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| Titel |
Resistivity measured by direct and alternating current: why are they different? |
| VerfasserIn |
V. Y. Zadorozhnaya |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7340
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| Digitales Dokument |
URL |
| Erschienen |
In: Geophysical monitoring of the near-surface by electromagnetic and other geophysical methods ; Nr. 19 (2008-11-14), S.45-59 |
| Datensatznummer |
250014152
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| Publikation (Nr.) |
 copernicus.org/adgeo-19-45-2008.pdf |
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| Zusammenfassung |
Mathematical modeling of a little known model of IP referred to as "induced
polarization caused by constrictivity of pores" was developed. Polarization
occurs in all types of rocks if surface areas and transfer numbers are
different for connected pores. The duration of the polarization process
depends on two parameters: pore radii of connected capillaries and transfer
numbers. During the polarization process all contacts between pores of
different transfer numbers will be blocked and the electrical current will
flow through the remaining canals. Two phenomena control the amplitude of
potential difference at time-on: 1. Successive blockage of pores increases
the resistivity of sediments and results in increased measured potential
difference. 2. Excess concentration of electrolyte at the boundaries between
pores with different radii provides an additional potential. The amplitude
of the potential difference (voltage) of such rocks not only depends on
solutions filling pore spaces, porosity and tourtuosity of pores channels,
but also on ion mobility, diffusion coefficient, and difference of transfer
numbers. During time-on a voltage is occurred due to flowing current
Ucurr (t) and excess concentration Uexcess (t) at the contacts.
However during the time-off only the excess of concentration Uexcess (t) is involved in the diffusion process which tends to level the ion
concentration along the pores. It was found that the measured chargeability
is proportional to the porosity. Blockage of pores and excess/loss ions at
the contacted pores control this physical parameter.
However the relationship between resistivity and porosity is very
complicated. Mathematical modeling and laboratory measurements both
confirmed the membrane IP effect diminishing with increasing salinity of
fluid filled pores of rocks. Membrane polarization does not exist on high
frequency of electrical current. As a result the resistivity measured by
direct and alternating current is different. The new algorithm was tested on
laboratory measurements data showing its good agreement with theory. The
calculation of pore size distribution using IP laboratory data has been
presented. The definition of the membrane IP effect is: "Membrane IP is the
successive blockage of inter-pore connections due to the excess distribution
of ions during current flow". |
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