摘要：
The effect of current turn-off (T/O) time is a knotty problem in TEM, which could affect the inversion results. By setting dif-ferent T/O time, a study on variation of transient electromagnetic field has been carried out. The study has shown that the primary field decreasing along with the increasing of T/O time, which has directly influenced the secondary field intensity. In order to obtain high-quality observation data, the quality of the primary field is also very important. During the current shutdown, affected by the primary field and the delay of initial sampling time, the secondary field intensity has decreased along with the T/O time. After the current shut-down, effected by signal-to-noise ratio decreasing, and deep part electrical property contrast, on the secondary field intensity curves will arise some jumps, but the difference is not significant. If the T/O time is too small, the inversion depth will be shallow, or may be superimposed with the primary field, thus will cause inversion results distortion. If the T/O time is too large, the shallow information will be easily lost and produced greater prospecting blind zones. Therefore, in order to obtain stable and high-quality observed data, it is necessary chosen best T/O time to meet requirements of prospecting depths, and without shallow part information losing.%在非理想关断情况下,关断时间大小将影响反演结果.通过设置不同的关断时间,开展全程信号的瞬变电磁场变化规律研究.结果表明:一次场随着关断时间的增加呈现逐渐减小的趋势,并影响到观测的二次场强度.因此要获得较高质量的反演数据,发送高质量的一次场尤为重要.电流关断前,受一次场减弱和起始采样时间延迟的影响,二次场强度随关断时间的增加而逐渐减小.电流关断后,受信噪比降低、深部电性差异等影响,二次场强度曲线存在跳跃,但差异不大.若关断时间过小,则反演深度较浅,且可能叠加一次场,导致信号失真;若关断时间过大,则易丢失浅部信息,造成较大的勘探盲区.因此现场探测时,需选择最佳的关断时间,既要满足勘探深度的要求,又尽可能不损失浅部信息,才能获得稳定、高质量的数据.
