基于随机电磁信号的地层电导率勘探理论  

Formation conductivity exploration theory based on random electromagnetic signals

作　　者：沈建国 陈锐 SHEN JianGuo;CHEN Rui(Tianjin University,Tianjin 300072,China)

机构地区：[1]天津大学微电子学院,天津300072

出　　处：《地球物理学进展》2023年第3期1386-1396,共11页Progress in Geophysics

基　　金：国家重点研发计划(2016YFC0802008)资助。

摘　　要：在地面用线圈和电极均能够接收到比较强的随机电磁信号,其中携带地层电导率信息.本文用白噪声模型,结合瞬变电磁场在导电介质中的扩散规律,对随机电磁信号的功率谱和空间相关系数进行了理论推导,给出了计算不同深度地层电导率的方法.与地震面波不同,地表激发的瞬变电磁场在地层和地表中扩散时,地层电导率既引起相位延迟还引起幅度衰减,两者均与频率有关.脉冲激发时,各个频率成分在地层中扩散以后幅度和相位改变,不同源距接收到的响应波形有时间延迟还有形状变化,相关性变差,空间相关系数计算时需进行衰减因子校正.振荡激发时,频带窄,不同源距的接收波形差异小,相关性好.从空间相关系数获得的波数是频率和电导率的非线性函数.频率恒定时,地层电导率越大,波数越大,探测深度越浅;电导率恒定时,频率越高,趋肤深度越小;频率越低,趋肤深度越大.瞬变电磁相移等效的速度随频率的增加而增加,与地震微动获得的速度随频率变化趋势相反.在地面用阵列线圈对瞬变电磁信号进行采集,计算出其互功率谱随频率变化规律与理论公式一致,初步验证了该方法的可行性,为进一步深入研究提供了基础.Coils and electrodes on the ground can receive relatively strong random electromagnetic signals,which carry information of formation conductivity.In this paper,the power spectrum and spatial correlation coefficient of random electromagnetic signals are theoretically deduced by using the white noise model and the diffusion law of transient electromagnetic fields in conductive media,and a method for calculating the formation conductivity at different depths is given.Different from the earthquake surface wave,the transient electromagnetic field excited by the surface excitation is spread in the formation and the surface,the formation conductivity causes both phase delay and amplitude attenuation,related to frequency.When the pulse is excited,the amplitude and phase of each frequency component change after diffusion in the formation.The response waveforms received at different source distances have time delay and shape change,and the correlation is poor.The attenuation factor correction is needed when calculating the spatial correlation coefficient.When the oscillating is excited,the frequency band is narrow,the difference between the received waveforms of different source distances is small,and the correlation is good.The wavenumber obtained from the spatial correlation coefficient is a nonlinear function of frequency and conductivity.When the frequency is constant,the larger the stratum conductivity,the larger the waves,and the shallower the detection depth;When the conductivity is constant,the higher the frequency,the smaller the skin depth;the lower the frequency,the larger the skin depth.The equivalent velocity of the transient electromagnetic phase shift increases with frequency,which is the opposite of changing the speed of microtremor exploration.The transient electromagnetic signal is collected by the array coil on the ground,and the variation law of the cross-power spectrum with frequency is calculated to be consistent with the theoretical formula,which preliminarily verifies the feasibility of the method a

关 键 词：瞬变电磁 随机信号 白噪声 微动 相位移动 空间相关系数 波数 

分 类 号：P631[天文地球—地质矿产勘探]