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3. The induced polarization The phenomenon of induced polarization
(IP) of the rocks (equivalent of frequency dispersion of their conductivity),
is caused by set of the electro physical and electrochemical processes
proceeding in heterogeneous and polyphase media. Those are processes of
macroscopical separation of charges caused by an applied electric field in
electrical non-uniform monophase media (Maxwell-Vagner effect) or in the
polyphase media having different permeability to positive and negative ions
of a solution ("membrane" potentials); changes of ion-diffusion
potentials within double layers on an interface of solid and liquid phases;
various kinds of the electrochemical phenomena arising on interfaces of media with electronic and ionic
conductivities and a some other processes. The IP is observed in a wide range
of times from microseconds up to tens s. As well as SE effect of
2 kind, IP is closely associated with structure of rocks, their mineral
structure and petrophysical properties. By virtue of it the basic attention is given
to joint studying of these phenomena and their joint petrophysical
interpretation. Work of Laboratory in IP area is
focused on its application in oil-field, hydro-geological and ecological
geophysics. As well as in case SE, it combines theoretical, experimental and
laboratory researches. Theoretical researches. Mutual influence of
the induction and polarizing phenomena is studying in Laboratory. It is
established, that IP processes differing on the nature are shown most
strongly in different time (frequency) ranges. So Maxwell-Vagner effect
develops on earliest times (microseconds), and electrochemical processes - on
the latest (seconds). It allows to describe frequency dispersion of
conductivity as the sum of Cole-Cole formulas with different time constants,
responsible for different processes. Broadband (in time or frequency areas)
studying of IP processes allows to receive the separate information on these
processes. Methods and programs of definition of parameters Cole-Cole of
measured IP processes are developed. These parameters at the further
interpretation are related to petrophysical properties of media (on the basis
of laboratory studying of samples). The program of calculation
of EÌ field of an grounding
cable of final length above horizontally layered polarized media is
developed. It is used in interpretation of field measurements. The similar
program for cylindrically layered medium (with reference to IP logging) also
is developed. Development of the program of numerical calculation of EÌ field in 2D axisymmetric models, containing cylindrical and
horizontal boundaries (by a method of final differences and integral
equations) is begun. The program 1D interpretation of
received VES-IP data on different separations and in several time ranges
allows to pick up a layered section (up to 10 layers) with complex frequency
dependent conductivity of layers. Frequency dispersion of conductivity is
presumed to be in form Cole-Cole. Conductivities, polarizabilities, time
constants and exponents of layers are defined. The opportunity to receive and
analyze some independent polarizing parameters as against apparent
polarizability traditionally used in IP method, essentially raises geological
information of the method. IP equipment and
experimental works. New IP equipment is
developed in Laboratory/ It allows to register electric and magnetic fields
(2 channels) excitated by an grounded cable in 4 different time ranges and to
study IP processes from the first microseconds up to tens seconds. The generator
develops a sign-variable sequence of rectangular pulses of the given duration
with the porosity equal 1. Duration of pulses of a current and pauses in a
measuring instrument are synchronized by the radio channel on distance up to
1.5 km. The generator works from the accumulator 24 V and gives the
stabilized current from 4 mÀ up to 1 A. The
duration of pulses of a current and pauses are equal to: 1 range - 256
microseconds, 2 range - 30 ms, 3 range - 1.28 s, 4 range - 100 s. Measuring
IP instrument represents the separate block, the data from it enter in a
computer. Measurements are carried out during pulses and pauses with the
different factors of amplification and
with 1024 readings for the period. It is used 16 digit ÀDT. The maximal amplification provides sensitivity 0.15 microvolt on
the least significant digit. The number of the periods of measurements for
accumulation can vary from 1 up to 1024. At field experiments the orthogonal
arrays, allowing essentially to weaken an induction component of a signal,
are applied. Maximum separation is some hundreds meters. The field works
carried out on various hydrological and engineering objects. Laboratory researches on samples of rocks: The developed field equipment allows to carry
out researches of the IP on samples of rocks. Researches on
sandy-argillaceous samples and on cores of sandstones and limestones are
carrying out. Processes IP are studied in the same time ranges, as at field
researches. The carried out laboratory measurements allow to apply IP
dependences on petrophysical properties to interpretation of the field
data. Laboratory is
interested: In realization of experimental works on objects where set of IP
parameters and definition of petrophysical parameters can be more
significant for the solution of a
geological problem, than classical interpretation only on specific resistance
and polarizability. It can be hydrological and engineering problems,
cryolite-zone and ore problems, studying kimberlite tubes etc. The basic publications: 1. V.V.Ageev, B.S.Svetov. Influence of polarizability of rocks on
results of electromagnetic soundings. //Physics of the Solid Earth (Russian), 1999, N1, p.
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