Host medium effects on the response of subsurface conductors in helicopter borne time domain electromagnetic exploration

Modification in the decay behaviour of vertical magnetic fields recorded by a typical HTEM system is studied considering a plate conductor in a conducting host. Good to excellent conductors sustain the induced eddy currents and thus yield slower decay rates in the recorded TEM response. For plate (or sheet like) conductors this behaviour can be seen clearly in the conductance aperture diagrams for various TEM systems. As the plate conductance increases, the amplitudes get smaller and yield slower decay rates. Thus it becomes difficult to discriminate between very good to excellent conductors. Measurements of ‘on time’ response or ‘B’ field instead of the ‘dB/dt’ field may improve the conductance discrimination to some extent. Present study reveals that current channelling due to host significantly modifies the decay of fields recorded over good to excellent conductors. For comparison, the responses of the host medium alone and the plate conductor in the absence of conducting host (air) is also computed. As the host medium conductivity increases its response dominates the response from the plate conductor to later times. Response in the presence of a very resistive (10,000 Ω.m) host is found close to that from the plate in air. However, the conductance aperture diagrams for various host medium resistivity values reveal that there is a divergence in various channel amplitudes for very good to excellent conductors. This is particularly observed for cases when the resistivity of the host medium is towards the higher values. For the lower values of the host medium resistivity, however, the response of even very good plate conductors is dominated by the currents in the host medium. The study suggests that a mildly conducting host medium may help in discriminating the conductivity of very good to excellent plate conductors.