FCSEM场源附近三维不均匀体的畸变特征及校正方法

【作者】 孙娅；

【导师】 柳建新； 何展翔；

【作者基本信息】 中南大学 ， 地球探测与信息技术， 2010， 硕士

【摘要】 众所周知,随着人工源频率域电磁法的发展,场源效应包括近场效应、场源复印效应以及阴影效应的影响变得越来越复杂而备受关注,尤其是在三维复杂条件下。罗延钟、何继善等在针对这一问题发表了多篇论文,并在近场校正等方面形成了自己的理论,在加拿大凤凰公司提出的近场校正方法——过渡三角形法的基础上,利用迭代法和数值逼近法建立了新的近场校正方法,校正效果较好。但是在高维情况下的场源附加效应以及阴影效应的研究仍存在挑战,目前仅仅知道它们的存在,其影响规律还不完全清楚以及校正方法尚待进一步研究。于是本文基于犹他大学CEMI推出的三维积分方程正演软件对场源附近存在三维不均匀体时所产生影响进行模拟研究。以期了解其规律特征,进而提出初步的校正方法。首先,对场源复印效应进行了研究,根据地电异常不均匀体所处的位置、其大小,埋深以及其电阻率与背景场的差异,通过正演计算水平电场分量的全区视电阻率,分析异常特征,总结出了场源附近的三维不均匀体所产生的畸变效应规律,总体来说,随着不均匀体的位置、大小、埋深及电阻率变化,场源复印效应也随之变化。综合分析可以看出该畸变效应的特征与我们传统中所说的静态位移有着相似的特征,但两者之间又有一定的差别。只有当场源AB与不均匀体相当时产生的效应和测点处的静态位移才基本一致；当场源AB远大于不均匀体时,这时的场源复印效应是局部不均匀体和静态位移的综合反映。由于有了这种似静态位移的特征,因此将以上这种场源效应称之为场源静态位移,以便我们能够采用常规静态位移校正的方法来压制这种场源静态位移的影响。在本文中还分析了移动场源下产生的异常特征,并采用中值滤波方法对移动场源下的静态位移进行了滤波处理。接着对阴影效应进行了模拟研究,通过设计不同模型,从不均匀体的大小、埋深、离场源的距离以及不均匀体与背景围岩的电阻率比,总结出了该畸变效应的产生规律以及影响因素。阴影效应的产生原理是由于在近区或过渡区的非平面波区测量时,地层波的成分占主导而引起的。从而根据地层波与地面波之比来判断产生阴影效应的这一条件,提出了压制地层波的方法达到滤除阴影效应的目的。当地层波与地面波之比大于10%时就用总场减去K倍比值的地层波,在本文中K取0.1。最后给出理论模型和实测数据,分别采用中值滤波法和压制地层波法进行了试验和探索。同时认识到对于这类效应目前还没有很好的校正方法,需要多场源激发、三维观测以及全三维反演才能得到好的效果,这才是可控源电磁法以后的发展方向。更多还原

【Abstract】 As we all know, with the controlled source frequency Electromagnetic development, the "source effects", including near-field effect, transmitter overprint effect, shadow effect, become more and more complicted, expecially in the responses of three-dimensional(3-D) structures due to a grounded electric bipole confirms. Many domestic scientists brought forward much good methods to deal with the near-field effect, and formed their own theoretics. Iterative method and the digtal approximated were introduced, all those was based on the method of transition triangle of company of phenix in canada. However, the reseach is on going to research source overprint effect and shadow effect in the high dimension. Now we only knew it was exist, but the law of influence and the method of correction would be studied further.Therefore, it was very significant to investigate the response of three-dimensional with uneven body near the field in the controlled source frequency Electromagnetic. So the modeling software "FCSEM "which was based on integral equation was introduced.The distortion characteristics of three-dimensional inhomogenous which were laid near the field through simulating the anomaly body size, depth, and difference in resistivity between inhomogenous and the background have been summerized, which were studied by the all-time apparent resistivity of horizontal electric field component. With the inhomogenous of the location, size and resistivity changes, the shift source effects, could be seen from the analysis of the characteristics of the distortion effect which was described in our traditional static shift have similar characteristics, but this kind of effect and the traditional static shift was somewhat difference. Only when the excitation sources A, B were all laid on the inhomogenous, the characteristics of source effect and the traditional static shift are consistent. when the source AB was partly laid on the inhomogenous, the effect was shown comprehensive influence by the local inhomogenous and static shift. Due to this feature looks like static shift, so we will call this above effects as source static shift so that we can deal with static shift of sources with suppressing the traditional static shift method.Then, the source shadow effect were analysed. And the distortion characteristics of three-dimensional inhomogenous have been summerized through design different models from their size, resistivity, burying and the distance away from the source. All those phenomenon was attributed to the nonsurface wave, it means the ground wave. Further more, a new approach of correctiving source shadow effect was introduced in this paper. Because shadow effect distorts was caused by the ground wave, and the potential magnitude of the distortion could be estimated by the proportion of ground to surface waves. Letβ=ground wave/surface wave. When the ratio was greater than 10%, then we must subtract the k times ratio of ground wave to suppresse it from the total power. Then the purpose of filtering out the shadow effect was carried out. In this paper, K was 0.1.Finally, theoretical models and experimental data, were used in the filtering method and the suppression of ground wave method. While recognizing that such effects is still not well correction method unless more than the source excitation, three-dimensional observation, all three-dimensional inversion is the future development.更多还原