基于等效源法的低纬度地区曲面磁异常化极、分量及张量转换研究
doi: 10.19762/j.cnki.dizhixuebao.2024006
张义蜜1,2 , 熊盛青3 , 何涛1,2 , 王皓4,5 , 王万银1,2,6 , 王林1,2,7
1. 长安大学地质工程与测绘学院,长安大学西部矿产资源与地质工程教育部重点实验室,陕西西安, 710054 ,中国
2. 海洋油气勘探国家工程研究中心,北京, 100028 ,中国
3. 中国自然资源航空物探遥感中心,自然资源部航空地球物理与遥感地质重点实验室,北京, 100083 ,中国
4. 西安测绘研究所,陕西西安, 710054 ,中国
5. 地理信息工程国家重点研究实验室,陕西西安, 710054 ,中国
6. 中国科学院海洋研究所,中国科学院海洋地质与环境重点实验室,山东青岛, 266071 ,中国
7. 那不勒斯费德里克二世大学,地球科学、环境与资源系,80126,意大利
基金项目: 本文为长安大学中央高校基本科研业务费专项资金(编号300102264106) ; 自然资源部航空地球物理与遥感地质重点实验室课题(编号2023YFL06) ; 海洋油气勘探国家工程研究中心2022年开放基金课题(编号CCL2022RCPS0794RQN)联合资助的成果
Reduction-to-the-pole, component and tensor conversion of surface magnetic anomalies at low latitudes based on the equivalent source method
ZHANG Yimi1,2 , XIONG Shengqing3 , HE Tao1,2 , WANG Hao4,5 , WANG Wanyin1,2,6 , WANG Lin1,2,7
1. School of Geology Engineering and Geomatics, Chang'an University, Key Laboratory of Western Mineral Resources and Geological Engineering, Ministry of Education, Chang'an University, Xi'an, Shaanxi 710054 , China
2. National Engineering Research Center of Offshore Oil and Gas Exploration, Beijing 100028 , China
3. China Aero Geophysical Survey and Remote Sensing Center for Natural Resources, Key Laboratory of Airborne Geophysics and Remote Sensing Geology, Ministry of Natural Resources, Beijing 100083 , China
4. Xi'an Research Institute of Surveying and Mapping, Xi'an, Shaanxi 710054 , China
5. State Key Laboratory of Geo-Information Engineering, Xi'an, Shaanxi 710054 , China
6. Key Laboratory of Marine Geology and Environment, Chinese Academy of Sciences, Qingdao, Shandong 266071 , China
7. Dipartimento di Scienze Della Terra, Università Federico II di Napoli, Napoli, 80126, Italy
摘要
化极、分量及张量转换是磁异常解释的重要基础,在低纬度地区尤其磁赤道附近的曲面磁数据观测,化极、分量及张量转换存在着不稳定等问题。本文提出一种基于等效源法的低纬度地区曲面磁异常化极、分量及张量转换方法,该方法利用位场与场源之间的物理关系,并行计算核函数矩阵,建立线性方程组,再辅以正则化灵敏度矩阵加权约束构建目标函数,使用共轭梯度法求解目标函数极小化问题可获得等效源物性分布,之后将其代入相关待重构或转换的磁异常分量或张量正演公式中,即可得到任意曲面的化极磁力异常、分量及张量转换结果,具有较好的稳定性和较高的计算精度。合成模型试验表明,该方法对低纬度地区曲面观测磁异常有着较高的计算精度,对于含噪数据也表现出较好的抗噪性。将该方法应用于卡罗琳板块与太平洋板块交界处Magur Islands研究区的磁异常数据化极和分量转换中,取得了良好的应用效果。
Abstract
Accurate interpretation of magnetic anomalies relies heavily on reduction-to-the-pole (RTP), component conversion, and tensor transformation. However, these processes often encounter instability issues in low-latitude regions, particularly near the magnetic equator. This study introduces a novel approach based on the equivalent source method to address this challenge. Our method performs RTP, component conversion, and tensor transformation of magnetic anomalies over curved surfaces in low-latitude regions. The proposed method leverages the physical relationship between the potential field and its source, employs parallel computation for the kernel function matrix, and establishes a system of linear equations. To enhance stability, we incorporate regularized sensitivity matrix weighting constraints into the objective function. The conjugate gradient method is then used to solve the objective function minimization problem, yielding the equivalent source physical property distribution. These results are subsequently substituted into relevant formulae to reconstruct or convert magnetic anomaly components and tensors, producing stable and highly accurate RTP, component, and tensor transformation results for any surface. Synthetic model tests demonstrate that this method achieves high computational accuracy for surface-observed magnetic anomalies in low-latitude regions, as well as its robustness against noise. Practical application to magnetic anomaly data from the Magur Islands study area, located at the boundary of the Caroline Plate and the Pacific Plate, has yielded highly satisfactory results.
磁测数据是重要的地球物理基础资料,通常测量的ΔT异常是地质体产生的磁异常沿地磁场方向投影的标量异常。然而,受到斜磁化影响,ΔT异常中心在空间位置上并不对应磁性体上方,这对磁测资料解释带来了困难。特别是在低纬度地区,观测所得的磁场形态十分复杂,负异常明显,伴生异常增多。化极(reduction-to-the-pole,RTP)工作就是将倾斜磁化情况下的磁异常转换为垂直磁化情况下的垂直磁异常,使磁异常的解释相对简单化。同时,地磁场和磁性体目标引起的磁异常都是矢量场,通常用3个坐标轴上的分量表示。理论分析和实践表明,磁异常的矢量信息能更全面地反映磁性体的磁场特征(卞光浪等,2011曾小牛等,2016)。
磁异常数据化极、分量及张量转换广泛应用在磁数据处理和解释中(Carvalho et al.,2008Shirman et al.,2015Milano et al.,2016;Nguyen et al.,2016;Plattner and Simons,2017Kolawole et al.,2018Tzanis et al.,2018Zuo Boxin et al.,2020)。化极最早由Baranov(1957)提出,通过空间域褶积的形式计算磁异常数据的化极场,用于处理磁异常投影中非垂直磁化的影响,它将磁异常直接换算到垂直磁化情况下,以获得直观的结果。一般来说,化极方法分为波数域和空间域两类。在波数域主要采用滤波类方法(Macleod et al.,1993姚长利等,2003Li Xiong,2008石磊等,2012Guo Lianghui et al.,2013Zhang Henglei et al.,2014Stewart,2019Ribeiro,2020)来提高化极的稳定性。主要包括维纳滤波法(Hansen and Pawlowski,1989Keating and Zerbo,1996)、视倾角法(Macleod et al.,1993Li Xiong,2008石磊等,2012Ribeiro,2020)、泰勒级数法(Mendonca and Silva,1993;刘卫民和邹新民,1998)、双曲正(余)弦法(张培琴和赵群友,1996)、阻尼因子法(姚长利等,2004林晓星和王平,2012荆磊等,2017)、正则化法(曾小牛等,2016张琪等,2018何涛等,2024)以及迭代方法(骆遥和薛典军,2010姚长利等,2012Hao Mengcheng et al.,2018He Tao et al.,20222024)。尽管波数域化极方法计算速度很快,但在处理低纬度磁数据时,对噪声具有放大作用,计算精度不高。此外,波数域化极方法处理起伏观测数据不如平面观测数据更有效(Zuo Boxin et al.,2021)。为了处理起伏观测面磁异常数据和低纬度问题,可以使用空间域的等效源法作为替代方案。
等效源方法是通过虚拟场源模拟实测异常的一种常用位场数据处理方法,其原理是通过构建等效场源,利用磁场与场源之间的物理关系,以等效场源正演得到的预测数据与当前实测数据的拟合差大小作为评价标准,如果两者拟合差小到可接受范围,那么此时的等效场源就可以用于正演计算其他目标参量。Dampney(1969)提出了经典的等效源方法,该方法使用积分方程计算一组点质量来拟合位场数据。Bhattacharyya and Chan(1977)提出了在任意起伏观测面上计算磁异常等效源的方法。Hansen and Miyazaki(1984)提出了一种等效层方法用来反演卫星磁数据。Blakely(1996)使用了一组规则网排列的等效层,用来计算场源外任意观测面的磁场数据。RTP的等效源方法是由Silva(1986)首次提出的,以处理波数域在低磁纬度地区工作时众所周知的困难。骆遥和薛典军(2009)基于概率成像技术提出了一种等效物性反演方法,先对地下等效源场成像进而计算化极结果。Li and Oldenburg(2010)提出利用小波变换构建磁场等效源的快速算法,以解决空间域等效源方法的计算时间成本,他们在研究中将化极视为反演问题并引入正则化项来抑制噪声的影响。Oliveira et al.(2013)将等效源划分为规则的网格结构,开发了一种具有节约空间成本的方法。Li Duan et al.(2020)提出了多层等效源方法以解决埋深较浅等效源存在的问题,该方法通过构建非规则网结构等效源提高了磁异常数据化极的精度。Zuo Boxin et al.(2020)利用多层等效源对任意观测面的总场磁异常进行向下延拓计算以及磁梯度张量转换。Zuo Boxin et al.(2021)提出了一种基于偏微分方程架构的磁异常低纬度化极和任意曲面间的向上延拓方法。
尽管等效源方法在低纬度化极和分量转换中得到了大量应用,但它依然面临着很多问题需要进一步研究。等效源方法对于存储空间和计算时间的需求则相对较大(Li Duan et al.,2020),近年来,国内外有许多学者对此进行了改进,但也存在提高速度的同时降低了计算精度的问题。另外,等效源方法在低纬度地区的张量转换少有人研究。本文提出了一种基于三维物性反演结构的等效源方法,可同时进行低纬度地区磁异常化极、分量及张量转换。为了提高等效源构建速度和精度,核函数正演采用并行计算,反演中使用灵敏度矩阵加权来自适应抵抗核函数随深度衰减问题,最后采用共轭梯度法求解器得到等效磁化率分布,从而可稳定地进行化极计算、分量及张量转换。通过合成模型试验表明,该方法在低纬度地区有着较高的计算精度,对于含噪数据也表现出较好的抗噪性。将该方法应用于卡罗琳板块与太平洋板块交界处Magur Islands研究区的磁异常数据化极和分量转换中,取得了较好的应用效果。
1 方法原理
1.1 基本理论与方程
根据位场理论,位场函数Fxyz)与场源物性分布函数mξηζ)在场源以外空间Ω的关系可使用积分方程表示
F(x,y,z)=Ω A(x,y,z,ξ,η,ζ)m(ξ,η,ζ)dξdηdζ
(1)
式中,A为积分核函数,是用来表征位场与场源之间物理关系的函数。若位场数据与场源物性分布是离散的,且每个离散单元内物性是均匀分布的,则公式(1)可写为(李端等,2018张义蜜等,2023):
Fxi,yi,zi=j=1N mξj,ηj,ζjGxi,yi,zi,ξj,ηj,ζj(i=1,2,3,,M)
(2)
其中:
Gxi,yi,zi,ξj,ηj,ζj=Γj Axi,yi,zi,ξj,ηj,ζjdξdηdζ
(3)
式中,Γj为第j个离散场源单元的积分区域。假设离散位场观测数据有M个,用di表示第i个观测点的位场值,则公式(2)可简化为:
di=j=1N Gijmj
(4)
将公式(4)写为矩阵形式:
d=Gm
(5)
式中,dM阶向量,mN阶向量,GM×N阶矩阵。
在实际正演核函数G时,由于观测数据是二维的,地下剖分单元达到三维,根据公式(3)可知需要计算五重循环。当剖分单元多,测点数据量大时,核函数矩阵G的计算量是相当大的。为了节省计算时间,本文采用OpenMP并行算法进行加速计算。OpenMP采用fork-join的执行模式,程序开始仅有一个主线程,需要进行并行计算时就会派生出多个分支线程来执行任务,自然提升了速度。
1.2 等效源物性反演
离散的等效源单元可以采用不同的形式,包括磁偶极子、磁偶层及均匀磁化规则形体等。本次选取长方体单元,其有助于“吸收”更多的深部场源信息,且随着等效源深度增加,可增大等效源单元体尺寸,以保持应有的敏感度并减少等效源单元体数量(李端等,2018)。假设等效源单元体受到均匀磁化,可以将等效源单元体的物性值m=(m1m2,···mNT作为未知量,其空间位置与几何形态信息作为已知量,再加上一组观测磁异常数据d=(d1d2,···dMT,即可构成方程。
为了提高方程求解的稳定性并且处理磁异常数据中的噪音,通过最小化数据项和模型项构成目标函数,求取正则化解:
φ=φd+λφs=Wd(Gm-d)22+λWmm22
(6)
式中,φd为数据拟合项;φs为模型拟合项(Lelièvre et al.,2012);λ为正则化因子(Lelièvre et al.,2012);Wd为数据加权矩阵,Wd=diag{1/σi,···,1/σN},其中σi是第i个观测值中包含随机误差的标准差;Wm为模型加权矩阵,取Zhdanov(2002)提出的灵敏度矩阵加权函数Wm=diagdiagGTG))。
为了加快收敛速度,在加权模型参数域求解上述目标函数(Portniaguine and Zhdanov,2002)。将目标函数式(6)转到加权模型参数域进行求解,转换公式为:
dw=Wdd
(7)
Aw=WdAWm-1
(8)
mw=Wmm
(9)
经过变换,目标函数式(6)可写为:
φ=Awmw-dw22+λmw22=Awmw-dwTAwmw-dw+λmwTmw
(10)
采用共轭梯度法(Pilkington,1997; Portniaguine and Zhdanov,1999,2002; Zhdanov et al.,2004)求解最优解mw时必须计算目标函数φ对加权模型mw的一阶导数,其公式为:
φmw=AwTAw+λImw-AwTdw
(11)
求得最优解mw后,再通过以下公式得到模型参数解
m=Wm-1mw
(12)
最终获得等效源物性分布minv
1.3 磁异常化极、分量及张量转换
将反演得到的等效源物性分布minv代入相关待重构或转换磁异常分量的正演公式中(李端等,2018),可得:
drec=G*minv
(13)
式中,drec表示重构或转换磁异常分量向量,可以是化极磁异常或不同方向上的分量,G*代表待计算参量相应的正演核函数矩阵。
2 合成模型试验
2.1 合成模型设计及磁异常正演
为了验证本文方法的有效性,设计了合成模型进行试验。场源由五个大小、埋深均不相同的直立六面体组成,埋藏深度在400~3000 m之间,且处在低纬度地区(磁倾角5°,磁偏角50°),组合模型体详细参数如表1所示(x1和x2分别为直立六面体沿x方向的范围,y1和y2分别为直立六面体沿y方向的范围,z1和z2分别为直立六面体沿z方向的范围),空间位置如图1b。观测面(图1a)为起伏面(z坐标方向铅垂向下为正),起伏高度为1127~1472 m。图1b模型体在图1a观测面上引起的磁异常ΔT、化极磁异常RTP、磁异常三分量HaxHayZa以及磁异常三个张量UxxUyyUzz图2所示。
1模型体空间位置坐标及物性参数
Table1Spatial location coordinates and physical parameters of the model body
1观测面高程及合成模型空间分布图
Fig.1Elevation of observation surface and spatial distribution of synthetic models
(a)—起伏观测面高程图;(b)—合成模型空间位置图
(a) —elevation map of the undulation observation surface; (b) —synthetic model space location map
2图1b模型体在图1a观测面上引起的磁异常、化极磁异常、磁异常三分量以及磁异常张量
Fig.2Fig.1b magnetic anomalies, chemotaxis magnetic anomalies, magnetic anomaly triple components, and magnetic anomaly tensor induced by the model body on the observation surface of Fig.1a
(a)—磁异常ΔT;(b)—化极磁异常RTP;(c)—磁异常水平分量Hax;(d)—磁异常水平分量Hay;(e)—磁异常垂直分量Za;(f)—磁异常张量Uxx;(g)—磁异常张量Uyy;(h)—磁异常张量Uzz
(a) —ΔT; (b) —RTP; (c) —Hax; (d) —Hay; (e) —Za; (f) —Uxx; (g) —Uyy; (h) —Uzz
2.2 不含噪磁异常化极、分量及张量转换
根据本文所提方法,对图2a磁异常进行等效源反演计算。本文反演地下深度为0~5000 m的等效场源,水平范围和观测面水平范围一致。经过等效源化极计算,由图3b可以看出,起伏观测面上的磁异常得到了很好的重构,拟合均方根偏差仅有0.012 nT(图3c),较好地“还原”了原始磁异常(图3a)。利用等效源计算了化极磁异常(图3e),并与理论化极磁异常(图3d)进行对比,计算得到的化极磁异常(图3e)结果与理论化极磁异常(图3d)形态一致,幅值相同,在绝大部分区域异常值偏差小于2 nT。
利用等效源转换得到了磁异常三分量(图4b、e、h),并与理论数据进行对比。转换得到的磁异常三分量(图4b、e、h)结果与理论数据(图4a、d、g)对比,形态特征吻合较好。统计了等效源转换磁异常、化极磁异常及三分量与理论结果偏差(表2),由表2可以看出,磁异常水平分量Hax偏差小于0.3 nT(图4c),均方根偏差仅有0.06 nT;磁异常水平分量Hay偏差小于0.28 nT(图4f),均方根偏差仅有0.05 nT;磁异常垂直分量Za偏差在绝大部分区域异常值偏差小于0.9 nT(图4i),均方根偏差仅有0.18 nT。
磁异常有9个张量,分别为UxxUxyUxzUyxUyyUyzUzxUzyUzz,限于篇幅,本文利用等效源转换得到了磁异常的三个张量,分别是UxxUyyUzz图5b、e、h),并与理论数据进行对比。转换得到的磁异常三个张量(图5b、e、h)结果与理论数据(图5a、d、g)对比,形态特征基本一致。统计了等效源转换磁异常张量与理论结果偏差(表3),由表3可以看出,磁异常张量Uxx计算值和理论值偏差(图5c)的均方根偏差仅有7.49×10-5nT/m;磁异常张量Uyy计算值和理论值偏差(图5f)的均方根偏差仅有8.09×10-5nT/m;磁异常张量Uzz计算值和理论值偏差(图5i)的均方根偏差仅有1.25×10-4nT/m。
2.3 含噪磁异常化极、分量及张量转换
对磁异常(图3a)加入均值为0,标准方差为5 nT的高斯噪声模拟含噪声磁力异常(图6a)。对图6a给出的含噪磁异常反演其地下0~5000 m深度范围内的等效源场。对比拟合磁异常(图6b)和原始磁异常(图6a)可以看出,两者形态一致,幅值相同,得到了很好的重构,误差(图6c)和加入的高斯白噪声水平相当,这说明该方法基本“滤除”了原始噪声的影响。利用等效源计算了化极磁异常(图6e),并与理论化极磁异常(图6d)进行对比,计算得到的化极磁异常(图6e)结果与理论化极磁异常(图6d)形态一致,幅值相同,均方根偏差约为4.4 nT,占原始化极磁异常(图6d)幅值的0.26%。
3理论数据和等效源计算数据对比图
Fig.3Comparison of theoretical data and calculated data from equivalent sources
(a)~(c)—分别为理论磁异常ΔT、等效源拟合磁异常以及两者拟合偏差;(d)~(f)—分别为理论化极磁力异常RTP、等效源计算化极磁力异常以及两者偏差
(a) ~ (c) —the theoretical magnetic anomaly ΔT, the equivalent source fitted magnetic anomaly, and the fitting deviation of the two, respectively; (d) ~ (f) —the theoretical RTP, the equivalent source calculated RTP, and the deviation of the two, respectively
4等效源转换得到磁异常三分量与理论数据对比图
Fig.4Comparison diagram of the three-component magnetic anomalies obtained from equivalent source conversion and theoretical data
(a)~(c)—分别为理论磁异常水平分量Hax、等效源计算磁异常水平分量Hax以及两者偏差;(d)~(f)—分别为理论磁异常水平分量Hay、等效源计算磁异常水平分量Hay以及两者偏差;(g)~(i)—分别为理论磁异常垂直分量Za、等效源计算磁异常垂直分量Za以及两者偏差
(a) ~ (c) —represent the theoretical magnetic anomaly horizontal component Hax, the equivalent source calculated magnetic anomaly horizontal component Hax, and their deviation, respectively; (d) ~ (f) —represent the theoretical magnetic anomaly horizontal component Hay, the equivalent source calculated magnetic anomaly horizontal component Hay, and their deviation, respectively; (g) ~ (i) —represent the theoretical magnetic anomaly vertical component Za, the equivalent source calculated magnetic anomaly vertical component Za, and their deviation, respectively
2等效源转换磁异常、化极磁异常及三分量与理论结果偏差
Table2Equivalent source-converted magnetic anomalies, RTP and three-component deviations from theoretical results
利用等效源转换得到了磁异常三分量(图7b、e、h),并与理论数据进行对比。转换得到的磁异常三分量(图7b、e、h)结果与理论数据(图7a、d、g)对比,形态特征吻合较好。统计了等效源转换磁异常、化极磁异常及三分量与理论结果偏差(表4),由表4可以看出,磁异常水平分量Hax偏差(图7c)的均方根偏差仅有3.04 nT;磁异常水平分量Hay偏差(图7 f)的均方根偏差仅有2.99 nT;磁异常垂直分量Za偏差(图7i)的均方根偏差仅有4.23 nT,相比于原始异常,这些均方根偏差值是较小的。
5等效源转换得到磁异常张量与理论数据对比图
Fig.5Comparison diagram of the magnetic anomaly tensor obtained from equivalent source conversion and theoretical data
(a)~(c)—分别为理论磁异常张量Uxx、等效源计算磁异常张量Uxx以及两者偏差;(d)~(f)—分别为理论磁异常张量Uyy、等效源计算磁异常张量Uyy以及两者偏差;(g)~(i)—分别为理论磁异常张量Uzz、等效源计算磁异常张量Uzz以及两者偏差
(a) ~ (c) —represent the theoretical magnetic anomaly tensor Uxx, the equivalent source calculated magnetic anomaly tensor Uxx, and their deviation, respectively; (d) ~ (f) —represent the theoretical magnetic anomaly tensor Uyy, the equivalent source calculated magnetic anomaly tensor Uyy, and their deviation, respectively; (g) ~ (i) —represent the theoretical magnetic anomaly tensor Uzz, the equivalent source calculated magnetic anomaly tensor Uzz, and their deviation, respectively
3等效源转换磁异常张量与理论结果偏差
Table3Deviation between the equivalent source converted magnetic anomaly tensor and the theoretical results
4等效源转换磁异常、化极磁异常及三分量与理论结果偏差
Table4Deviation between the equivalent source converted magnetic anomaly, RTP and three-component data compared to theoretical results
6理论数据和等效源计算数据对比图
Fig.6Comparison diagram of theoretical data and equivalent source calculated data
(a)~(c)—理论磁异常ΔT、等效源拟合磁异常以及两者拟合偏差;(d)~(f)—理论化极磁力异常RTP、等效源计算化极磁力异常以及两者偏差
(a) ~ (c) —theoretical magnetic anomaly ΔT, equivalent source fitted magnetic anomaly, and their fitting deviation, respectively; (d) ~ (f) —theoretical reduced-to-the-pole (RTP) magnetic anomaly, equivalent source calculated RTP magnetic anomaly, and their deviation, respectively
7等效源转换得到磁异常三分量与理论数据对比图
Fig.7Comparison diagram of the three-component magnetic anomalies obtained from equivalent source conversion and theoretical data
(a)~(c)—分别为理论磁异常水平分量Hax、等效源计算磁异常水平分量Hax以及两者偏差;(d)~(f)—分别为理论磁异常水平分量Hay、等效源计算磁异常水平分量Hay以及两者偏差;(g)~(i)—分别为理论磁异常垂直分量Za、等效源计算磁异常垂直分量Za以及两者偏差
(a) ~ (c) —theoretical horizontal magnetic anomaly component Hax, equivalent source calculated horizontal magnetic anomaly component Hax, and their deviation, respectively; (d) ~ (f) —theoretical horizontal magnetic anomaly component Hay, equivalent source calculated horizontal magnetic anomaly component Hay, and their deviation, respectively; (g) ~ (i) —theoretical vertical magnetic anomaly component Za, equivalent source calculated vertical magnetic anomaly component Za, and their deviation, respectively
同样,利用等效源转换得到了磁异常的三个张量,分别是UxxUyyUzz图8b、e、h),并与理论数据进行对比。转换得到的磁异常三个张量(图8b、e、h)结果与理论数据(图8a、d、g)对比,形态特征基本一致。统计了等效源转换磁异常张量与理论结果偏差(表5),由表5可以看出,磁异常张量Uxx计算值和理论值偏差(图8c)的均方根偏差仅有5.60×10-3nT/m;磁异常张量Uyy计算值和理论值偏差(图8f)的均方根偏差仅有5.93×10-3nT/m;磁异常张量Uzz计算值和理论值偏差(图8i)的均方根偏差仅有9.15×10-3nT/m。
8等效源转换得到磁异常张量与理论数据对比图
Fig.8Comparison diagram of the magnetic anomaly tensor obtained from equivalent source conversion and theoretical data
(a)~(c)—分别为理论磁异常张量Uxx、等效源计算磁异常张量Uxx以及两者偏差;(d)~(f)—分别为理论磁异常张量Uyy、等效源计算磁异常张量Uyy以及两者偏差;(g)~(i)—分别为理论磁异常张量Uzz、等效源计算磁异常张量Uzz以及两者偏差
(a) ~ (c) —theoretical magnetic anomaly tensor Uxx, equivalent source calculated magnetic anomaly tensor Uxx, and their deviation, respectively; (d) ~ (f) —theoretical magnetic anomaly tensor Uyy, equivalent source calculated magnetic anomaly tensor Uyy, and their deviation, respectively; (g) ~ (i) —theoretical magnetic anomaly tensor Uzz, equivalent source calculated magnetic anomaly tensor Uzz, and their deviation, respectively
5等效源转换磁异常张量与理论结果偏差
Table5Deviation between the equivalent source converted magnetic anomaly tensor and the theoretical results
3 实际资料应用
实例所用数据来自磁赤道附近的Magur Islands研究区,该研究区中心点处的磁化倾角为2.78°、磁化偏角为3.66°,是非常良好的低纬度地区试验数据。研究区海底地形起伏较大,深度范围在-7.7~-5444 m之间,Magur岛屿部分地形平坦,深度大约在-60 m左右(图9a)。Magur Islands研究区ΔT磁数据来源于EMAG2(V2)地磁网格数据(earth magnetic anomaly grid,2-arc-minute resolution),异常值范围在-262.8~332.3 nT之间(图9b)。Magur Islands位于卡罗琳板块与太平洋板块交界处,是卡罗琳热点的产物,属于火山岩岛屿,具有高磁特性(Wu et al.,2016),Magur岛屿上方磁异常呈现明显的低磁异常特征,呈现NE走向,磁异常大部分在-100 nT左右。Magur Islands属于火山岩岛屿,其上方应该为高磁异常,但实际确是低磁异常,这与已知地质认识不符,而高磁异常主要分布在火山南北两侧,这是典型的磁赤道附近磁性体磁力异常的表现。
为了降低倾斜磁化对地磁异常造成的影响,使磁异常形态变得相对简单,便于异常分析和解释,对磁异常(图9b)进行化极和分量转换处理,得到化极磁力异常RTP(图10a)、磁异常垂直分量Za图10b)、磁异常水平分量Hax图10c)以及Hax图10d)。化极后,化极磁力异常在Magur Islands处均对应高磁异常区,整体呈现NE向高低值相间分布的条带特征,这是太平洋洋壳磁条带的典型表现,研究区Magur Islands中间高,北东、北向低的磁场特征反映了区内火山岩不同期次、不同阶段的分布格局,以及此区域在火山活动时期受北东向构造的影响。磁异常垂直分量在Magur Islands平面位置处依然是以低磁异常为主,磁异常水平分量Hax高值区与Magur Islands北东向边界以及北西向边界对应较好,在Magur Islands平面位置中央附近,磁异常水平分量Hay有北东向条带特征,这与北东向洋壳磁条带吻合。
9Magur Islands研究区地形和磁异常图
Fig.9Topographic and magnetic anomaly map of the Magur Islands study area
(a)—地形图;(b)—磁异常图(图中黑色虚线为Magur Islands平面位置)
(a) —topographic map; (b) —magnetic anomaly map (the black dashed line in the figure indicates the planar position of the Magur Islands)
10化极和分量转换结果图
Fig.10Results diagram of reduction-to-the-pole and component transformation
(a)—化极磁力异常;(b)~(d)—磁异常垂直分量Za,水平分量HaxHay
(a) —RTP; (b) ~ (d) —vertical magnetic anomaly component Za, horizontal magnetic anomaly components Hax and Hay
4 结论与建议
本文提出了一种基于等效源法的低纬度地区曲面磁异常化极、分量及张量转换方法。该方法主要是利用位场与场源之间的物理关系,并行计算核函数矩阵,结合物性反演流程建立起目标函数,利用共轭梯度法求解目标函数极小问题,获得等效源物性分布。最终,将反演得到的等效源物性分布代入相关待重构或转换磁异常分量、张量的正演公式中,即可同时得到任意曲面的化极磁力异常、分量及张量转换结果。
通过合成模型测试,本文方法对低纬度地区曲面观测的磁异常有着较高的磁异常化极、分量及张量转换精度,且能同时得到任意曲面的磁异常化极、分量及张量转换结果,对于含噪数据也表现出较强的抗噪性。在实际资料处理中,本文方法表现出稳定性和实用性,很好地反应出Magur Islands平面位置下火山岩引起的高磁异常特性。
1观测面高程及合成模型空间分布图
Fig.1Elevation of observation surface and spatial distribution of synthetic models
2图1b模型体在图1a观测面上引起的磁异常、化极磁异常、磁异常三分量以及磁异常张量
Fig.2Fig.1b magnetic anomalies, chemotaxis magnetic anomalies, magnetic anomaly triple components, and magnetic anomaly tensor induced by the model body on the observation surface of Fig.1a
3理论数据和等效源计算数据对比图
Fig.3Comparison of theoretical data and calculated data from equivalent sources
4等效源转换得到磁异常三分量与理论数据对比图
Fig.4Comparison diagram of the three-component magnetic anomalies obtained from equivalent source conversion and theoretical data
5等效源转换得到磁异常张量与理论数据对比图
Fig.5Comparison diagram of the magnetic anomaly tensor obtained from equivalent source conversion and theoretical data
6理论数据和等效源计算数据对比图
Fig.6Comparison diagram of theoretical data and equivalent source calculated data
7等效源转换得到磁异常三分量与理论数据对比图
Fig.7Comparison diagram of the three-component magnetic anomalies obtained from equivalent source conversion and theoretical data
8等效源转换得到磁异常张量与理论数据对比图
Fig.8Comparison diagram of the magnetic anomaly tensor obtained from equivalent source conversion and theoretical data
9Magur Islands研究区地形和磁异常图
Fig.9Topographic and magnetic anomaly map of the Magur Islands study area
10化极和分量转换结果图
Fig.10Results diagram of reduction-to-the-pole and component transformation
1模型体空间位置坐标及物性参数
Table1Spatial location coordinates and physical parameters of the model body
2等效源转换磁异常、化极磁异常及三分量与理论结果偏差
Table2Equivalent source-converted magnetic anomalies, RTP and three-component deviations from theoretical results
3等效源转换磁异常张量与理论结果偏差
Table3Deviation between the equivalent source converted magnetic anomaly tensor and the theoretical results
4等效源转换磁异常、化极磁异常及三分量与理论结果偏差
Table4Deviation between the equivalent source converted magnetic anomaly, RTP and three-component data compared to theoretical results
5等效源转换磁异常张量与理论结果偏差
Table5Deviation between the equivalent source converted magnetic anomaly tensor and the theoretical results
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