【作者】 罗明璋；

【导师】 胡文宝；

【作者基本信息】 长江大学 ， 地球探测与信息技术， 2012， 博士

【摘要】 随着计算机技术与电子技术的发展,TEM方法近年来从仪器到方法均获快速发展,应用领域被迅速拓宽。为了进一步提高时间域电磁法在油气勘探和开发中的应用效果,需要通过研究不同的观测方式和资料处理算法以进一步提高观测效率、提高资料质量和成像的分辨能力。经典的瞬变电磁正演计算建立在阶跃脉冲激励的基础上,因此在瞬变电磁方法中,发射装置大多采用阶跃电流为初始激励源,理想的阶跃电流为瞬时关断,但由于仪器固有的过渡效应,实际上并非瞬时关断,关断电流均为斜阶跃波,这将影响二次场的观测,影响浅层的分辨率,甚至导致解释的错误。为了减小这种影响,要对观测到的晚期信号进行校正,常规的校正方法有坐标移动法、Fitterman法和Eaton法。这几种方法使用范围不同,也各有局限性。在实际操作中,需要尽量缩小关断时间,但这样就对仪器提出了更高的要求,实现也变得更困难,在大功率强电流勘探中表现尤为突出。另外,由于阶跃电流信号源的产生需要先建场,发射装置要求较高的功率供给,难以实现系统的小型化。因此,现用的时域电磁法普遍采用的观测方式是大功率发射机、大极距发射源、大偏移距、小道集观测仪器,野外资料采集时布设困难、资料采集速度慢、不能进行多偏移据观测和叠加。本论文针对目前瞬变电磁方法采用阶跃电流为初始激励源及观测方式的不足,提出采用新型大功率脉冲发射装置作为功率源的时域电磁勘探方法,利用多道电磁数据采集系统在地面进行阵列观测,构建脉冲时域电磁勘探系统。利用脉冲源轻便易布设的特点,利用电磁脉冲源与地震子波相似的特点,实现拟地震方式的观测和资料处理,诸如多偏移距观测、叠加处理与成像等,可望大大提高电磁勘探的构造分辨能力。本文首先在介绍瞬变电磁发射技术的基础上,分析了以阶跃信号为激励源的瞬变电磁方法所面临的问题；提出了以拟高斯脉冲信号为激励源的瞬变电磁方法,介绍了拟高斯电磁脉冲场源的原理,推导了拟高斯电磁脉冲场源时域和频域的电流方程。在前人研究的基础上,第三章推导了均匀半空间中脉冲源激励下频率域和时间域的电磁响应正演公式,推导了频率域视电阻率的表达式；编程实现了脉冲偶极源激励下均匀半空间模型频率域和时间域电磁响应的正演计算和频率域视电阻率计算；通过对算例结果进行分析,分别讨论了在电偶极源、磁偶极源激励下,不同地层电阻率对水平电场分量、磁场分量响应的感生电动势曲线影响的规律,定性给出在不同观测方式下不同电磁场分量受地层电阻率变化的敏感程度。第四章考虑层状地层模型,利用数值法求解偶极源激励下电磁场的分布特性,探求偶极源激励下电磁场在层状半空间中响应的基本规律；采用Laplace变换推导了脉冲源激励下时间域和频率域的电磁响应正演公式。编程实现了脉冲电偶极源激励下层状半空间模型时间域电磁响应的正演计算。通过对算例结果进行分析,详细讨论了在电偶极源、磁偶极源激励下,不同观测距及观测方式下电磁场分量对异常体进行识别的分辨率的影响以及对采集装置的要求。理论研究的结果表明,采用拟高斯脉冲信号为激励源的电磁方法是完全可行的。根据理论研究得出的频率范围以及对采集装置采样率的要求,提出了大功率电磁脉冲场源和地面电磁采集阵列的硬件实现方案和参数。大功率电磁脉冲场源需要重点研究解决以下问题：发射大功率电磁脉冲信号,支持接地和中心回线两种发射方式,最大瞬时功率不低于100kw：对发射电流信号进行16位/1M的记录；与采集装置GPS卫星同步,精度优于±100ns。地面电磁采集阵列重点解决以下问题：采集盒子采用网络编址；使用GPS卫星信号实现大面积全系统、不间断同步；无线实时监测采集盒子的采集状态和质量；24位/300khz模数转换；SD卡数据存储；每个盒子均可独立工作。按照设计方案,本论文完成了原理样机的研制,在第五章和第六章给出了大功率脉冲场源和地面电磁采集阵列原理样机的详细实现过程。野外试验及资料分析结果表明：(1)地面大功率电磁脉冲场源实现了对地发射大功率的电磁脉冲发射,频谱丰富,支持接地导线和回线两种发射方式,与加拿大凤凰公式V8系统的成功配接,证明大功率脉冲场源软硬件设计的方案是合理的,发射信号的同步性和一致性达到了预定的功能和指标要求；(2)地面电磁采集系统与V8系统记录的感生电动势形态基本一致,通过对记录数据进行分析,得到了与理论分析吻合的波形和频率响应特征,验证了理论计算的正确性和原理样机设计方案的可行性。理论和实验研究的结果表明：采用拟高斯大功率电磁脉冲作为激励源,利用多道电磁数据采集系统在地面进行阵列观测的时域电磁勘探方法可行；研制的大功率脉冲场源和地面采集阵列系统体积小、易于铺设,很好地弥补了传统电磁方法采用阶跃信号作为激励源的不足,具有广阔的应用前景更多还原

【Abstract】 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.更多还原