Study on the influence and correction method of coast effect on magnetotelluric responses
LI Yongbo1,2,3, WU Qiong1,2,3, WANG Gang1,2,3, ZHANG Zhenyu1,2,3, WANG Shumin1,2,3, XI Yongzai1,2,3
1. Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences, Langfang, Hebei 065000, China; 2. National Research Center of Modern Geological Exploration Engineering Technology, Langfang, Hebei 065000, China; 3. Laboratory of Geophysical Electromagnetic Probing Technologies, Ministry of Natural Resources, Langfang, Hebei 065000, China
Abstract：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 (3D) forward mode-ling, and the coast effect was quantified by a mean-square-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 50km, 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 100km, 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.
李永博, 吴琼, 王刚, 张振宇, 王书民, 西永在. 海岸效应对大地电磁响应的影响及校正方法[J]. 石油地球物理勘探, 2021, 56(3): 631-644.
LI Yongbo, WU Qiong, WANG Gang, ZHANG Zhenyu, WANG Shumin, XI Yongzai. Study on the influence and correction method of coast effect on magnetotelluric responses. Oil Geophysical Prospecting, 2021, 56(3): 631-644.
Tikhonov A N. On determining electric characteristics of the deep layers of the Earth's crust[J]. Dokl Akad Nauk SSR, 1950,73(2):295-311.
[2]
Cagniard L. Basic theory of the magnetotelluric me-thod of geophysical prospecting[J]. Geophysics,1953,18(3):605-635.
[3]
Evans R L,Hirth G,Baba K,et al. Geophysical evidence from the MELT area for compositional controls on oceanic plates[J]. Nature,2005,437(7056):249-252.
[4]
Parkinson W D. Directions of rapid geomagnetic fluctuations[J]. Geophysical Journal of the Royal Astronomical Society,1959,2(1):1-14.
[5]
Cox C. Electromagnetic induction in the oceans and inferences on the constitution of the earth[J]. Geophysical Surveys,1980,4(1-2):137-156.
[6]
Ranganayaki R P,Madden T R. Generalized thin sheet analysis in magnetotellurics:an extension of Price's analysis[J]. Geophysical Journal of the Royal Astronomical Society,1980,60(3):445-457.
[7]
Chave A D,Cox C S. Electromagnetic induction by ocean current and the conductivity of the oceanic litho-sphere[J]. Journal of Geomegnetism and Geoelectri-city,1983,35(11-12):491-499.
[8]
Mackie R L,Bennett B R,Madden T R. Long-period magnetotelluric measurements near the Central California Coast:a land-locked view of the conductivity structure under the Pacific Ocean[J]. Geophysical Journal,1988,95(1):181-194.
[9]
李桐林,林君,刘福春,等.国内外海洋大地电磁测深的发展综述[J].国外地质勘探技术,1998,(4):1-10.LI Tonglin,LIN Jun,LIU Fuchun,et al. A summary of the development of marine magnetotelluric sounding[J]. Foreign Geoexploration Technology,1998,(4):1-10.
[10]
杨文采,方慧,程振炎,等.苏鲁超高压变质带北部地球物理调查(Ⅱ)——非地震方法[J].地球物理学报,1999,42(4):508-519.YANG Wencai,FANG Hui,CHENG Zhenyan,et al. Geophysical investigation of northern Sulu UHPM terrane in east China(Ⅱ):non-seismic methods[J]. Chinese Journal of Geophysics,1999,42(4):508-519.
[11]
Jones F W,Price A T. Geomagnetic effects of slo-ping and shelving discontinuities of earth conductivity[J]. Geophysics,1971,36(1):58-66.
[12]
Lines L R,Jones F W. The perturbation of alternating geomagnetic fields by an island near a coastline:reply[J]. Canadian Journal of Earth Sciences,1973,10(11):1703-1704.
[13]
Jones F W,Lokken J E. Irregular coastline and channeling effects in three-dimensional geomagnetic perturbation models[J]. Physics of the Earth and Planetary Interiors,1975,10(2):140-150.
[14]
Parkinson W D,Jones F W. The geomagnetic coast effect[J]. Reviews of Geophysics,1979,17(8):1999-2015.
[15]
Cox C S,Constable S C,Chave A D,et al. Controlled-source electromagnetic sounding of the oceanic lithosphere[J]. Nature,1986,320(6057):52-54.
[16]
Olsen N,Kuvshinov A. Modeling the ocean effect of geomagnetic storms[J]. Earth Planets & Space,2004,56(5):524-530.
[17]
Key K,Constable S C. Coast effect distortion of marine magnetotelluric data:Insights from a pilot study offshore northeastern Japan[J]. Physics of the Earth and Planetary Interiors,2011,184(3-4):194-207.
[18]
Tamara W,Marion J,Andrei S. Approximations for the 2-D coast effect on marine magnetotelluric data[J].Geophysical Journal International,2012,189(1):357-368.
[19]
Malleswari D,Veeraswamy K. Numerical simulation of coast effect on magnetotelluric measurements[J]. Acta Geodaetica Et Geophysica,2014,49(1):17-35.
[20]
Wang S G,Constable S C,Reyes V,et al. A newly distinguished marine magnetotelluric coast effect sensitive to the lithosphere-asthenosphere boundary[J]. Geophysical Journal International,2019,218(2):978-987.
[21]
魏文博,邓明,谭捍东,等.我国海底大地电磁探测技术研究的进展[J].地震地质,2001,23(2):131-137.WEI Wenbo,DENG Ming,TAN Handong,et al. Development of marine magnetotelluric prospecting technique in China[J]. Seismology and Geology,2001,23(2):131-137.
王东坡,曾效箴,薛林福,等.海洋阵列大地电磁测深法在辽东湾滩海深部地质构造研究中的应用[J].石油与天然气地质,2000,21(4):293-299.WANG Dongpo,ZENG Xiaozhen,XUE Linfu,et al. Oceanic array magnetotelluric sounding used in study of deep structures in beach sea of Liaodong Bay[J]. Oil & Gas Geology,2000,21(4):293-299.
[24]
邓明,刘方兰,张启升,等.海陆联合大跨度多点位海底大地电磁同步数据采集[J].科技导报,2006,124(10):28-32.DENG Ming,LIU Fanglan,ZHANG Qisheng,et al. Long-span and multi-point synchronizing data acquisition for seafloor magnetotelluric based on union of marine and land[J]. Science & Technology Review,2006,124(10):28-32.
[25]
于鹏,王家林,吴健生,等.海陆联测的大地电磁数据处理与综合解释[J].同济大学学报(自然科学版),2006,34(9):1264-1269.YU Peng,WANG Jialin,WU Jiansheng,et al. Processing and integrated geophysical interpretation of land-sea synchronously-observed magnetotelluric sounding data[J]. Journal of Tongji University (Na-tural Science),2006,34(9):1264-1269.
[26]
丁建荣,王勇,于鹏,等.浅海电磁道分离同步采集的大地电磁测深观测试验[J].石油地球物理勘探,2006,41(1):107-110.DING Jianrong,WANG Yong,YU Peng,et a1.MT surveying test of synchronous acquisition by separated electric and magnetic field receiving stations on shallow sea[J]. Oil Geophysical Prospecting,2006,41(1):107-110.
[27]
张全胜,王家映,叶景艳,等.雷琼地区滩海MT勘探可行性试验研究[J].地质与勘探,2006,42(1):64-67.ZHANG Quansheng,WANG Jiaying,YE Jingyan,et al. A feasibility test of offshore magnetotelluric prospecting in the Leiqiong area[J]. Geology and Prospecting,2006,42(1):64-67.
[28]
杨生.水域MT勘探电磁信号分离采集的视电阻率计算[J].石油地球物理勘探,2006,41(5):592-595.YANG Sheng. Computation of apparent resistivity for separately acquired electromagnetic signals in MT exploration of water domain[J]. Oil Geophysical Prospecting,2006,41(5):592-595.
[29]
杨进,魏文博,王光锷,等.海水层对海洋大地电磁勘探的影响研究[J].地学前缘,2008,15(1):217-221.YANG Jin,WEI Wenbo,WANG Guang'e,et al. The effect of seawater in the marine magnetotelluric sounding[J]. Earth Science Frontiers,2008,15(1):217-221.
[30]
许建荣,柳建新,李爱勇,等.水域大地电磁测深中水体对电磁场的影响[J].中南大学学报(自然科学版),2007,38(3):567-573.XU Jianrong,LIU Jianxin,LI Aiyong,et al. Liquid effect upon electromagnetic fields in water area MT exploration[J]. Journal of Central South University(Science and Technology),2007,38(3):567-573.
[31]
张帆,魏文博,金胜,等.海岸效应对近海地区大地电磁测深数据畸变作用研究[J].地球物理学报,2012,55(12):4023-4035.ZHANG Fan,WEI Wenbo,JIN Sheng,et al. Ocean coast effect on land-side magnetotelluric data in the vicinity of the coast[J]. Chinese Journal of Geophy-sics,2012,55(12):4023-4035.
[32]
李飞,董浩,崔志强,等.滩涂区综合地球物理调查技术在栟茶河断裂空间展布中的应用[J].地质与勘探,2020,56(3):566-579.LI Fei,DONG Hao,CUI Zhiqiang,et al. Application of integrated geophysical survey technology in the spatial distribution of Benchahe fault in Rudong mudflat area,Jangsu Province[J]. Geology and Exploration,2020,56(3):566-579.
[33]
Nolasco R,Tarits P,Filloux J H,et al. Magnetotelluric imaging of the Society Islands hotspot[J]. Journal of Geophysical Research Atmospheres,1998,103(B12):30287-30309.
[34]
Snatos F M,Nolasco M,Almeida E P,et al. Coast effects on magnetic and magnetotelluric transfer functions and their correction:application to MT soundings carried out in SW Iberia[J]. Earth Planetary Science Letters,2001,186(2):283-295.
[35]
Baba K,Chave A D. Correction of seafloor magnetotelluric data for topographic effects during inversion[J]. Journal of Geophysical Research Atmospheres,2005,110(B12):1-16.
[36]
Yang J M,Lee C,Yoo H. Correction of the sea effect in the magnetotelluric(MT) data using an iterative tensor stripping during inversion[J]. Journal of Korean Geophysics and Geophysical Exploration,2008,(11):286-301.
[37]
Yang J M,Min D J,Yoo H S. Sea effect correction in magnetotelluric(MT) data and its application to MT soundings carried out in Jeju Island,Korea[J]. Geophysical Journal International,2010,182(2):727-740.
[38]
Unsworth M,Soyer S,Tuncer V,et al. Hydrogeolo-gic assessment of the Amchitka Island nuclear test site(Alaska) with magnetotellurics[J]. Geophysics,2007,72(3):B47-B57.
[39]
Lee T J,Song Y H,Toshihiro U. Three-dimensional magnetotelluric surveys for geothermal development in Pohang,Korea[J]. Exploration Geophysics,2007,38(1):89-97.
[40]
Sébastien de la K de R,Rune M. 3D marine magnetotelluric modeling and inversion with the finite-diffe-rence time-domain method[J]. Geophysics,2014,79(6):E269-E286.
[41]
王培杰,胡华,徐菲,等.基于四叉树网格的MT二维正演[J].石油地球物理勘探,2019,54(3):709-718.WANG Peijie,HU Hua,XU Fei,et al. Magnetotelluric data 2D forward modeling based on quadtree grid[J]. Oil Geophysical Prospecting,2019,54(3):709-718.
王鹏飞,王书明.改进的布谷鸟MT反演算法[J].石油地球物理勘探,2020,55(1):217-225.WANG Penfei,WANG Shuming. MT data inversion based on improved cuckoo search algorithm[J]. Oil Geophysical Prospecting,2020,55(1):217-225.
[44]
Egbert G D,Kelbert A. Computational recipes for electromagnetic inverse problems[J]. Geophysical Journal International,2012,189(1):251-267.
[45]
Kelbert A,Meqbel N,Egbert G D,et al. ModEM:A modular system for inversion of electromagnetic geo-physical data[J]. Computers & Geosciences,2014,66:40-53.
[46]
Constable S C,Parker R L,Constable C G. Occam's inversion:A practical algorithm for generating smooth models from electromagnetic sounding data[J]. Geophysics,1987,52(3):289-300.
[47]
Groothedlin C D D,Constable S C. Occam's inversion to generate smooth,two-dimensional models from magnetotelluric data[J]. Geophysics,1990,55(12):1613-1624.
[48]
Constable S C,Key K,Lewis L. Mapping offshore sedimentary structure using electromagnetic methods and terrain effects in marine magnetotelluric data[J]. Geophysical Journal International,2009,176(2):431-442.
