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.

由CGG在MEGATEM调查期间完成的原始或流式机载电磁（AEM）数据的记录，可以提取也存在但无法正常处理的无源EM信号。其中包括（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）在25 Hz–25频率范围内的自然源音频电磁（AFMAG）和VLF响应从单个大气放电（铁氧体）中提取的kHz。对于所讨论的数据集之一，后一种方法设法在音频大地电磁（AMT）死区（1-5 kHz）中提取良好的信号。三成分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/水平磁场比（倾斜）数据的推导和建模。可以从自卸车数据中导出电导率信息，并进行明显的电导率转换，更严格地说，可以使用考虑地形地形的2D和3D反演。在两个数据集上演示了被动EM响应的提取和建模。从加拿大蒂明斯附近的MEGATEM调查获得的电力线视在电导率网格显示的电导率结构与相应的有源源EM数据相似。来自南美MEGATEM数据的VLF和AFMAG响应显示出与地形的强烈相关性。这些数据已成功通过2D和3D反演建模，