基于三维正演的音频大地电磁阻抗相位不变量校正技术

AMT impedance phase invariant correction based on 3D MT modeling technology

复杂地形、地质条件的大地电磁数据解释容易出现假象,采用三维正演技术模拟地形和地表不均匀体的背景响应,对实测数据阻抗相位不变量进行校正,实现更准确的定性分析;对三维异常体模型的合成数据进行一维、二维多参数反演试算,以确定地形剧变区选择反演技术的最佳方案.合成数据的试反演结果显示一维反演水平切片假异常较多,二维反演能压制测向假异常,但不能压制走向的假异常,水平切片多出现测向条带.使用本文提出的阻抗相位不变量校正法扣除地形、地表背景响应,结合一维、二维反演,能使实际资料解释成果更加可靠.

Qualitative analysis using impedance is necessary in geological interpretation of real acoustic magnetotelluric（AMT）data with heavy fake structures in inversion results caused by strong static shift effects.But impedance itself would lose the veracity due to violent changing of three-dimensional topography or non-homogeneity of the shallow subsurface.This work studies how to remove the effects of topography and non-homogeneity from real AMT data,test the ability to recover the deep anomaly locations using this phase correction technique,and evaluate the its conribution to improve the reliability of geological interpretation. Using staggered grid finite difference MT modeling technology,we can compute two models′AMT responses with or without predefined anomalies.The background responses can be used as correction values to correct measured data.The high values of corrected impedance phase invariants correspond to the lowered resistivity,while the low values to rising resistivity.We do many inversions in different parameters and modes of anomaly model synthetic data both in 1D and 2D.The anomaly locations would be in the places which have high coherence between inversion results and corrected impedance phase invariant,meanwhile the inconsistency of them shows fake structures influenced by static shift.It is more important to obey the coherency principle of inversion result and corrected impedance phase invariant when interpreting real data whatever inversion technology（1Dor 2D）is used. Both the model data and real data are related with a kimberlitic exploration project in southwest China.The corrected impedance phase invariant recovered the horizontal locations of predetermined anomalies for modeling study,true anomaly location is right in the area with high consistency between inversion and impedance,while there are many small trap-shape fake structures shown in 1Dinversion and belt-shape fake structures shown in 2Dinversion（better than 1D）.Such correction of measured data reveals three high-phase traps（meaning decreasing of resistivity）.We chose a low resistivity trap following the coherency principle and interpret it as a most likely kimberlitic pipe,which has been proved by drilling data. The impedance phase invariant correction technique proposed in this work is easy to perform and can recover the response of deep anomalies under complex topography and geology conditions.This technique can be an important supplementary interpretation method or verification tool.