瞬变电磁测井原理研究I:Doll几何因子

The Theory Study on Transient Electromagnetic Logging I:Doll Geometric Factor

瞬变电磁测井是一种通过井内大功率瞬变电磁激发测量井内电磁感应电动势的方法。与电极、侧向与感应等电测井方法不同,瞬变电磁激发的电磁场频率是连续的,采集到的感应电动势也是连续波形,测量的原始数据量大、所携带的地层信息丰富。瞬变电磁激发的电磁场在地层中会产生二次场,该二次场可以用Doll几何因子进行描述。给出瞬变电磁激发信号为阶跃函数时井内二次感应电动势的响应曲线。从曲线上发现,在激发波形发生突变的位置,井内电磁感应电动势响应信号的幅度最大,当激发波形恒定后,响应信号从最大值连续变化到0。二次感应电动势曲线的幅度与地层电导率成正比。实际的瞬变激发波形不会突变,会有一定的延迟和过渡过程,用一阶网络的指数衰减模拟激发波形后发现,其响应波形的幅度减小,指数衰减越慢,其二次场响应幅度越小。

Transient electromagnetic logging is a method of measuring electromagnetic induction electromotive force with high-power transient electromagnetic excitation in the well. Different with the electric logging methods, such as electrode, lateral and induction and so on, electromagnetic frequency excited by transient electromagnetic is continuous, induced electromotive force collected is also a continuous waveform, the amount of raw data measured is enough, stratum information carried is abundant. According to the principle of electromagnetic induction, electromagnetic field excited by transient electromagnetic can produce secondary field in the stratum which can be described with the Doll geometric factor. In this paper, the response curve of induced electromotive force in the well is get when transient electromagnetic excitation signal is step function. In the response curve, the signal amplitude of electromagnetic induction electromotive force is maximum at the mutation of the excitation waveform and the response signal changes continuously from maximum to 0 as the excitation waveform is constant. Theamplitude of the secondary induced electromotive force curve is proportional to the formation conductivity. Instead of mutation, the actual transient excitation waveform has a certain delay and transition process. By simulating excitation waveform with the exponential decay of first- order network, it is obtained that the exponential decay is slower and the response amplitude is smaller.