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Processing of passive EM fields acquired during active-source airborne EM surveys
Exploration Geophysics ( IF 0.9 ) Pub Date : 2021-02-04 , DOI: 10.1080/08123985.2021.1882846
Daniel Sattel 1 , Eric Battig 2
Affiliation  

The recording of raw or streamed airborne electromagnetic (AEM) data, as done by CGG during MEGATEM surveys, allows for the extraction of passive EM signals also present, but not normally processed. These include (1) powerline responses, (2) responses in the very low frequency (VLF) range due to radio transmissions and (3) natural-source audio-frequency magnetic (AFMAG) and VLF responses in the frequency range 25 Hz–25 kHz extracted from individual atmospheric electrical discharges (sferics). The latter approach manages to extract good signal in the audio-frequency magnetotelluric (AMT) dead band (1–5 kHz) for one of the discussed data sets. The recording of the three-component AEM data allows for the vector processing of these passive EM responses, including the derivation and modelling of the vertical-to-horizontal magnetic field ratio (tipper) data. Conductivity information can be derived from the tipper data with an apparent conductivity transformation and, more rigorously, with 2D and 3D inversions that take into account the terrain’s topography. The extraction and modelling of passive EM responses is demonstrated on two data sets. A powerline apparent-conductivity grid derived from a MEGATEM survey near Timmins, Canada indicates conductivity structures similar to those in the corresponding active-source EM data. VLF and AFMAG responses derived from South American MEGATEM data show a strong correlation to topography. These data were successfully modelled with 2D and 3D inversions, and the derived shallow conductivity structures confirm and complement the information extracted from the active-source EM data.



中文翻译:

在主动源机载电磁测量期间获取的被动电磁场的处理

原始或流式机载电磁 (AEM) 数据的记录,如 CGG 在 MEGATEM 调查期间所做的那样,允许提取也存在但未正常处理的无源 EM 信号。这些包括 (1) 电力线响应,(2) 由于无线电传输而在极低频 (VLF) 范围内的响应以及 (3) 自然源音频磁 (AFMAG) 和 VLF 响应在 25 Hz–25 频率范围内从单个大气放电 (sferics) 中提取的 kHz。后一种方法设法在所讨论的数据集之一的音频大地电磁 (AMT) 死区 (1–5 kHz) 中提取良好信号。三分量 AEM 数据的记录允许对这些被动 EM 响应进行矢量处理,包括垂直与水平磁场比(倾卸)数据的推导和建模。电导率信息可以通过表观电导率转换以及更严格地考虑地形地形的 2D 和 3D 反演从倾卸车数据中获得。在两个数据集上演示了被动 EM 响应的提取和建模。来自加拿大蒂明斯附近的 MEGATEM 调查的电力线视电导率网格显示的电导率结构类似于相应的有源电磁数据中的电导率结构。来自南美 MEGATEM 数据的 VLF 和 AFMAG 响应显示出与地形的强相关性。这些数据通过 2D 和 3D 反演成功建模,

更新日期:2021-02-04
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