摘要
瞬变电磁测井满足Maxwell方程,包含电磁波传播过程及其影响。从Maxwell方程出发,分析了无限大均匀介质电导率对电磁波传播速度的影响,与空气中的电磁波传播速度是常数不同,一般介质的电磁波传播速度随频率改变,低频时变化很大。利用裸眼井模型对井内的电磁波传播特征进行研究,给出了描述井内电磁波传播的二维谱及其频散曲线。在高频段(大于100 MHz)与声波测井相似,井内传播的电磁波也具有频散效应,速度随着频率的增加从地层的电磁波速度连续改变到井内液体的电磁波速度,在有些频率段,其速度与地层的电磁波速度一致。在这些频率测量电磁波能够得到地层的电磁波速度,从中获得地层的介电常数,这是由井的圆柱形边界导致的。与声波测井不同,在低频段地层的电磁波传播速度随频率改变,在井中所测量的电磁波速度也随频率改变,这种现象主要由电磁感应引起,与地层的电导率有关。这一部分的响应特征构成了瞬变电磁测井电阻率测量的基础。
The basic equation of transient electromagnetic logging is Maxwell equation which includes the electromagnetic wave propagation process. In this paper, electromagnetic wave propagation velocity of infinite homogeneous medium from Maxwell equation is obtained. Instead of that the electromagnetic wave propagation speed is constant in the air, the propagation velocity of general medium changes with frequency, and is very slower at low frequency. The electromagnetic wave propagation velocity in the borehole is studied, the two-dimensional spectrum describing electromagnetic propagation characters in the well and its dispersion curve are obtained. At high frequency range （〉100 mHz）, the electromagnetic wave propagation velocity changes with frequency, it is called dispersive effects, and is the same as acoustic logging. At a certain frequency band, the speed is consistent with the velocity of stratum. In this frequency range, measuring electromagnetic wave can get the stratum＇s electromagnetic wave speed, and measuring dielectric constant of the stratum. It is caused by the cylindrical boundary of the well. Different from acoustic logging, the propagation speed of the stratum＇s electromagnetic wave changes with frequency at low frequency band, yet the speed of electromagnetic wave measured in borehole also changes with frequency. It is caused by electromagnetic induction, and is related to the conductivity of the stratum. These response characteristics constitute the foundation of measuring resistivity by transient electromagnetic logging.
出处
《测井技术》
CAS
CSCD
北大核心
2014年第5期530-534,共5页
Well Logging Technology
关键词
瞬变电磁
电导率
电磁波
频散
二维谱
transient electromagnetic, conductivity, electromagnetic wave, dispersion, two-dimensional spectrum
