起伏地表頻域/時域航空電磁系統(tǒng)三維正演模擬研究

張博, 殷長春, 劉云鶴, 蔡晶

吉林大學(xué)地球探測科學(xué)與技術(shù)學(xué)院， 長春　130026

起伏地表頻域/時域航空電磁系統(tǒng)三維正演模擬研究

張博, 殷長春*, 劉云鶴, 蔡晶

吉林大學(xué)地球探測科學(xué)與技術(shù)學(xué)院， 長春130026

摘要由于航空電磁系統(tǒng)具有工作頻率低、時間延遲短等特點，地形對航空電磁響應(yīng)有很大影響，忽略地形影響會給航空電磁數(shù)據(jù)解釋造成很大誤差.本文將基于非結(jié)構(gòu)化網(wǎng)格的矢量有限元法應(yīng)用于模擬起伏地表條件下頻域/時域(FD/TD)三維航空電磁系統(tǒng)響應(yīng).該方法由于采用非結(jié)構(gòu)網(wǎng)格，與傳統(tǒng)的結(jié)構(gòu)化網(wǎng)格電磁正演算法相比，能更好地擬合地形和地下不規(guī)則異常體，提高對不規(guī)則地形和地下介質(zhì)航空電磁響應(yīng)的計算精度.通過將計算結(jié)果與半空間模型的半解析解及已發(fā)表的結(jié)果進(jìn)行對比，檢驗了本文算法的精度.通過對典型山峰和山谷地形航空電磁響應(yīng)分析對比，總結(jié)了地形對航空電磁響應(yīng)的影響特征.研究結(jié)果對航空電磁地形效應(yīng)的識別和校正具有指導(dǎo)意義.關(guān)鍵詞航空電磁； 三維正演； 地形效應(yīng)； 矢量非結(jié)構(gòu)有限元法

Starting from the frequency-domain Maxwell′s equations, we obtain a vector Helmholtz equation for a stable solution when air is present. We use the Galerkin method to discretize the Helmholtz equation and obtain the final finite-element equations. To accelerate the calculation speed, the source response in a free-space is used as the primary field, while the direct solver is used to deal with the multiple-source problem. After calculating the frequency-domain AEM responses, a Hankel′s transform is applied to obtain the time-domain EM responses.

To check the accuracy of the presented algorithm for AEM topographic modeling,we compare our results with both the semi-analytical results and those from published literatures.After that, we calculate the EM responses for 3D models with only topography and with both topography and anomalous targets.

From profiled AEM responses, we draw similar conclusions to those of Yin (2015), while from area AEM responses, we find that: 1) for frequency-domain AEM systems, the real part of AEM response contains more information to deep earth than the imaginary part; 2) for time-domain AEM system, the magnetic induction dB/dtreveals the underground conductivity distribution better than theBfield. These features provide the theoretical basis for identification and correction of topographic effect from the AEM measurements.

1引言

航空電磁法由于其高效、經(jīng)濟(jì)、且無需地面人員接近勘查區(qū)等特點，在地質(zhì)填圖、油氣勘探、礦產(chǎn)普查及工程和環(huán)境調(diào)查等方面得到了廣泛的應(yīng)用(Wolfgram and Goiden, 2001; Paine and Collins, 2003;Smith et al., 2006; Pfaffhuber et al., 2009;Tan et al., 2009; Yang and Oldenburg, 2013).國內(nèi)外眾多學(xué)者研究表明，地形效應(yīng)對航空電磁響應(yīng)的影響巨大(Liu and Becker, 1992; Annetts et al.，1998；Mitsuhata, 2000; Baba and Seama, 2002; Sasaki and Nakazato, 2003; Nam et al.，2007；劉云鶴和殷長春，2013；蔡晶等，2014).忽略這種影響會給航空電磁數(shù)據(jù)解釋帶來很大誤差(殷長……