- A - Physics of the Earth's Interior
- B - Seismology
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Online First
Modelling of Crack Induced Resistivity Changes - Applications to Earthquake Studies
Volume: 274
Series: A-24
An anisotropic resistivity model of rocks that contain spatially oriented crack systems is presented. Each system consists of open cracks filled in with material of different resistivity than that of the rock frame; the cracks in the system have the same orientation and shape. Generally speaking, the resistivity change caused by inclusions (cracks) is intermediate between two extreme cases: inclusion effect expressed by connection in series, and inclusion effect expressed by parallel connection. Resistivity changes in three directions are attributed to geometry of the cracks, so the resistivity tensor is calculated for a specified crack geometry, by adding serial and parallel part of the change.
The resistivity of medium is, in general, a function of time. Two classes of cracks having opposing effect on a medium resistivity are considered: cracks filled in with gas, and those filled with aqueous solution. A numerical simulation of resistivity changes in a medium with cracks has been performed, based on the dilatancy-diffusion model of earthquake precursors.
Arificial samples have been constructed in the laboratory, with oriented inclusions that model cracks in a medium. Electric resistance of the samples has been measured with alternating current, in the frequency range 5 Hz - 13 MHz. The experiments corroborate the existence of resistivity anisotropy, in conformity with the numerical simulation.
A review of results of resistivity monitoring in the field is added, with emphasis on those results which indicate the presence of electric anisotropy in the medium. A similarity between the results from seismic areas, mines, laboratory experiments and numerical simulations is underlined.
The obtained results clearly indicate that the presently applied techniques of resistivity surveying in seismically active areas are inadequate to include anisotropic character of crack distribution, and should be supplemented by differently oriented measuring sites and lines. The present study also shows that resistivity surveying should be very useful in complex geophysical methods for an analysis of processes in focal zones of earthquakes that are of both tectonic and induced origin.
CONTENTS
Abstract, ...3
1. Introduction, ...4
1.1 Apparent electric resistivity in monitoring of earthquakes, mine shocks and rupture of rock samples, ...4
1.2 Mechanisms of preseismic electric resistivity changes, ...8
1.3 Oriented crack structures in the future focal zone, ...9
1.4 Material containing inclusions of different resisitivities, ...11
2. Model of the anisotropic rock resistivity, ...12
2.1 Principal features of the modelled medium, ...12
2.2 Analogies of electrical circuit laws for continuous anisotropic media, ...13
2.3 Contrast distribution functions, ...16
2.3.1 High resistivity cracks - the electric current deviation, ...16
2.3.2 The influence of low resistivity cracks size, ...17
2.4 Geometrical coefficients - general case, ...18
2.5 Numerical simulations, ...20
2.5.1 Essential characteristics, ...20
2.5.2 Apparent resistivity simulation, ...34
2.5.3 Medium resistivity variations when all cracks are either wet or dry, ...34
2.5.4 Concurrent wet and dry sub-systems, ...36
2.5.5 The influence of primary anisotropy on resistivity, ...37
2.5.6 Effect of unsynchronous variations in dry and wet crack concentrations - simulation of tectonic-origin resistivity
changes, ...37
2.5.7 Simulated resistivity variations due to drying or rainfall, ...38
2.6 Geometry of cracks with complicated shape, ...38
2.7 The problem of connections between cracks, ...39
3. Experiments with artificial samples, ...39
3.1 Resistivity anisotropy - laboratory results, ...39
3.2 Methodology of the author´s experiments, ...41
3.3 Influence of current frequency and sample moisture on electric resistivity, ...42
3.4 Influence of anisotropic inclusions system, ...42
4. Anisotropic resistivity changes - field results, ...48
4.1 Resistivity anisotropy in seismic zones, ...48
4.2 Anisotropy revealed in monitoring in the mines, ...52
5. Discussion, ...55
5.1 Remarks on the main results of the author´s experiments, ...55
5.2 Different resistivity models, ...56
5.3 Practical inferences from the present modelling, ...59
6. Conclusions, ...60
References, ...61