海岸效应对大地电磁响应的影响及校正方法  

Study on the influence and correction method of coast effect on magnetotelluric responses

作　　者：李永博[1,2,3] 吴琼 王刚[1,2,3] 张振宇 王书民[1,2,3] 西永在 LI Yongbo;WU Qiong;WANG Gang;ZHANG Zhenyu;WANG Shumin;XI Yongzai(Institute of Geophysical and Geochemical Exploration,Chinese Academy of Geological Sciences,Langfang,Hebei 065000,China;National Research Center of Modern Geological Exploration Engineering Technology,Langfang,Hebei 065000,China;Laboratory of Geophysical Electromagnetic Probing Technologies,Ministry of Natural Resources,Langfang,Hebei 065000,China)

机构地区：[1]中国地质科学院地球物理地球化学勘查研究所,河北廊坊065000 [2]国家现代地质勘查工程技术研究中心,河北廊坊065000 [3]自然资源部地球物理电磁法探测技术重点实验室,河北廊坊065000

出　　处：《石油地球物理勘探》2021年第3期631-644,I0015,共15页Oil Geophysical Prospecting

基　　金：中央级公益性科研院所基本科研业务费项目“海水对海岸带大地电磁响应影响研究”(AS2017J08)资助。

摘　　要：在近海地区,受海岸效应的影响,当测点到海岸线的距离小于目标频率的趋肤深度时,大地电磁数据会发生畸变。为此,通过大地电磁三维正演模拟,采用误差估计量化海岸效应的影响,分析不同电性结构条件下海岸效应的变化规律,并利用迭代校正法消除海岸效应。海岸效应的影响范围主要取决于测点到海岸线的距离及浅层的电阻率:随着测点远离海岸线,影响范围逐渐减小;浅层电阻率越大,影响范围也越大。海岸效应的强度主要取决于测点到海岸线的距离、海水深度及其深部地层的电阻率:海岸效应强度随着测点远离海岸线而减小,随海水深度增加而增大,随着深部地层电阻率的变化在距海岸线不同的区域有所不同。研究表明,在近海区(距离海岸线小于50km),海岸效应较强,TM和TE模式的海岸效应随深部地层电阻率的减小分别呈现增大和减小的特征;在远离海岸线的区域(距离海岸线50~100km),海岸效应较弱,随深部地层电阻率的减小而减弱。同时,TM模式的海岸效应对低阻软流层较敏感,即软流层越浅,TM模式的海岸效应越强,而TE模式的海岸效应则相对较弱。数值模拟结果表明,在近海区域,海岸效应不可忽视,在数据处理过程中须加以校正;在远离海岸的区域可选择海岸效应较弱的模式进行反演。经验证,迭代校正法可以在一定程度上消除海岸效应。对海岸效应变化规律和校正方法的研究,为近海地区大地电磁数据的校正和反演提供了参考。Influenced by coast effect,distortions occur on land-side magnetotelluric(MT)data when the distance from coastline is less than the skin depth of target-related frequency.In this study,the variation of coast effect on various types of electrical structures were analyzed and summarized via magnetotelluric three-dimensional(3 D)forward modeling,and the coast effect was quantified by a meansquare-relative-error(MSRE).An iterative correction method was used to correct the coast effect.Coast effect typically depends on the distance from coastline and the resistivity of subsurface layers.It decreases with the increase of the distance,and increases with the increase of the layer resistivity.The intensity of coast effect typically depends on the distance from coastline,sea water depth and the resistivity of deep layers.It decreases with the increase of the distance,and increases with the increase of sea depth.When the distance from coastline is approximately less than 50 km,coast effect increases with decreasing resistivity of deep layers in a transverse magnetic(TM)mode,which is on the contrary in a transverse electric(TE)mode.When the distance from coastline is approximately more than 50 to 100 km,coast effect is weak,and the smaller the resistivity of deep layers,the weaker the coast effect.Additionally,coast effect in TM mode is sensitive to conductive asthenosphere,and the shallower the asthenosphere,the stronger the coast effect.Modeling results indicate that the coast effect in an offshore area should be corrected,the inversion model can be selected based on the coast effect in the area far from the coastline,and the iterative correction method can be better able to eliminate the coast effect.In this paper,the study on the variation and correction methods of coast effect provides a basis for MT data correction and inversion in offshore areas.

关 键 词：海岸效应 大地电磁 三维正演 迭代校正法 

分 类 号：P631[天文地球—地质矿产勘探]