瞬变电磁法中关断电流的响应计算与校正方法研究

Research on response calculation and correction technique of turn-off current in the transient electromagnetic method

关断电流的存在直接影响仪器的探测能力和数据资料的解释.为了分析和校正这种影响,文中给出了关断时间的计算方法,根据多匝回线源产生的瞬变场理论解析式,计算了线性和半正弦关断电流影响下瞬变场表达式,计算半正弦关断电流产生的瞬变场时将正弦函数进行泰勒展开;分析了关断时间在2μs^100μs之间两种关断模式产生的瞬变响应.结果显示:同一关断时间下线性关断电流和半正弦关断电流对瞬变场的影响主要表现在"早期",并且随观测时间的增加影响逐渐减弱.同时,关断时间越长,关断效应对瞬变场的影响越向瞬变"晚期"延伸;半正弦关断电流对瞬变场的影响大于线性关断电流.当关断时间为100μs时,关断电流产生的瞬变场曲线约在0.5 ms之后与理论曲线重合.研究结果为数据处理和仪器开发提供参考.

Both the detectivity of instruments and the interpretation of data are influenced by turn-off current. In order to analyze and correct this effect, the method to calculate turn-off time is given. According to theoretical TEM response of multiturn loop source, TEM response affected by linear and half-sine turn-off current was calculated, and Taylor＇s series expansion was used by calculating TEM response of half-sine turn-off current. The two mode＇s responses, linear mode＇s response and half-sine mode＇s response, were analyzed in turn-off time 2μs-100μs. The resuits indicate that, both effects happen in “early times” mainly, which are due to linear mode and half-sine mode in a same turn-off time, and gradually attenuate as the observation time increase. Meanwhile, the longer turn-off is, the later the time that transient field is affected by turn-off current in “later times” is. The effect of half-sine turn-off current to transient field is relatively stronger than that of linear current. The transient curve of turn-off current is coincident with the theoretical curve after time 0. 5ms, when turn-off time is 100μs. The results provide interpretation guidelines for data processing and instrument development.