低温SQUID瞬变电磁法中去除地磁场影响的方法研究
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摘要
采用低温SQUID(Superconducting Quantum Interference Device)进行瞬变电磁测量,具有高灵敏度的优势,但易受到地磁场的影响.由于地磁场变化为nT量级,而低温SQUID灵敏度可达■.使用传统的双极性脉冲激励叠加对地磁场的抑制作用有限.当晚期道二次场信号叠加了地磁场信号后,导致晚期道信号发生整体抬升或降低,甚至出现负值,使得晚期道信号无法直接利用,从而直接影响勘探深度.本文研究不同噪声环境下信号叠加对噪声压制的机理,并结合地磁场的特点,对off-time晚期道噪声段进行一次线性拟合,获得近似地磁场变化规律,并通过外推得到整个off-time地磁场干扰,最终去除off-time信号中的地磁场.去除地磁场后晚期道瞬变电磁信号信噪比明显改善,勘探深度得到较大提高.
Using Low-Temperature Superconducting Quantum Interference Device(LT SQUID)as B-sensor in transient EM(TEM)measurements, higher magnetic sensitivity has been achieved than the conventional sensors. However, the geomagnetic field can be a serious issue because the geomagnetic field range in nT, while the sensitivity of LT SQUID is about ■. The suppression to the geomagnetic field is limited by the conventional stacking of bipolar pulse excitation, thus, when the secondary field signal is mixed with the geomagnetic field signal, it is found that there always exist an offset in the late off-time channels after stacking, making the off-time signal unusable. In this paper, we first study the noise suppression by stacking in different noise environments. Then, we analyze the features of geomagnetic field and after that we propose a polynomial fitting method to correct the geomagnetic field for the whole off-time channels. In this way, we can extend the usable signal to very late time channels, so that the depth of exploration of TEM systems can be improved.
Using Low-Temperature Superconducting Quantum Interference Device(LT SQUID)as B-sensor in transient EM(TEM)measurements, higher magnetic sensitivity has been achieved than the conventional sensors. However, the geomagnetic field can be a serious issue because the geomagnetic field range in nT, while the sensitivity of LT SQUID is about ■. The suppression to the geomagnetic field is limited by the conventional stacking of bipolar pulse excitation, thus, when the secondary field signal is mixed with the geomagnetic field signal, it is found that there always exist an offset in the late off-time channels after stacking, making the off-time signal unusable. In this paper, we first study the noise suppression by stacking in different noise environments. Then, we analyze the features of geomagnetic field and after that we propose a polynomial fitting method to correct the geomagnetic field for the whole off-time channels. In this way, we can extend the usable signal to very late time channels, so that the depth of exploration of TEM systems can be improved.
引文
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CHEN Bin, LU Cong-De, LIU Guang-Ding. 2014. A denoising method based on kernel principal component analysis for airborne time domain electromagnetic data [J]. Chinese Journal Of Geophysics(in Chinese), 57(1): 295-302, doi: 10.6038/cjg20140125.
Chwala A, Smit J P, Stolz R, et al. 2011. Low temperature SQUID magnetometer systems for geophysical exploration with transient electromagnetics [J]. Superconductor Science and Technology, 24(12): 4535- 4537.
GUO Feng-Xia, ZHANG Yi-Jun, YAN Mu-Hong. 2007. The secular variation characteristics and mechanisms of geomagnetic field [J]. Chinese Journal Of Geophysics(in Chinese), 50(6): 1649-1657.
Ji Y, Du S, Xie L, et al. 2016. TEM measurement in a low resistivity overburden performed by using low temperature SQUID [J]. Journal of Applied Geophysics, 135: 243-248.
Macnae J C, Lamontagne Y, West G F. 1984. Noise processing techniques for time-domain EM systems [J]. Geophysics, 49(7): 934-948.
WANG Chi-jun, CHEN Xiao-dong, ZHAO Yi, et al. 1999. application of 77 K squid magnetometer in TEM. Chinese Journal of Geophysics(in Chinese), 42(S1): 161-166.
常凯. 2015. 低温SQUID瞬变电磁大深度勘探关键技术研究[博士论文]. 上海: 中国科学院上海微系统与信息技术研究所.
陈斌, 陆从德, 刘光鼎. 2014. 基于核主成分分析的时间域航空电磁去噪方法[J]. 地球物理学报, 57(1): 295-302, doi: 10.6038/cjg20140125.
郭凤霞, 张义军, 言穆弘. 2007. 地磁场长期变化特征及机理分析[J]. 地球物理学报, 50(6): 1649～1657.
李实, 宋建平, 李创社. 2001. 瞬变电磁仪中几种干扰的消除方法[J]. 西安交通大学学报, 35(4): 373-376, 381.
刘家富, 雷宛, 张扬, 等. 2014. 瞬变电磁信号去噪方法对比分析与评价 [J]. 工程勘察, 42(11): 88-93.
刘卫强, 陈儒军, 向毕文. 2015. 稳健 M估计用于瞬变电磁数据抽道叠加与噪声压制[J]. 物探与化探, 39(6): 1238-1244.
牛之琏. 2007. 时间域电磁法原理[M]. 长沙:中南大学出版社.
裘伟, 李享元, 程衍富. 2014. 基于小波和曲线拟合对瞬变电磁信号去噪的优化[J]. 现代电子技术, 418(11): 61- 64.
王赤军, 陈晓东, 赵毅, 等. 1999. 77 K SQUID磁强计在TEM测量上的应用研究. 地球物理学报, s1(42): 161-166.
CHEN Bin, LU Cong-De, LIU Guang-Ding. 2014. A denoising method based on kernel principal component analysis for airborne time domain electromagnetic data [J]. Chinese Journal Of Geophysics(in Chinese), 57(1): 295-302, doi: 10.6038/cjg20140125.
Chwala A, Smit J P, Stolz R, et al. 2011. Low temperature SQUID magnetometer systems for geophysical exploration with transient electromagnetics [J]. Superconductor Science and Technology, 24(12): 4535- 4537.
GUO Feng-Xia, ZHANG Yi-Jun, YAN Mu-Hong. 2007. The secular variation characteristics and mechanisms of geomagnetic field [J]. Chinese Journal Of Geophysics(in Chinese), 50(6): 1649-1657.
Ji Y, Du S, Xie L, et al. 2016. TEM measurement in a low resistivity overburden performed by using low temperature SQUID [J]. Journal of Applied Geophysics, 135: 243-248.
Macnae J C, Lamontagne Y, West G F. 1984. Noise processing techniques for time-domain EM systems [J]. Geophysics, 49(7): 934-948.
WANG Chi-jun, CHEN Xiao-dong, ZHAO Yi, et al. 1999. application of 77 K squid magnetometer in TEM. Chinese Journal of Geophysics(in Chinese), 42(S1): 161-166.
常凯. 2015. 低温SQUID瞬变电磁大深度勘探关键技术研究[博士论文]. 上海: 中国科学院上海微系统与信息技术研究所.
陈斌, 陆从德, 刘光鼎. 2014. 基于核主成分分析的时间域航空电磁去噪方法[J]. 地球物理学报, 57(1): 295-302, doi: 10.6038/cjg20140125.
郭凤霞, 张义军, 言穆弘. 2007. 地磁场长期变化特征及机理分析[J]. 地球物理学报, 50(6): 1649～1657.
李实, 宋建平, 李创社. 2001. 瞬变电磁仪中几种干扰的消除方法[J]. 西安交通大学学报, 35(4): 373-376, 381.
刘家富, 雷宛, 张扬, 等. 2014. 瞬变电磁信号去噪方法对比分析与评价 [J]. 工程勘察, 42(11): 88-93.
刘卫强, 陈儒军, 向毕文. 2015. 稳健 M估计用于瞬变电磁数据抽道叠加与噪声压制[J]. 物探与化探, 39(6): 1238-1244.
牛之琏. 2007. 时间域电磁法原理[M]. 长沙:中南大学出版社.
裘伟, 李享元, 程衍富. 2014. 基于小波和曲线拟合对瞬变电磁信号去噪的优化[J]. 现代电子技术, 418(11): 61- 64.
王赤军, 陈晓东, 赵毅, 等. 1999. 77 K SQUID磁强计在TEM测量上的应用研究. 地球物理学报, s1(42): 161-166.