大地电磁非线性共轭梯度反演试验——以内蒙古扎鲁特盆地数据为例

Magnetotelluric nonlinear conjugate gradient inversion experiments:an example from data acquired in the Jarud Basin,Inner Mongolia,China

近年来,随着三维深部地质调查和非常规能源勘探项目的展开,大地电磁这种非震地球物理方法的重要性越来越突显.尽管大地电磁三维反演方法在近十年得到蓬勃的发展,但是还处在试验阶段,没有得到普遍应用.因此生产实践中普遍使用二维反演方法.大地电磁反演方法主要有非线性共轭梯度法,奥卡姆法和数据子空间法.由于占用内存少、速度快,非线性共轭梯度法得到更广泛应用.该方法在理论上介绍比较多,但在实践中如何应用的介绍却非常少.本文利用内蒙古扎鲁特盆地采集的一条大地电磁剖面数据,选择不同电阻率的初始模型,正则化参数,底板误差做一系列共轭梯度反演方法试验.试验结果表明:在通常取值范围内,一般参数对反演结果影响不大,但是在个别参数数值,反演结果有一定差别.本文还使用TM模式,TE模式,TM+TE模式做共轭梯度反演试验.试验结果表明不同模式反演结果差别较大.TM+TE模式反演电阻率模型综合TM和TE单独反演特征,更好地反映地质结构.然而反演结果应该参考视电阻率和阻抗相位的曲线图、拟断面图做综合分析.这样做能减少对地质的错误解释.本文试验结果对大地电磁实践者具有一定参考意义.

In recent years, with the launch of 3-D deep geological survey and non-conventional resources exploration projects,the magnetotelluric sounding gets more and more important in contrast with other non-seismic methods.Even if 3-D magnetotelluirc sounding was developed dramatically over the last decade,it still remains in the phase of immature,and has not been applied widely.Therefore,the magnetotelluric 2-D inversion methods have to be pursued. They comprise dominantly nonlinear conjugate gradient,Occam,and data subspace methods.Due to less memory and fast speed,the nonlinear conjugate gradient method got wide applications.Although it is described well in theory,the publications on application of it in the real data are scare.The article will address these problems.Under the support of the project Deep geological survey at the periphery of the Songliao Basin,we have collected five NW magnetotelluric profile data in the Jarud Basin,Inner Mongolia,China in 2012.One of them,D-D＇profile was adopeted for experiments since it is of good quality and representive in the study area. Based on different initial resistivity models, different regularization parameters,different error floors,a series of nonlinear conjugate gradient nversion experiments were carried out.It turned out that the inversion parameters have slight impacts on inversion results generally. However,for certain parameter,it has some differences somewhere in the inversion sections.In addition,TM mode, TE mode and TM ＋ TE modes inversion experiments were performed.It turned out that there are large differences among distinct modes.TM ＋ TE mode inversion results incorporate the features from TE and TM mode inversion results,and image the subsurface structures well relatively.On selecting an appropriate inversion model,it should refer to the curves and pseudo-sections of apparent resistivity and impedance phase whereby to circumvent inappropriate geological interpretation.The experiment results are instructive for magnetotelluric practitioners.