Advantages and limitations of helicopter borne TEM systems employing flying loop and grounded cable transmitters

The helicopoter borne TEM (HTEM) methods have now emerged as the most popular EM exploration tool. Most of the HTEM systems fly a large loop transmitter with a small receiver coil placed either at its centre or in close vicinity. Such systems employ different types of primary pulse shapes and can have dipole moment varying over a large range. There is only one HTEM system, namely the GREATEM system, which utilizes a long grounded cable as source transmitter. Response characteristics of these two generically different classes of HTEM systems employing inductive loop and grounded cable transmitters are studied for various values of the earth resistivity. Numerically modelled transient decay responses, appropriately normalized to account for the pulse shapes and the dipole moment, are analysed to study the capability of various systems in resolving the earth resistivity. The results emphatically bring out the basic difference in the EM excitation of the earth by the inductive and galvanic systems. While the inductive systems generate only TE modes, the galvanic excitation generates both TE and TM modes. This provides a much better resolution capability to the GREATEM system. This advantage, however, should be viewed in terms of the operational flexibility of the two classes of the HTEM systems. The results are nevertheless significant as they clearly indicate the additional benefits of TM mode excitation suggesting new areas of applications. We have also analysed the ‘Tau’ (time constant) images obtained by the two classes of HTEM systems. It is found that the GREATEM system also images the effect of source cable on the host medium due to continuous ground energization. This influence dominates the ‘Tau’ imaging at late times.