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. 19-27.