考虑激发极化效应的瞬变电磁一维Occam反演

大地的感应激发极化效应有时会在瞬变电磁晚期响应上产生负值变号现象。常规实电阻率瞬变电磁反演由于没有考虑激发极化效应,对于观测数据负值部分的解释一直存在问题,这影响了反演解释的真实性和可信度。对此,本文首先将Cole-Cole复电阻率模型加入正演计算,并验证了计算的准确性。然后在阻尼最小二乘法的基础上加入Occam平滑约束来构建反演方程,能够同时反演出零频电阻率、充电率、时间常数以及频率相关系数,实现了一维瞬变电磁复电阻率反演算法。最后,建立具有不同程度激发极化效应的典型层状理论模型进行反演试算与结果分析,结果表明,在激发极化效果明显的低阻高极化地层中,复电阻率反演效果更好。与实电阻率反演结果的对比说明,瞬变电磁复电阻率反演既可以达到实电阻率的常规反演效果,也能解决实电阻率无法实现的负值拟合问题。

1-D Occam Inversion of Transient Electromagnetic in Consideration of Induced Polarization Effect

The transient electromagnetic response sometimes becomes negative at late times, which may be caused by the induced polarization(IP) effect.Because of the fact that there is no consideration of the IP effect in the traditional real resistivity inversion, the interpretation of the negative part of the observed data has always been a problem which affects the accuracy and reliability of the inversion.In this paper, we first introduced the Cole-Cole complex resistivity model into the forward modeling code and tested its accuracy.Then, one-dimensional TEM Occam inversion algorithm was implemented based on the damped least-squares method.We constructed the inversion equations involving Occam smoothness constraint matrix and inversed simultaneously the four complex resistivity parameters, i.e.DC resistivity, chargeability, time constant and frequency dependence.In order to test the inversion algorithm, we established two different typical theoretical polarized layered models and analyzed their inversion effects.The results show that the complex resistivity parameters can be better recovered in the layer with low resistivity and high chargeability.Comparing with the traditional real resistivity inversion, the complex resistivity inversion can not only perform like the real resistivity inversion, but also solve the fitting problem in the case of the negative response.