考虑关断时间的瞬变电磁视电阻率计算及不同波形浅层分辨特征分析  

Transient electromagnetic apparent resistivity calculation considering turn-off time and shallow resolution analysis of different waveforms

作　　者：曹华科 戚志鹏[1] 李貅[1] 齐彦福 程旺盛 CAO HuaKe;QI ZhiPeng;LI Xiu;QI YanFu;CHENG WangSheng(College of Geology Engineering and Geomatics,Chang\an University,Xi\an 710054,China)

机构地区：[1]长安大学地质工程与测绘学院,西安710054

出　　处：《地球物理学进展》2022年第4期1704-1716,共13页Progress in Geophysics

基　　金：国家自然科学基金重点项目(41830101);国家自然科学基金面上项目(42074168)联合资助。

摘　　要：发射电流波形与关断时间会影响瞬变电磁早期响应,导致瞬变电磁浅部信息缺失形成探测盲区.为了分析发射波形对瞬变电磁探测盲区的影响,文中实现了考虑发射波形影响的瞬变电磁全域视电阻率定义方法,并基于该算法对常用的发射波形瞬变响应对浅层电性介质的分辨特征进行分析.首先,采用电流波形与阶跃响应的褶积实现了任意发射电流的全波形瞬变电磁响应计算,将地下电阻率作为全波形响应的函数,利用反函数原理实现了全波形响应的视电阻率计算.然后,以航空瞬变电磁装置为例,利用文中视电阻率计算方法分析了层状介质不同发射波形瞬变电磁视电阻率曲线特征.根据不同盖层厚度视电阻率曲线可知,当覆盖层较厚时,不同发射波形的视电阻率成像结果与模型电性特征基本相近;当盖层厚度较小时,半正弦波、三角波和梯形波等off-time较长的发射波形瞬变电磁响应不能反映浅层的电阻率信息,早期道视电阻率受到深部电性影响,形成了一定范围的探测盲区.最后,利用文中视电阻率计算方法对不同发射波形的复杂三维模型正演的数据进行分析,半正弦波、三角波和梯形波的瞬变电磁响应都能反映异常的地表位置,但会丢失浅部地层信息,深部异常出现时间提前且纵向拉伸.这是由于关断时间增加,会丢失发射信息的高频成分,造成浅部信息丢失.在实际工作中,尽量对高频信息进行补偿或者采用校正方法获得完整的地电剖面.The emission current waveform and turn off time will affect the early response of transient electromagnetic,resulting in the lack of shallow information of transient electromagnetic,forming a detection blind area.In order to analyze the influence of transmitting waveform on the blind area of transient electromagnetic detection,a transient electromagnetic full field apparent resistivity definition method considering the influence of emission waveform is realized,and the resolution characteristics of common emission current waveforms transient response to shallow electrical media are analyzed based on this algorithm.Firstly,the convolution of current waveform and step response is used to calculate the full waveform transient electromagnetic response of arbitrary emission current.Taking the full waveform response as a function of underground resistivity,the apparent resistivity of the full waveform response is calculated by using the inverse function principle.Then,taking the airborne transient electromagnetic device as an example,the characteristics of transient electromagnetic apparent resistivity curves of different emission waveforms in layered media are analyzed by using the method presented in this paper.According to the apparent resistivity curves of different cover thickness,when the overburden is thick,the apparent resistivity imaging results of different emission waveforms are basically similar to the electrical characteristics of the model.When the overburden is small,the transient electromagnetic response of off time long emission waveforms such as half sine wave,triangular wave and trapezoidal wave can not reflect the resistivity information of shallow layer.The apparent resistivity of early trace is affected by the deep electrical property,forming a certain range of detection blind area.Finally,the apparent resistivity calculation method in this paper is used to analyze the forward modeling data of complex three-dimensional models with different emission waveforms.The transient electromagnetic response

关 键 词：发射电流波形 关断时间 视电阻率定义 探测盲区 

分 类 号：P631[天文地球—地质矿产勘探]