基于等效源的总强度磁异常非线性处理方法

Nonlinear equivalent source method for transformation and inversion of total-field magnetic anomaly

总强度磁异常(ΔT1)常规处理方法通常将其近似当作磁异常矢量在地磁正常场方向上的投影(ΔT2).然而对于高磁环境如磁铁矿处的磁异常场幅值可达104 nT甚至更大的情况,上述近似假设则不再成立,如采用常规处理方法可能会带来明显的误差.本文针对该问题,提出一种基于等效源的总强度磁异常非线性处理方法;该方法根据总强度磁异常获取流程,直接反演实测地磁场总强度幅值与正常场强度幅值之差来求取等效场源.本文首先通过模型试验,分析ΔT1与ΔT2的差异;然后将ΔT1当作ΔT2采用常规处理方法以及ΔT1采用新方法得到的结果作对比分析,结果表明新方法处理结果与理论值的差异为常规方法处理结果的1/5甚至更小,充分说明了新方法的有效性;最后将该方法应用于铁矿实测总强度磁异常处理实例中来转换计算磁异常总模量,其实际应用效果进一步体现了高幅值总强度磁异常数据处理过程中采用新方法的必要性.

Total-field magnetic anomaly(ΔT1)is generally considered as one component(ΔT2)of the anomalous magnetic field onto the direction of geomagnetic reference field.This approximation betweenΔT1 andΔT2 is valid when the amplitude of the anomalous field is much weaker than the strength of the reference field.However,the amplitude of the anomalous field can be more than 104 nT in the presence of highly magnetic environments like BIF(Banded Iron Formation)iron deposits,which would let the above approximation be invalid and the generally existing methods be no longer applicable.We present a nonlinear equivalent source method for inversion of total-field magnetic anomaly to solve this problem.The data-misfit function in this method is constructed based on the measured and pre-processed procedure of total-field anomaly,that is,the subtraction of total-field intensity and the strength of reference field.Firstly,we design a synthetic model to analyze the error betweenΔT1 andΔT2.We then compare the differences between the results fromΔT1 using general methods and the new equivalent source method,respectively.These tests indicate that the difference between results using new method and their corresponding true value is less than one fifth of the difference between results using general method and the same true values.This new method is applied to measured total-field anomaly over an iron deposit,and the result indicates that it is necessary to use this nonlinear equivalent source method for transformation and inversion of total-field magnetic anomaly in the presence of highly magnetic environments.