矿井瞬变电磁法电阻率偏低原因分析与校正方法

矿井瞬变电磁法由于采用多匝小回线工作装置进行发射和接收,实测视电阻率与大地中实际岩层电阻率相比存在严重偏低问题,给资料解释带来困难,探测深度也随之降低。通过数值模拟和现场试验,分别研究了半空间和矿井全空间条件下瞬变电磁法偶极装置观测信号随收发距的变化规律,发现收发距在20 m内变化二次场信号变化很小;随收发距增大,实测信号在双对数坐标系中呈线性衰减。理论分析表明,实测信号主要为发射线圈与接收线圈之间的互感信号和大地中感应二次场的叠加,其中互感信号强度远大于大地中感应的二次场,是造成电阻率偏低的主要原因。然后,基于互感电动势随收发距变化的规律,考虑到二次场信号在20 m收发距内变化很小,提出了利用两次不同收发距观测数据进行互感消除和二次场提取方法,推导了相关计算公式,实现了偶极装置矿井瞬变电磁法实测视电阻率偏低的校正。最后,通过半空间和全空间探测实例应用,验证了方法的有效性。

Reason and correction of low resistivity problem in mine transient electro-magnetic method

Mine transient electromagnetic method has a low resistivity problem for using minor transmitter and receiver loops,which leads to data interpretation difficulties and decreases detection depth.Firstly,this paper studied the variation of secondary field with transmitter-receiver distance through numerical simulation and field test,and found that the simulant secondary field was almost unchanged with transmitter-receiver distance within 20 m while measured data decreases linearly in logarithm coordinate system.Theoretical analysis shows that the mutual induced electromotive force is the main component of measured data,which leads to low resistivity problem.Furthermore,the paper proposed a resistivity correction method based on the different properties of secondary field and mutual induced electromotive force.The method eliminates the mutual induced electromotive force by using two measured data with different transmitter-receiver distances.Finally,the proposed method was successfully used in a half-space field case and a whole-space field case to demonstrate its practical usefulness.