Passive EM Processing of MEGATEM and HELITEM Data

Abstract

The recording of raw or streamed data, as done by CGG during MEGATEM and HELITEM surveys, allows for the extraction of passive EM responses, inadvertently recorded during AEM surveys. These include powerline responses in data sets acquired in the vicinity of strong powerlines, VLF responses in data sets recorded with sufficiently high sampling frequencies and potentially AFMAG responses in the frequency range 25-600 Hz. 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 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 of passive EM responses is demonstrated on a number of data sets. A powerline apparent-conductivity grid derived from a MEGATEM survey near Timmins, Canada indicates conductivity structures not evident in the corresponding active-source EM data. VLF responses derived from South American MEGATEM and North American HELITEM data show a strong correlation to topography. The former 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.