54-th Meeting of European Association of Exploration Geophysicists.
June 1-5 Paris.
FIELD OF AN UNDERGROUND POINT CURRENT ELECTRODE.
Alfred FRASHERI
Polytechnical University, Faculty of Geology and Mining.
Tirana ALBANIA
ABSTRACT
The borehole underground IP survey presents one of the main
directions to increase the depth investigation of
Electrical Prospecting for copper ore deposits. The IP anomal
effects are strongly increased when the current source is
settled close to the underground polarizable ore bodies.
Moreover the outlook of spatial distribution and the
intensity of this effect are quite different in comparison
with cases when current electrodes are settled into earth's
surface.
Through mathematical modelling and checking up the
experimental results in physical models in laboratory and in
situ, over the known geological conditions, the study of this
effect is presented. The measurements were carried out with
IPR- 10A, IPR-11 receivers and IPC-7/15 KW transmitter
(SCINTREX) in Time Domain. The models has been carried out
for any geometrical body shape with the same resistivity as
surrounding medium. One of the current electrodes was placed
underground, in borehole or mine works, while the other one,
on the surface. Calculation of Bleil's integral has been
performed utilizing some notions of the finite element
method.
From the modelling has been defined that IP anomaly is
accentuated many times when one of the current electrodes is
placed underground, against the ore body, in comparation with
the case when this electrode is situated on the surface. This
is particulary evident for bodies at great depth (600-800 )m.
1. INTRODUCTION
Albanides are part of the Mediterranean Alpine orogenic
belt. Those extend in Albanian's territory and are placed
between Dinarides and Helenides. The ophiolites are one of
the most important elements of this belt, covering a
territory of 2600 km. Within ophiolites a lots of chrome and
copper ore deposits are found. Their exploration in Albania
has reached up to 1000 m deep and more. The increasing of
depth exploration from 200-300m up to 600-800 m through
geophysical prospecting has became a necessity in Albania.
One of ways to reach such a great depth exploration in
electrical prospecting is the studing of the anomal effect of
IP method setting one or both current electrodes in
boreholes. Through such settlements the current source
approches to the ore body and IP effect is obviously
increased.
To investigate this kind of IP surveys we used the
mathematical models. For measurements in terrain, powerful
transmitters and high sensitivity receivers were necesary,
so we used IPC-7/15Kw transmiter and IPR-10A and IPR-11
receivers (SCINTREX). The last one permited us to obtain
spectral IP parameters as well (Alikaj P. 1989).
The same instrumentation was used in IP electrical
soundings (IP-ES) with long spacing (up to AB=4400m), results
of which were coordinated with hole-hole or hole-surface
arrays.
2. MATHEMATICAL MODELLING.
Mathematical models for anomalous effects of resistivity
and IP are carried out using different ways and algorithms.
Das V.C. and Parasnis D.S. (1987) have used the solutions of
Fredholm's integral equation of the second kind. The results
are presented for a dipole-dipole array. Eskola L. et al.,
(1984) have done their modellings in the Frequency Domain.
The integral in this case is solved by means of the method of
subsections.
But the modellings of anomalous effects for the surveys
with underground arrays begin with the study of Waag D.M. and
Seigel H.O. (1983) where beside the mathematical models the
results of field surveys are given. For these models there
are papers from Komarov V.A. (1972) and Draskovits P. and
Simon A. (1992).
In order to fulfill the demands of the development of
prospectings for copper ore deposits situated at great depths
up to several hundred meters we realised another mathematical
modelling for the anomalous effects of IP in the time Domain
by using the method of finite elements, or even only its
principles. This modelling was done for surveys with gradient
array.
The mathematical modelling was carried out to study the
anomal effect of IP caused by ore bodies of any geometrical
shape.
The ore body was supposed to have the same electrical
resistivity (o) as the surrounding rocks. Such ore bodies are
for example disseminated sulphides or chromites which contain
secondary magnetite. These ores have a volume polarizability
higher than the surrounding rocks. Chargeability has a value
n. The relief was supposed to be flat. One of the current
electrodes is situated underground in the point A with
coordinates (X ,Y ,h) at a depth h from the surface. The
other electrode is settled at point B (X ,Y ,0) on the
surface. The IP anomal effect was analysed by determining
the potentials of the IP electrical field and the polarizable
field in the points M and N which are moved on the surface
(Fig. 1).
The induced polarization effect Uip is calculated with
the well known formulae (Bleil D., 1953; Seigel H.O., 1059):
ō 1
Uip=C³VU.VÄdv (1)
õ R
V
where:
Uip - the potential of induced polarization field,
Uo - the potential of primary electric field,
U=Uo+Uip - the potential of resultant electric field,
R - the vector from body point to measurement point,
C - a constant value determined by electric properties of
medium.
For the 3D models, because we considered that the ore
body and surrounding rocks have the same electrical
resistivity, the integral depending from Uo can be calculated
directly by discretizing the surface of the body with small
elements. In the 2D case, when the chargeability is a
voluminous one, the potential Uo is calculated by the finite
element method, as well as VUo and the vector of
chargeability C.VUo .
It is known that in field conditions of electrical
survey we have Uip< . The results of physical
modelling are shown in fig.4. It is clearly seen that they
are a good proof for the results of mathematical modelling.
The position of current electrode A in relation with the
target determines the anomaly amplitude (fig.4,5). The
highest amplitude is observed in cases when current electrode
A is placed in front of the middle of the target and the
lowest ones when this electrode is on both edges of the
target. Based on this fact a methodoligical conclusion may
be drawn: the measurements should be carried out for
different depths of the current electrode in borehole,
because the optimal depth of the target is not known. If the
underground electrode is situated in face of the middle of
the target, but in variable distances from it (the positions
3,4 of the electrode), it is seen that there are anomalies
and for considerable distances from the target, about 300m
for the given model, or until the ratio l/d÷0.7. In longer
distances the anomaly becomes indistinguishable.
As for the all types of geophysical anomalies, and for
the IP anomalies in the case we are studing, the amplitude
depends on the dimensions of the target as well. The anomaly
is distinguishable and when the target has a ratio l/h÷1/4.
For the ratios l/h÷1/10, the anomaly becomes negligible.
On map, the IP anomaly is extended out of the target's
extremities (Fig. 6), but in these sectors the anomaly
configuration is different from that over the target: the
positive part of anomaly is diminished and the amplitude of
negative part becomes higher. In cases when the target is
located out of the the current dipole (AB) the anomaly
presents the highest amplitude on target's edge. The anomaly
is more intensive in the central survey line when the target
occurs between the current electrodes. During interpre-
tation, one should consider the extention of anomaly in
dependence on the electrode array position in relation with
the target. The target is located sidelong of the positive
epicenter of anomaly but inside its
negative part
It is very important to discover rapidly in which side
of the borehole is situated the ore body. To solve this
problem two pairs of potential electrodes M1,N1 and M2, N2
are fixed on the surface and two sets of measurement for
variable positions of the current electroe (A) in borehole
are carried out. The anomaly recorded with potential dipole
over the target is more intensive and without negative
sectors (Fig. 7). Through this methodology is determinated as
well the depth of location of the current electrode in front
of target and the position of the second current electrode on
the surface for the future IP ground survey.
5. CASE HISTORIES.
The control of the mathematical and physical modelling
results were carried out in our field experimental surveys
(Avxhiu R. 1989, Frasheri A., Avxhiu R., Alikaj P. 1990).
In Fig.8 is presented a geological section where the
drillhole has intersected the volcano- sedimentary series in
which is located the sulphide mineralization (Alikaj P.1989).
IP anomal effect is negligible when the current electrodes
are placed on the surface. it becomes intensive when the
current electrode A is immersed on a borehole, at depth of
620 m. In this case, the anomal effect is caused by sulphide
mineral zone intersected by borehole. This is also proved
through the measurements carried out with fixed potential
dipole ( M, N ) on the surface and moving current electrode
on the ground. The mineralized zone is reflected in the IP
anomaly from the depth 320 m and its highest amplitude
reaches at depth 620 m, where the highest sulphide grade
intersected by borehole ocurrs.
The case shown in fig.9 is more complicated. The surface
measurements carried out with gradient array with spacing
AB=3000 m, MN=100 m present an anomal sector (plot 1),
beginning from the station 60, where the polarizable
serpentinites outcrop, up to the proximity of the station 92,
in limestones rocks. A local anomaly is fixed between the
stations 78- 92, over the volcano- sedimentary series. The
measurements have been repeated with the current electrode A
placed on the ground at the depth 212. At the station 82 a
minimum of chargeability ( M3 ) was obtained (plot 2). The
maximum of M3 on the left should be related with the presence
of serpentinites. The maximum on the right would be a
sulphide ore body at depth. To verify this interpretation
some other boreholes were projected. In this Figure the
profile of chargeability M3, recorded by the fixed potential
dipole on the surface and by moving on the ground the current
electrode., is shown too (plot 3). This profile presents an
anomaly at depth 170- 220 m.
We have also carried out borehole IP measurements with
array MNA for spacing AM = MN = a = 2.5m, 5m, 10m, 20m, 40m
(Langora L. et al. 1989). Based on mathematical models a
depth investigation of such array was carried out. In fig.10
there are presented the results of such measurements over a
geological section, where massive copper sulphide ore body,
related with diabase rocks is located near the tectonic
contact with serpentinized hatzburgites. The IP contours very
cleary outline the ore body tought by fault tectonics.
Based on above mentioned treatment one may draw into
conclusion that IP surveys carried out with current
electrodes placed on the ground is an effective way to
increase the depth exploration of polarizable targets. Of
course, these surveys need for powerful transmitters and high
sensivity receivers. In our studies these requests were
properly fulfiled by IPC-7/15 KW transmitter and IPR-10A or
IPR-11 receivers, which we used both in borehole-surface
measurements and in deep IP ground surveys with spacing up to
AB=4000 m. This combination of the ways to inccrease the
depth of investigation has shown good results (Avxhiu R.1989)
In fig.11 there is presented an electrical Real-section
in one of copper sulphide deposits in Albania, together with
IP contours carried out with gradient arrays of different
spacings (Alikaj P.1989). Here the T=4 sec, t=2 sec and a
current of 11 Amps were used. The surveys were done using
three gradient arrays with lengths of 600m, 1200m and 2000m.
As it is seen in fig.11, the short array was used to
investigate in a depth of 75-100m and with an IP
chargeability M3=6-10mV/V the western edge of the upper
mineralized level was fixed. When the array is increased up
to 2000m, the depth of investigation is up to 300-350m and
the anomaly with a chargeability over 16mV/V with epicenter
in the point 108 was interpreted as connected with a deep
mineralized zone. The borehole projected over that anomaly
met this zone.
In the fig.12 there is given another electrical section
with IP contours carried out with deep IP electrical
soundings (Avxhiu R. 1989, Avxhiu R. et al. 1989). The deep
sondings of IP carried out using arrays with a length up to
AB=6000m have a depth of investigation up to 800-1000m. The
anomalies with a chargeability over 24mV/V were fixed over
profiles Pr.2-Pr.4 at the depth. The borehole DH-6 over this
anomaly met with the mineralized zone at depth of 520m.
CONCLUSIONS
1. The IP anomal effect is strongly amplified if one of
the current electrodes is placed in borehole, because the
current flow density, passing through the ore body is increa-
sed.
2. The configuration of anomaly is determined by the po-
sition of the electrical array in relation with ore body
and by its spatial position .
3. The IP anomaly, in plan contours, is longer extended
than the target's edges. But, however, its character is
different from the target projection.
4. The underground IP survey is one of the ways of in-
creasing of depth exploration, at least up to depth 600-800m.
5. Our POLARELF-F (POLARELF-3), POLARPRIZ-2, POLARPRIZ-
ZP and POLARPRI-3P programmes allow an accurate calculation
of IP anomal effect of the polarizable targets of any shape
with the same resistivity with surrounding rocks, for
underground current electrodes.
REFERENCES
Alikaj P. 1989. The study of Spectral IP characteristics in
the search for rich sulphide ores. M.Sc.Thesis.
Polytechnic University of Tirana, ( in Albanian ).
Avxhiu R. 1989. A study on the ways of increasing of the
depth exploration for copper sulphides through the IP
method in the northen and central part of Mirdita tec-
tonic zone. Ph.D. Thesis. Polytechnical University of
Tirana, ( in Albanian )
Avxhiu R., Frasheri A., Zajmi A., Alikaj P. 1989. Some direc-
tions on the prefection of the electrical methods for
the prospection of copper sulphide ores. Bulletin of
Geological Sciences No. 4, pp. 213- 221. In Albanian,
summary in English.
Bleil D. 1953. Induced polarization: a method of geophysical
prospecting. Geophysics 18(3), pp. 636- 662.
Das V.C. and Parasnis D.S. 1987. Resistivity and induced
polarization reponses of arbitrary shaped 3-D bodies
in a two layered earth. Geophysical Prospecting 35,
pp.98-109.
Draskovitz P. and Simon A. 1990. Application of geoelectrical
models using buried electrodes in exploration and
mining. Geophysical Prospecting 40, pp.573-86.
Eskola L., Eloranta E. and Puranen R. 1984. A method for
calculating IP anomalies for models with surface
polarization. Geophysical Prospecting 32, pp.78-87.
Frasheri A. 1987. The study of the scattering of electric
field in heterogeneous media. In Albanian. Ph.D.
Thesis. Faculty of Geology and Mining, Polytechnic
University of Tirana, Albania.
Frasheri A, Avxhiu R., Frasheri N. 1987. The influence of
current electrode position in connection with the ore
body in the configuration of IP anomalies in seach for
copper and chrome mineralizations. Bulletion of
Geological Sciences No. 3, pp. 143- 154. In Albanian,
summary in English.
Frasheri A. 1989. An algorith for the mathematical modelling
of the IP anomal effect over the rich copper ore bo-
dies of any geometrical shape. Bulletin of Geological
Sciences No. 1, pp. 115- 126, in Albanian, summary in
English.
Frasheri A., Avxhiu R., Alikaj P. 1990. The modelling of
anomalous effect of IP over a target placed in the
electrical field of a point source. Bulletin of
Geological Sciences No.1, pp.135-46 (in Albanian,
summary in English).
Kamarov V.A. 1972. Electrical Prospecting for Induced Pola-
rization method. In russian. Published by Njedra.
Langora Ll., Alikaj P., Gjevreku Dh. 1989. Achievements in
the copper sulphide exploration in Albania with IP and
EM methods. Geophysical Prospecting No.37, pp.975-993.
Seigel H.O. 1959. Mathematical formulation and type curves
for induced polarization. Geophysics 24, pp. 547- 565.
Waag D.M. and Seigel H.O. 1963. Induced Polarization in Drill
Holes. Canadian Mining Journal, April, Gardenvale
Quebec, pp.1-7.
Zienkievitcz O. 1977. The Finite Element Method. London.
LIST OF CAPTIONS
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
Fig 1. 3-D geoelectric model of a target of a random
geometrical shape (a) and of a prismatic body with a
random section (b).
Fig 2. The results of the program POLARPRIZ-2 compared with
the results obtained by means of a theoretical
formula. 1 - the anomaly calculated by POLARPRIZ-2;
2 - the anomaly calculated by theoretical formula.
Fig 3. The normal electric field of an array with current
electrode A placed on the ground. 1,2 - are potential
lines; 3 - current lines; 4 - target and polarization
vector.
Fig 4. The IP calculated anomaly using POLARPRIZ-2P program
over a polarizable prismatic target, with underground
current electrodes. The IP plot is calculated for
arrays: 1 - A B ; 2 - A B ; 3 - A B .
Fig 5. The IP surveyed anomaly in physical modelling over a
polarizable prismatic target, with underground current
electrodes. The IP plot surveyed with array: 1 - A B ;
2 - A B .
Fig 6. Dependence of configuration, amplitude and position of
anomaly, from placement of the current electrode in
relation with the target. The IP plots calculated with
arrays: 1 - AB ;2 - AB ; 3 - AB ; 4 - AB ; 5 - AB ;
6 - AB ; 7 - AB ; 8 - AB ; 9 - IP plot surveyed by
moving the current electrode B in the borehole and
measurement electrodes MN fixed on the surface.
Fig 7. The modelling map of the IP anomaly according the
measurements carried out with a current electrode
placed underground. The position of electrodes is A(-
100,0), B(-4,0.4), 1 - Target.
Fig 8. The configuration and amplitude of anomaly in depende-
nce of the position of potential dipole in the Earth's
surface.
Fig 9. Increase of depth investigation of IP method setting
the current electrodes into boreholes. The M3 IP plots
surveyed: 1 - in the surface when the current
electrode A was placed in the borehole; 2 - using the
array AMNB on the surface; 3 - with fixed MN on the
surface and with the current electrode A moving in the
borehole. 4 -amphibolite; 5 - clay_siliceous schist;
6 - diabase; 7 - limestone; 8 - sulphide mineral zone.
Fig 10. A geological section with surface and borehole -
surface IP surveys. The M3 IP plot surveyed: 1 - with
the array AMNB on the surface; 2 - on the surface when
the current electrode A was placed in the borehole;
3 - with MN fixed on the surface and the currrent
electrode A moving in the borehole. 4 - deluvions;
5 - volcanic_sedimentary pack; 6 - serpentinites;
7 -limestones; 8 - disjunctive fault; 9 - sulphide
mineral zone.
Fig 11. Geological section with IP contours according to the
measurements carried out in boreholes with three
electrode array. 1 - diabase; 2 - serpentinized
hartzburgites; 3 - sulphide target; 4 - disjunctive
fault; 5 - the M3 IP contours (in mV/V)surveyed using
the array AMN,B-> (AM=MN=2.5m) moving in the
borehole; 6 - mine works.
Fig 12. Increase of depth investigation using greater
gradient array separations. 1 - volcanic rocks; 2 -
detritic argilaceous pack; 3 - sulphide ore body; 4 -
disjunctive fault; 5 - M3 IP contours in mV/V.
Fig 13. A longitudinal geological section with IP contours
provided by deep IP electrical soundings.
1 - detritic_argilaceous pack; 2 - volcano-
algomeratic rocks; 3 - pillow lava; 4 - gabbro;
5 - sulphide mineral zone: a-verified, b-predicted;
6 - the IP contours in %; 7 - the M3 IP contours in
mV/V; 8 - boreholes; 9 - lines of integration survey.