Research On Pulsed Electromagnetic Method And The Hardware Implementation

The transient electromagnetic method (TEM) has made considerable progress due to rapid development of computer and electronic technique. In order to achieve better performance in oil and gas exploration and development, it need to research different observe ways and data processing algorithms of TEM to further improve observation efficiency, improve data quality and the resolution of imaging.The forward Modeling of classic TEM method based on the excitation of step electromagnetic pulse signal, so most transmitter of TEM instruments transmitting step current waveform. The ideal step current needs turning off instantaneous, but it is no way to get ideal step current for the transition effect of the transmitter instrument. In fact, the transmitting waveform of most transmitters are ramp wave, which is disadvantageous for observing the secondary electromagnetic field and raising the resolution of shallow, or even cause inexplicable errors. In order to reduce the adverse effect, it is necessary to reduce the turn-off time as short as possible. But the shorter of the turn-off time, the transmit instrument become more difficult to realize, especially in the demand of transmitting high-power and large current. Although some calibration methods such as Shifting coordinate, Fitterman, Eaton, which had been proposed to reduce the adverse effect by correcting the late signal, but it is impossible to select one method adapt to any circumstances. In addition, transmitting step electromagnetic pulse signal need to build high electric field firstly, so it is difficult to achieve system miniaturization for the demand of higher power supply. Accordingly, almost all of the time-domain electromagnetic detecting e instruments composed of a high power transmitter with long polar distance and several recording devices with long offset, which follow with high difficulties of laying devices and inefficient of recording data, so it is impossible to observe with Multi-offset and data processing such as multiple stacking.In this paper, aim to overcome the defects of the traditional TEM method based on the step pulse electromagnetic excitation, a novel TEM method based on high power quasi-gauss-pulse excitation is presented. The exploration system of quasi-gauss-pulse TEM composed of a high power quasi-gauss-pulse source and multi-channel electromagnetic data acquisition boxes. For the quasi-gauss-pulse source's important characteristics of light and similar to the seismic wavelet, some mature technology of seismic exploration can be used in the new TEM method, such as multi-offset observation, data stack processing and imaging, by which the proposed method may achieve better performance than the state-of-the-art methods, in terms of the detecting resolutionIn the first two chaps, by introducing the transmitting technology of TEM, the problem of the TEM method based on the step current excitation is analyzed; Then, by introducing the transmitting principle of quasi-gauss-pulse electromagnetic signal and deriving the current equation in frequency domain and time domain of the source, the TEM method based on high power quasi-gauss-pulse excitation is presentedIn Chapter3, based on the studies of previous, the forward formulas both in frequency domain and time domain of quasi-gauss-pulse electromagnetic response in the homogeneous half-space are derived, by which apparent resistivity equation is derived; Then, by programming and analyzing the results of forward formulas and apparent resistivity equation, the changing law of electric field component and induced electromotive force of magnetic field component correspond to different resistivity of media is revealed, and the sensitivity of electromagnetic components' changing correspond to different resistivity of media under different observation modes is summarized qualitatively.In Chapter4, the quasi-gauss-pulse electromagnetic response in the half-space layered media model is studied by numerical methods. Firstly, the forward formulas both in frequency domain and time domain are derived by Laplace algorithms; Then, by programming and analyzing the results of forward formulas, the resolution of detecting abnormal media is discussed in detail under different conditions.The conditions including electric dipole source or magnetic dipole source, different observing distances and method. The requirements of data acquisition system is presented at last.The results of above study show that it is feasible to adopt quasi-gauss-pulse electromagnetic signal as excitation source in TEM method. According to the frequency range and the requirements of data acquisition system, the development scheme and technology specification of high-power pulsed-field source and ground electromagnetic acquisition arrays is designed. The high-power pulsed-field source solving the following problems:transmitting large power electromagnetic pulse signal, supporting two transmitting modes of grounding conductor and loop-line, the maximum instantaneous power is not lower than100kw; recording the emission current signal in the feature of16bit/1M; GPS timing and synchronization (the precision of synchronizing with the GPS superior to±100ns). Ground electromagnetic acquisition arrays resolving the following problems:network addressing; GPS timing and synchronization (the precision superior to100ns); telemetry for real time status control and QC;24bit/300kHz data acquisition; data stored in CF card; each unit can work independently.According to the design scheme, the theory prototype of the instruments had been developed, which is introduced in detail in chapter5and chapter6. Field experiments and the results of analyzing data show that:(1) The High-power pulsed-field source can transmit high-power electromagnetic pulse to the ground with rich spectrum, supporting two transmitting modes of grounding conductor and loop-line. It is also successfully connected with the V8system developed by Canadian Phoenix Co., which prove that the plan of High-power pulsed-field source is reasonable.(2) The Ground electromagnetic acquisition system achieved the same performance with the the V8system in the aspect of recording waveform, which are consistent with the result of theoretical study.In this paper, a new TEM method based on the excitation of quasi-gauss-pulse electromagnetic pulse signal is proposed and several theoretical results are presented, through which a series of TEM instrument is developed. Compared to the traditional TEM detecting instrument based on the excitation of quasi-gauss-pulse electromagnetic pulse signal, the proposed TEM method achieved enhanced performance with respect to signal-noise-ratio,weight and bulk, which have good prospect.