Study On Method And Instrumentation For Non-contact Electrode Detection

In the investigation and evaluation of national resources, electrical detection methods play an increasingly important role. With the progress of the technology, they also used to solve some engineering detection problems such as reservoir dam leakage, highway roadbed detection and so on. Traditional electrical methods adopt grounded electrode to measure resistivity, that is, using copper electrode or other non-polarized electrodes. In the good conditions, grounded electrode can provide the necessary power supply to improve detection depth and resolution. But in the bedrock exposed areas and other cases in which the contact electrode is forbidden, the non-contact electrode will be adopted to meet the requirements and be expected to improve the work efficiency based on traditional electrical methods. Therefore, we proposed a project Application of non-contact electrode methods and got the support of the Department of National Resource.Current non-contact electrode measurements transmit fixed frequency and adopt the open capacitance pole plate to make a resistance-capacitance coupling network with the ground. But the resistivity measurement methods cannot distinguish the deference between those targets which have similar resisitivity but quite different in dielectric parameter. So we proposed the non-contact electrode detection to distinguish the deference the complex impedance.1. Development of transmitter and receiver for non-contact electrode detectionTransmitter is an indispensable equipment of active source electromagnetic methods. Time domain electromagnetic methods (TDEM), frequency domain electromagnetic methods (FDEM), inspired polarized methods (IP), charging methods and controlled sources audio frequency magnetotellurics technologies (CSAMT) require power or current from the transmitters as active source. The performance of the transmitter will directly affect the accuracy of the interpretation results. After studied the Ohm-Mapper system, we developed and tested the transmitter system with intermittent transmitting sine current.In complex impedance measurement methods, the major observation parameters are the amplitude△Uf , the apparent dispersion rate Ps and the phase of voltage signal with different frequency. There are two measurement methods: one is dual frequencies measurement, that is, calculating the apparent dispersion rate Ps through measuring the signal amplitude and phase with dual frequencies; the other method is called harmonic wave measurement, that is, through measuring the amplitude and phase of base wave and odd harmonic waves to get the PS, and it only need the base wave current. The absolute phase measurement is sample but it is very difficult to transfer the synchronization signal in fieldwork and it will increase the complexity of the electric circuit. The harmonic wave measurement is sample too, but it is also difficult to detect the weak harmonic wave signal according to the Fourier transform. So we adopted dual frequencies pulse as transmitting resource and use Insulated Gate Bipolar Transistor (IGBT) bridge circuit. We developed the transmitter prototype that can transmit single frequency pulse and dual frequencies pulse.2. Development of the receiver for non-contact electrode detectionTo satisfy acquisition of field data, the receiver should be stability, low power consumption, portable, large storage capacity and high accuracy. According to above requirements, a receiver was developed based on DSP techniques. DSP chips integrate real-time processing and control capacity. They are flexible to manipulation with low power consumption and computing capacity with high speed. These features provide ideal solution to the problems of immense computation and field interpretation.Laboratory tests and experiments were made focused on measurement of noise level of receiver, depressing of power frequency disturbance and removing of error data. These improvements ensure the overall properties of receiver to satisfy field detection.Non-contact electrode method means injection the current power into the ground through the capacitance coupling. In the apparent resistivity measurement, the ground can be equivalent to a resistance network. The ground and the electrodes can be equivalent to a series of parallel capacitor. The voltage obtained from the receiver electrodes divided by the transmitting current is apparent resistivity. The dissertation adopted the transmitter electrodes and receiver electrodes as"cable electrodes"and deduced the configuration coefficient K of the"cable electrodes".After testing experimental parameters of the circular electrodes, rectangular electrodes and cable, it can be concluded that the obtained voltage will increase as the increase of area of plats and length of the cable. The obtained voltage will decrease as the increase of distance between receiver electrodes and transmitter electrodes. The experiment results indicated that the performance of the cable electrodes were better than other electrode forms. So we adopted the cable as the transmitter electrodes and receiver electrodes.3. Simulations and tests of non-contact electrodes method for apparent resistivity detectionThe principle of non-contact electrodes method for apparent resistivity detection was studied. The expressions electromagnetic field excited by electric dipole in infinite homogenous space and half homogenous space are presented. The expressions of Harmonic wave excited by electric dipole put on the surface of half homogenous space are presented. The expressions of electromagnetic field in half space including high resistivity layer excited by electric dipole are presented. The electromagnetic fields excited by electric dipole in infinite homogenous space, half homogenous space, half space with high resistivity layer and half space with high resistivity globe were simulated employing MATLAB. Based on apparent resistivity detection method, transmitter electrodes and receiver electrodes were proved to be'line electrodes'according to experiments. The configuration coefficient of non-contact electrodes method was presented.The configuration coefficient of non-contact cable electrode K is:In laboratory, tests of axial arrayed non-contact electrodes were made. In the suburb of Changchun city, the method was used to detect active fault. Detection results have been compared with results from other electrical methods like transient electromagnetic method (employing GDP-32), composite profile method, and frequency electromagnetic method (employing GEM-2). These results have good agreements, which verified the feasibility of non-contact electrodes method.4. Detection of non-contact electrode method for complex impedanceBased on the theory of non-contact electrode for apparent resistivity, we deduced the related calculation formula and interpretation formula. We simulated the correlation computation of complex sequence in advance for the actual field detection.The resistance dispersion is that the resistivity of rock changes with the excitation frequency. It is one important character of the rock medium. In general, frequency electromagnetic detection belongs to complex resistivity detection. Using the dispersion of electric parameter for geological evaluation is the major character of complex resistivity detection. Some oversea researchers conducted the complex resistivity measurement in lab, but did not mention the instrument development. The dissertation stated the theory of complex resistivity measurement, developed the relevant instrument system, simulated and interpreted the experimental data. At last, it was proved the feasibility and validity of the methodology.In general, we think rock has its resistance and capacitance. We can introduce the resistance and capacitance network model to study the dispersion of rock and deduce the complex resistivity of rock:In general, we interpret the resistivity spectrum based on the Cole-Cole model assumptions, the spectrum can be described as:The dissertation adopted the cole-cole model to validate the relationship between complex resistivity and excitation frequency.The complex resistivity can be obtained from the parameters:ρ0,η,c andτ, it also can be derived from the amplitude and phase angle.Principle of non-contact four-electrode method is presented. The amplitude and phase of normalized complex resistivity, especially whenεr varies linearly, were simulated employing MATLAB. Based on the interpretation of data and computer simulation, phase correlation detection was implemented. Detection result of active fault detection in suburb of Changchun city verifies feasibility and validity of non-contact electrodes method for complex resistivity detection.5. ConclusionsMajor innovations:(1). Propose the non-contact electrode detection for complex resistivity.The dissertation presented the non-contact electrode detection for complex resistivity based on the apparent resistivity detection and deduced the calculation and interpretation formulas, and the simulation is conducted using MATLAB. It simulates, the earth model of liear changingεr. It not only keeps the advantages of apparent resistivity detection such as real-time and efficiency, but also can detect the targets with different dispersion character. It can be used in environmental pollution and engineering detection for detecting the objects of differences in permittivity such as plastic and cement broading the application of this method.(2). Double-frequency transmitting technology in electrical method using non-contact electrodeThe non-contact electrode method utilized dual frequency pulse synchronized transmitting to detected the phase difference and measure the dispersion rate, which is difficult for single frequency pulse transmitting to measure the dispersion rate with correlation detection because of the weak harmonic energy.(3). Employing the electrical method of non-contact electrode to detect city faultageFaultage is the source of various kinds of geological disaster. City faultage detection is important for engineering security evaluation and disaster prevent. The field results comparison of detecting city faultage between non-contact electrodes method, and TEM, composite profile method, and FEM respectively shows this method is feasible in faultage detecting.Major existing problems:(1). The equivalent circuit model is the simplified form to study the relationship between the resistivity and frequency. It is also used to introduce the conductive mechanism and dispersion of rock medium. The dissertation presented the resistance-capacitance network model to simulate the ground medium, but actually the situation is much more complicated, there no deeper research in the dissertation.(2). The instrument is not commercialized yet.