electrical sounding (VES). Based on VES, the
boundary is located at a depth of 25 to 35 m. Accord-
ing to the results of RES2DINV, it is found at 30 m,
and according to ZondRes2D, at about 35 m. Both the
programs indicate a horizontal change in resistivity
values suggesting a facies change of sediments in the
second layer as well as in the third layer. This change
is better evident in the uncut resistivity cross sections
in the lower part of the figure.

The conditions for measuring ERT in other figures
(the site of PVP/underground research facility/Bukov,
testing chamber ZK-1) were opposite to that in the
preceding case. Whereas in measurement from the
survey for the motorway it can be said that the condi-
tions were ideal (measurement took place on natural
ground with normal moisture content, so the contact
resistance values on electrodes ranged from about
200 to 1 110 Ω), underground measurement was much
more complicated. From the monitoring of contact
resistance values, it was detected that the contact
resistance values between the electrodes and the rock
massif were significantly higher. Specifically, in the
studied profile (lower right, PD) contact resistance val-
ues in 2019 changed from 3 000 to 120 000 Ω. During
processing, the results of such high contact resistance
values on these electrodes were eliminated from further
processing.

When using electrical resistivity tomography under-
ground, we still verified the effect of the origin and
version of the interpretation program on the resulting
resistivity field. We carried out an extensive analysis
on measurements at the beginning of cross-cut BZ-
XIIJ on its left side just behind the beginning of the
research facility, where copper electrodes of 10 mm

20

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vol. 45/2023 Geophysics for Underground Engineering

Figure 1. Comparison of ERT interpretation using the RES2DINV and ZONDRES 2D programs.

in diameter were used and sunk into the side of the
mine working to a depth of 5–10 cm. The resistivity
fields detected by different versions of the program
are given in Figure 2.

As in the preceding example from measurement in
ideal conditions, the basic geological structure identi-
fied by both the programs is the same. But the details
of each anomaly are different not only by areal exten-
sion, but also by the values of resistivity. In addition to
the basic environment designated “A”, five anomalies
“B” to “F” were detected using the ZondRes2D pro-
gram; the RES2DINV program identified six anoma-
lies “B” to “G”. Anomalies “B” to “G” have relatively
small dimensions and their maximum resistivity values
range between four and six thousand Ω m in the Zon-
dRes2D program, whereas in the RES2DINV program
they range between four and forty thousand Ω m. The
area of high-resistivity anomalies is approximately the
same in both the versions. An exception is the lower
limitation of anomalies “B” and “D”. A larger differ-
ence is in their maxima. In the “Russian” version
it is 4 200, 5 700, 5 300, 4 000 and 6 000 Ω m and in
the “Danish” version it is 4 500, 4 200, 5 500, 7 000
and 30 000 Ω m. The basic environment “A” in the
“Russian” version is visually more homogeneous than
in the “Danish” version. This is due to the fact that

the blue colours of the low-resistivity environment of
the “Russian” version, in a range of 500 to 1 700 Ω m,
are practically indistinguishable to the human eye.
The basic environment has the values of resistivity
in the “Russian” version 500 to 1 700 Ω m and in the
“Danish” version 400 to 1 500 Ω m.

3. Measurement and
interpretation of ERT in 2D
and 3D versions underground

The first example of the use of electrical resistivity
tomography in the 3D version is from measurement in
testing chamber ZK-1 at the site of PVP Bukov. In
this chamber, several stages of repeated measurements
were carried out, with six profiles being measured: two
of them always on the sides of the chamber, one on the
footwall of the chamber and one on its top wall. The
profiles were measured both separately and on the
opposite profiles, i.e., one series of electrodes was, for
example, on the lower left profile (LD) and the other
on the upper right profile (PH). In all, measurement
in testing chamber ZK-1 took place five times, in
January 2016, then in April 2017, in September 2019,
in February 2020 and in May 2021. For those in
September 2019 and in February 2020, measurement
took place on the lower right profile (PD) always three

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P. Bláha, R. Duras, J. Gebauer Acta Polytechnica CTU Proceedings

Figure 2. Comparison of the results of ERT interpretation using RES2DINV and ZondRes2D programs in ZK-1.

Figure 3. Resistivity cross sections through testing chamber ZK-1.

times immediately one after the other. The results
of measurement from 2019 processed by the GENIE
program for 3D interpretation and put together at
the Technical University of Liberec, are depicted in
Figure 3.

The interpreted resistivity values range from 200
to 25000 Ω m. The interpreted resistivity values are
unusually (even improbably) high. On two sub-
horizontal cross sections and on one vertical cross
section, we can distinguish eight blocks. Blocks Y and
Z lie at the margins of the measured space and cannot
be considered as reliable. The reason is a very low
number of measured points from which it is possible
to count resistivity values. These blocks cannot be
assigned a geological meaning. The other blocks by
their position allow us assign their geological, or more
precisely, their geotechnical meaning.

4. Measurement and
interpretation of ERT in 2D
and 3D versions underground

The first example of the use of electrical resistivity
tomography in the 3D version is from measurement in
testing chamber ZK-1 at the site of PVP Bukov. In
this chamber, several stages of repeated measurements
were carried out, with six profiles being measured: two
of them always on the sides of the chamber, one on the
footwall of the chamber and one on its top wall. The
profiles were measured both separately and on the
opposite profiles, i.e., one series of electrodes was, for
example, on the lower left profile (LD) and the other
on the upper right profile (PH). In all, measurement
in testing chamber ZK-1 took place five times, in
January 2016, then in April 2017, in September 2019,

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vol. 45/2023 Geophysics for Underground Engineering

Figure 4. ERT cross sections perpendicular to ZK-1.

in February 2020 and in May 2021. For those in
September 2019 and in February 2020, measurement
took place on the lower right profile (PD) always
three times immediately one after the other. The
results of measurement from 2019 processed by the
GENIE program for 3D interpretation, put together
at the Technical University of Liberec, are depicted
in Figure 3.

The interpreted resistivity values range from 200
to 25 000 Ω m. The interpreted resistivity values are
unusually (even improbably) high. On two sub-
horizontal cross sections and on one vertical cross
section, we can distinguish eight blocks. Blocks Y and
Z lie at the margins of the measured space and cannot
be considered as reliable. The reason is a very low
number of measured points from which it is possible
to count resistivity values. These blocks cannot be
assigned a geological meaning. The other blocks by
their position enable us to assign their geological or
more precisely, their geotechnical meaning.

Block A lies on the right northern wall of the cham-
ber. This block can be considered as a block of rocks
in natural state, not being affected in any way by
mining work or any other investigation that could
have taken place in this space. Its resistivity values
range between about 10 and 25 thousand Ω m. Block
B can also be characterised similarly, being divided
into two sub-blocks B1 and B2. Sub-blocks B1 and
B2 are probably separated by a discontinuity, or by
a conductive zone of fracturing of the rock mass (δ).
This zone is also manifested on the right side as a
boundary between blocks A and Z. The zone of frac-
turing defined by resistivity probably corresponds to
joints of the NNW-SSE to N-S strike. According to
all three resistivity cross sections, one mega-block can
probably be defined, comprising blocks A, B and C
and corresponding to the original rock mass.

Interesting information can be obtained from cross

sections drawn perpendicular to the axis of ZK-1 (Fig-
ure 4). The cross sections were constructed only on
the basis of ERT measurement in September 2019. In-
terpreted resistivity values in the cross sections range
from 300 to 15 000 Ω m. The cross sections were con-
structed under the stations of 7.5 and 9.5 m perpendic-
ular to the axis of the working. The most interesting
on the perpendicular cross sections are anomalies of
low resistivity values above the top wall of the cham-
ber and beneath its footwall. These low-resistivity
anomalies indicate the places of the massif loosening
after the chamber excavation and the re-distribution of
the stress field. We proceed from an assumption that
the micro-fissures were formed in the loosened massif
and filled by groundwater. The moisture content in
the massif may not be at all high. It is sufficient for a
decrease in rock resistivity when the fissures create a
conductive network.

The drop in resistivity values is different above the
top wall of the chamber and beneath its footwall. The
explanation must be found in the moisture content of
the rock above and beneath the working. It is possible
to expect full saturation of fissures in the massif by
groundwater beneath the footwall of the chamber,
whereas above the top wall there remain places in
which pores in the rock are not fully saturated. This
fact is shown by small anomalies of low resistivity
values, whereas beneath the footwall the decrease in
resistivity values affects the whole massif concerned.

If we proceed from a generally accepted assumption
that the fractured massif in the surroundings of the
stope has the form of an ellipse in the perpendicular
cross section [2], then it is possible to estimate what
the ellipse definition of the fractured massif will look
like in the vicinity of ZK-1. It is interesting that the
ellipses do not have their long axes vertical, but that
the axis of the ellipse above the top wall is turned to
the north. On the cross section at 7.5 m, the ellipse is

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P. Bláha, R. Duras, J. Gebauer Acta Polytechnica CTU Proceedings

Figure 5. Cross sections from ERT between boreholes S1 and S8.

turned by about 25 ◦ and on the cross section at 9.5 m
by 10 ◦. In relation to the possibilities of the method,
the given numbers cannot be taken as absolute num-
bers, but it can be stated that the ellipse is turned by
roughly 15 ◦ to the north. Similarly, the stress should
also be turned. Interesting is the ratio of semi-axes of
the ellipse. The resistivity data show that the ratio
of semi-axes of the outlined ellipses of fracturing is
about 1:2.

Another testing of possibilities of 3D ERT and a new
program for 3D interpretation of ERT (GenieERT)
took place on the results of measurement between
boreholes S1 and S8 at the same site. In this case, all
electrodes were placed in the borehole. In relation to
the conditions of measurement, it was an ideal situa-
tion. The electrodes in the borehole have a minimum
contact resistance that changed from 930 to 6 500 Ω.

The comprehensive interpretation of measurement
in boreholes S1 and S8 comprises the processing of
electrical resistivity tomography (2019), resistivity log-
ging (carried out by Aquatest, a.s.) and the resistivity
values obtained from ERT. The grey curve depicts
the interpreted resistivity values of the first series of
ERT elements from the sections of one metre size.
The outcome of this comparison of all results is shown
in Figure 5. The interpreted resistivity values range
between 300 and 20 000 Ω m. On the cross section
between boreholes S1 and S8, it is possible to iden-
tify four blocks A to D2. The high resistivity values
characterise blocks A and B. The other radiographed
area is characterised by low resistivity values. Blocks
A and B are of special significance, differing from the
other blocks by their area and the size of the maxima
of resistivity values, which reach about 20 000 Ω m.
The delineation of the blocks is also evident in the

results of down-hole logging. These anomalies consti-
tute blocks of rocks which are not practically affected
by fracturing. The pictures of distribution of the in-
terpreted resistivity values obtained in 2019 and 2020
bring factually the same results. Certain differences
can be found in the form of anomalies between 2019
and 2020. The anomalies in 2019 are in area larger
and the margins of anomaly A in 2019 are not so
rugged. Anomalies C and D in 2019 are smaller than
was the case in 2020.

5. Physical documentation of mine
walls

Below the RQD output in Figure 6, graphs of the
results of measurement using “speedy” methods are
presented one after another, i.e., measurement of tem-
perature, radioactivity and magnetic susceptibility.
The results of “speedy” methods provide valuable re-
sults, particularly in comparison with other measure-
ments, e.g., resistivity methods. On the curves of dose
rate, certain increases of values are evident, located
under the stations between about 9 and 18 m, having
a significant response in ERT measurement (the last
graph). The extent of the “radioactive” anomaly is
very close to the extent of the resistivity anomaly. An
interesting fact appears that a local significant de-
crease in the values of interpreted resistivity occurred
in the inflection point of the resistivity elevation (un-
der the station of about 13.5 m). Another interesting
fact is a correlation of magnetic susceptibility with
ERT under the stations of about 3.5 and 17 m, which
have a response in resistivity minima on the curves of
ERT measurement. Whereas the geomagnetic anoma-
lies are in.

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vol. 45/2023 Geophysics for Underground Engineering

Figure 6. Physical documentation of a mine wall.

Figure 6 presents the results of 2D and 3D geophys-
ical and geotechnical measurements on the wall of
chamber ZK-1. The first graph illustrates the mechan-
ical state of the rock environment on the left side of the
working, represented by the RQD (Rock Quality Des-
ignation) coefficient, and also expressed in the form
of the frequency of fissures per the unit of distance.
Both measurements were conducted by geomechanics
from the Institute of Geonics of the Czech Academy
of Sciences (IG CAS). The RQD coefficient expresses
the quality of rock and is usually determined on drill
cores. Geomechanics of the IG CAS developed proce-
dures to determine RQD from measurement on the
surface of mine workings. When viewing the shape of
curves, it is evident that the quality of the measured
rocks is good to excellent. An exception is a section
under the stations between about 7.5 and 9.5 m, where
the value of RQD diminishes from about 95–100 % to
about 55 %, i.e., the quality of rocks is classified by
a medium grade. The given fact probably indicates

the existence of structural discontinuities. The RQD
value 55 accompanies a higher frequency of fissures
(joints).

Relatively narrow, the decrease in apparent resis-
tivity values is more gradual and more extensive in
the measured line. It is possible that this concerns
the manifestations of local lithological changes (veins
penetrating the base rock perhaps?), but for explana-
tion it is again necessary to have more information of
a non-geophysical character.

The next-to-last graph in Figure 6 shows graphs
of already mentioned resistivity measurements.
Specifically, these are profiling curves of spacings
A0,5M0,25N0,5B; A1M0,25N1B; A2M0,25N2B. The
data were extracted from a cloud of values obtained by
the ERT method, carried out with a detailed interval
(0.25 m) and to a sufficient extent (about 22 m). The
curves of resistivity profiling enable the basic quasi-
homogeneous blocks to be delineated; they are charac-
terised by the extent and value of apparent resistivity.

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P. Bláha, R. Duras, J. Gebauer Acta Polytechnica CTU Proceedings

The resistivity curves are similar by their character
to the curves obtained from the measurement of dose
rate, where a small difference was recorded only in the
initial part of the profile – a local minimum under the
station of about 2.5 m on the curve of dose rate has
the equivalent shifted to the station of about 3.5 m
on the resistivity curves. This difference is probably
determined by the oblique course of the plate (vein),
which then causes the anomaly. It is necessary to
realise that the dose rate characterises the space the-
oretically to 0.5 of a metre, but in reality even less.
The range of geoelectrical measurement can be many
times higher. The remaining sections correlate very
well.

In the last cross section, two resistivity curves are
plotted, each of them from a different depth level,
indicating the assumed state of the rock environment.
The grey curve reflects the resistivity situation at the
level to 0.25 m behind the wall of the working and
the black curve to 1 m behind the wall of the working.
The given values, however, are only indicative because
the determining parameters are the size and shape of
cells from the 3D model, which are not the same. The
displayed data, however, are sufficiently representative
for orientation and the basic distribution of the values
of resistivity along the wall of the working.

6. Conclusions
The comparison of programs for 2D interpretation
of electrical resistivity tomography (RES2DINV and
ZondRes2D) has brought forth a number of interesting
findings. We proceeded from our previous knowledge
that the resulting resistivity field was not only a pic-
ture of distribution of resistivity values in the rock
mass, but it could also be affected by the setup of
input parameters for iterations. We have obtained
for testing the ZondRes2D program in a functional
demo version, and thanks to the cooperation of Czech
and Uzbek specialists, a possibility has arisen to use
both the programs to their full extent [3] and [4], Dur-
ing testing work, it appeared that it was possible to
change the resulting resistivity field, similarly as in
the RES2DINV program, by setting up parameters in
the ZondRes2D program as well.

When evaluating digital programs of interpretation,
we must be aware that we are trying to solve math-
ematically a not quite correct problem. In this case,
we determine a substantially larger number of param-
eters than the input data we actually have. When
comparing the results of both the programs, we began
with the interpretation of results in simple geological
conditions. In this case, the results were satisfactory
because the differences in interpretation using both
the programs were comparable with the differences
which are obtained when changing the setup of input
parameters of iteration. In more complicated sur-
face conditions, the differences in interpretation were

higher, but still comparable with changes given by the
setup of programs. The largest difference occurred
in the application with mine conditions. There, the
differences between both the programs even increased,
and we even recorded an example where interpretation
by the ZondRes2D program was more reliable and the
calculated model approached the reality more than in
the case of the output from RES2DINV.

Geophysical measurement using the system of 3D
and its 3D evaluation brings quite new possibilities
when describing the rock mass in general and this also
applies to special conditions underground. In mine
workings, the possibility to choose the arrangement
of profiles and points of measurement on profiles is
limited as compared to surface applications of geo-
physical measurements. To find a suitable system
of points measurement is usually very difficult and
requires many compromises. An advantage of 3D
processing of geophysical measurements is the spatial
information on the geological structure of the area
of interest. A disadvantage is the problem of obtain-
ing a sufficient number of high-quality data for a 3D
interpretation to bring reliable results. This paper
describes several successful examples of the use of
3D geophysical measurement in different lithological
types of rocks.

Acknowledgements
Geophysical work published in this paper was largely car-
ried out in the underground research facility (PVP) Bukov.
It was constructed, and is operated by, the Radioactive
Waste Repository Authority (SÚRAO). To accomplish any
activity in the spaces of the deep mine is relatively costly,
complicated and requires a coordination of many profes-
sions and activities (working facilities, technical support,
etc.). Thanks to the coordination with SÚRAO, which sup-
ports research activities in PVP to the maximum extent
possible in the form of technical support and by providing
services, it was possible to manage very well the planned
measurements and all the other activities. For all this our
thanks and appreciation are due to SÚRAO.

References
[1] P. Tábořík, J. Gebauer, R. Duras, P. Bláha. The

mystique of ERT. Exploration Geophysics, Remote
Sensing and Environment (XXIV.2):29–39, 2017.
https://doi.org/10.26345/EGRSE-028-17-204X

[2] G. Lanyon. Excavation damaged zones assessment.
Report of Fracture Systems Ltd–NWMO
DGR-TR-2011-21, 2011. 95 p.

[3] P. Bláha. Verified technology, Development of
geotechnical and geophysical methods for obtaining 2D
and 3D images of the geological structure. GEOtest a.s.,
2021. 80 p.

[4] P. Bláha, R. Duras, J. Gebauer, et al. Development of
geotechnical and geophysical methods for obtaining 2D
and 3D images of the geological structure. Final Report,
GEOtest a.s., 2021. 53 p.

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https://doi.org/10.26345/EGRSE-028-17-204X

	Acta Polytechnica CTU Proceedings 45:0–0, 2023
	1 Introduction
	2 The issue of measurement and interpretation of 2D and 3D ERT
	3 Measurement and interpretation of ERT in 2D and 3D versions underground
	4 Measurement and interpretation of ERT in 2D and 3D versions underground
	5 Physical documentation of mine walls
	6 Conclusions
	Acknowledgements
	References