| The seismic georesistivity method originates from the direct current(DC)resistivity method in geophysical prospecting(electrical prospecting),which is designed to monitor the electrical properties of the earth medium over time at fixed locations,so as to discover the time or dynamic changes that may be related to seismogenic processes.Ground resistivity has been observed since Xingtai earthquake in 1966.With the continuous progress of observation technology and the accumulation of observation data,very good observation results were obtained.In the long-term practice of observation,many precursor anomalies related to the prominent of large and strong earthquakes have been recorded,which plays an important role in earthquake prediction.With the development of social economy in our country,the observation of ground resistivity might be influenced by various environmental factors,which leads to the reduction of observation accuracy,especially the stray current interference caused by the operation of subway,light rail and other urban rail transit.As the power system applies DC supply,there is a large leakage to the ground during operation.The DC observation of ground resistivity also uses DC supply,so that the influence of stray current interference is difficult to eliminate.At the same time,the stray current signal has large amplitude and long propagation distance,and its influence range can reach tens or even hundreds of kilometers,which seriously affects the DC observation of ground resistance near cities.Experimental results have shown that the observation method using alternating current(AC)power supply can solve the stray current interference well.In the longterm AC observation of earth resistivity,it is found that the variation trend is basically consistent with that using DC observation.However,the observed values using AC method differ in variable power supply frequencies compared to that using DC method.In addition,the difference increases with power supply frequency.Therefore,to explore the root cause of the difference and eliminate its influence is of great significance for the practical application of AC observation of ground resistivity.It may also provide technical support for the ground resistivity observation to continue to play an important role in earthquake monitoring and prediction.In this thesis,we explored the difference between AC and DC observations of ground resistivity based on theoretical research,simulations and experimental analysis.Mathematical modeling method was used to simulate and analyze several factors that affect the observed AC value of ground resistivity.By this approach,we attempt to figure out the observation technique method and data processing method to eliminate the difference.Through laboratory and station experiment,the characteristics of the observed value using AC method with variable power supply frequencies were analyzed.Moreover,the effects of device parameters and system parameters on coupling resistivity values were analyzed.The reliability of line coupling and the effectiveness of the elimination of inductive coupling effect were further verified.The main research work and innovation of this thesis are as follows:Firstly,we make it clear that inductive coupling effect is the main reason for the changes in observed values during the AC observation with varying frequencies of power supply signal.In the following,we consider four possible reasons for the difference in the AC observation values of ground resistivity caused by different frequency of power supply signal.(1)Polarization effect of rocks and ores in the AC field;(2)Different skin depths for different AC frequencies;(3)Inductive coupling effect caused by the interaction of magnetic fields between the power supply and measurement lines;(4)Capacitive coupling effect caused by the interaction of electric fields between the power supply and measurement lines.By comparing the AC observation data of multiple stations,we found the changing characteristics of difference were consistent with that caused by inductive coupling effects,while the influence of the other three factors was insignificant.Therefore,the inductive coupling effect was identified as the fundamental reason for the changes in the observed values during the AC observation of ground resistivity with different frequencies of the power supply signal.Secondly,a circuit coupling model for AC observation of ground resistivity affected by inductive coupling effect was established.Starting from the Maxwell equations and Faraday’s law of electromagnetic induction,a formula for calculating the mutual inductance coefficient between parallel lines was derived.The model was established for the circuit coupling in AC observation of ground resistivity affected by inductive coupling effects.It provides a theoretical basis for the simulation and application of AC observation of ground resistivity.Thirdly,simulation of ground resistivity with AC method was carried out.Based on the circuit coupling model established in this thesis,the impact of different power supply signal frequencies,subsurface resistivity,and the distance between power supply and measurement lines on AC observation was analyzed.The results show that the increase of frequency of power supply signal and the coupling length or the decrease of distance between lines can enhance the intensity of the inductive coupling effect.In addition,with the increase of the resistivity value of underground medium,the intensity of inductive coupling effect first increases and then decreases.Finally,an effective method to eliminate the inductive coupling effect in AC resistivity observation was proposed.It was found that the circuit coupling model can be reduced to an equation with four unknowns.Therefore,by using spectrum analysis on the same period of time with power supply signals of two frequencies,the real and imaginary parts of the observation results could be obtained respectively,so as to establish a system of equations with the four unknows.The ground resistivity without inductive coupling effect could be obtained by solving the equations with computer. |
