electrical prospecting method,
vertical electric current soundings (VECS) are discussed here. This method is based on using
a new source, the circular electric dipole (CED). The source is installed in the following way.
One of the transmitter poles is grounded in the central point. The other pole is uniformly
grounded around a radius determined by the depth of investigation desired. It can be defined
as a noninductive source, generating a transverse magnetic (TM) non-stationary field. The
most interesting CED properties in the low frequency regime are as follows. CED has no
magnetic field of its own. It is a pure galvanic source, which differs from a loop (a pure induc-
tive source) and from a line, which is both galvanic and inductive (a "line" here means a cable
or insulated wire grounded at its end points). The normal magnetic field on the earth's surface
is absent (within the quasi-static approximation), and only a radial electric component exists.
Taking into consideration

the pronounced vertical character of the currents under the central
electrode and current circulation in the vertical planes we propose to

term the electrical pros-
pecting technique using the CED as the vertical electric current soundings method (VECS).
At present, real measurements of the magnetic components with an attached CED-array are
the most promising. The authors consider results of the field tests in the Tatarstan (oil pool).
THEORETICAL. So, why CED? The variety of transmitter systems located at the Earth's
surface or at any other boundary and formed by wire and grounding segments, can be for-
mally described with the use of the surface density of synchronously varying excitation cur-
rent. In the general case the field (any component) is represented by two members generated
by the inductive and galvanic modes (H and E fields, H and E modes, TE and TM fields,
etc.). These members correspond to two types of transient processes, which contribute to the
total field, depending on various properties of the excitation current distribution. The contri-
bution of the TE (transverse electric) field is determined by the rotor of the excitation cur-
rent, and that of the TM (transverse magnetic) field by the divergence of the excitation cur-
rent.
Explaining the approach developed in this paper, we can say that an ungrounded loop is the
purely inductive source generating only a transverse electric field (H mode). Another conven-
tional source - horizontal electric (grounded) dipole is the combined source actually consist-
ing of three sources: a segment of linear current (inductive source) and two point groundings
(galvanic sources). Note that because of the more rapidly attenuating TM process the expo-
nential attenuation in a medium (with the insulating base), the horizontal grounded line (com-
bined source) behaves mostly as inductive, particularly in the late stage. This situation is dem-


New electrical prospecting for oil

VLADIMIR MOGILATOV

Russian Academy of Sciences, Institute of Geophysics,
Pr. Ak. Koptyug 3, 630090, Novosibirsk, Russia
Ph.:+7-3832-344602, fax: +7-3832-333432, e-m.: vmogilat@uiggm.nsc.ru, www: lab6408.geophysics.uiggm.nsc.ru
2

demonstrated in Fig. 1. This two-uniform physical and mathematical model of electrical
prospecting shows:
1) all traditional electrical prospecting (in time or frequency domain) is founded only
on the

application of a TE polarized field;
2) the electrical prospecting (in time or frequency domain) with the use of a M field
has never been implemented until now.


Fig. 1. The physical and mathematical model of induction electrical prospecting
The behavior of an inductive source (loop) and a TE process are well known. The proper-
ties of the TM transient process have been poorly studied and little used (in the best case, only
in problems of applying a little suited vertical electric line). The most remarkable properties
of the field in this process include the absence of the normal (quasi-static) magnetic field at
the outer surface of a layered medium, as well as the dependence of the process on the vertical
geoelectrical structure (rather than on the overall longitudinal conductivity only, which is
typical of the processes excited by inductive means).
Now we supplement electrical prospecting,
offering to use the TM process. For this purpose first
of all it is necessary to offer transmitter, excitating a
pure M field. We should use such allocation of an
extraneous current, that

( , ) 0
rot j x y
e
z
=


It is possible to realize by a set of radial
Fig.2. current lines, grounded in the centre and on a circle
with a radius determined by the depth of investigation desired. As in Fig. 2. The circular
electric dipole (CED) is the purely galvanic source generating a transverse magnetic
non-stationary field (Mogilatov, 1992, 1996). The CED field is at right angles to the loop
field and has azimuthal symmetry. The CED field is always governed by a vertical medium
structure (at the latter transient stage as well) rather than by the total longitudinal conductiv- 3

ity. An interesting result was obtained: in marine electrical prospecting a sea water layer will
not play such a crucial role when a CED is used as in applying a loop or a line. In a medium
with a non-conducting basement the decay of the CED field is exponential. The transient
process is faster than in the case of a loop or a line. The CED can also be considered as a
ground analogue of another known source, a vertical electric line. Besides the CED as a pure
galvanic source does not excite a long-term transient field. Thus it seems to be a new useful
means to study IP processes.
VERTICAL ELECTRIC CURRENT SOUNDINGS (VECS). Mogilatov and Balashov
(1996) presented the first result of theoretical and experimental works on creating a new
method of transient soundings based on the circular electric dipole as the source. At present,
area measurements of the magnetic components with an attached CED-array are the most
promising.
As a rule an anomalous contribution of local objects or disturbances in the horizontal ho-
mogeneity of the section in transient field sounding methods is recorded against an intense
background of host rock mass response. On the one hand, this makes

recording the response
difficult and, on the other hand, this hampers the interpretation which consists in separating an
anomalous part of the response at the first stage. The problem of suppression of the normal
background of the host medium can be solved, among other factors, by optimizing the field
source. One can consider the circular electric dipole as the optimized source with the lack of
the normal magnetic field in mind.
The technique is illustrated in Fig. 3. The CED-array is placed on terrain. The impulse
feeding is used. The currents are aligned in radial lines by special devices. The measuring
system (one or several
)
is moved on a square grid. The system includes an inductive sensor
and meter for transient signal. The measuring system is synchronized with an impulse current
in CED and becomes located on terrain by means of signals from the satellite (GPS). For a
solution the oil problems, CED has a radius of 500 m, current in the each ray up to 10 A, the
measuring system has a maximum range of 5 radii of CED. As the result of field works, an
area map of signal intensity is formed for each temporal reference. On a contour of a signal it
is possible to determine a contour of a heterogeneity (oil reservoir, for example).


Fig. 3. Vertical electric current soundings (VECS) 4


Fig. 4. Shuganskaya and Akbyazovskaya pools


Fig. 5. Krasno-Oktyabrskaya oil pool. Plan and section.

REFERENCES. Mogilatov, V.S.,1992. A circular electric dipole as a new source in electric
survey. Izv. RAS. Ser. Fizika Zemli, 6, 97-105.
Mogilatov, V.S. and Balashov B.P., 1996. A new method of geoelectrical prospecting by
vertical electric current soundings: Journal of Applied Geophysics, Vol. 36, 31-41.



THE FIELD TESTS. We have
interesting results for various
objects (ore bodies, kimberlite
pipes). However, the greatest
success we have is in the con-
touring of oil pools. Such works
are conducted systematically in
Tatarstan now. Here we exhibit
results of field tests on known
deposits of oil. |The contour of
deposit (known from other data)
is selected by the boosted sig-
nals. On the pool Shugan-
skaya" three installation CED
were used. The pool "Krasno-
Oktyabrskaya" is disposed in
two horizons. In Fig. 5 the ap-
parent section is presented, on
which the double storied verti-
cal structure of the pool is really
visible. All these images in Fig.
4 and 5 are formed by filed sig-
nal directly.
CONCLUSION. The field of
the conventional sources (i.e
loop and horizontal electric di-
pole or line) is defined by the
horizontal currents. This means
that we use the TE field only. It
is safe to say that the traditional
electrical prospecting mainly
investigates the longitudinal
conduction. If you want to study
IP, permitivity, multiphase or
fractal structure of the medium
and other subtle items, yo