‘Earth–Ionosphere’ Mode Controlled Source Electromagnetic Method

charles t. horngren, srikant m. datar, madhav v. rajan;

In traditional artificial-source electromagnetic exploration, the effects of the ionosphere and displacement current (DC) in the air were neglected, and only the geoelectrical structure of the earth’s crust and upper mantle was considered, such as for controlled source audio frequency magnetotelluric (CSAMT).By employing a transmitter (less than 30 kW) to generate source fields, the CSAMT method overcomes the problems associated with weak natural electromagnetic (EM) fields used in magnetotellurics. However, the transmitter is moved
and the source–receiver offset is approximately less than 20 km, because of the limitation of emission energy. We put forward a new idea, that is, a fixed artificial source (greater than 200 kW) is used and the source location selected at a high resistivity region (to ensure a high emission efficiency), so theremay be a possibility that as long as the source strength magnitude is strong enough, the artificial EM signal can be easily observed within a distance of several thousand kilometres. Previous studies have provided the evidence to support this idea; they used the ‘earth–ionosphere’ mode in modeling the EM fields with the offset up to a thousand kilometres. Such EM fields still have a signal/noise ratio over 10–20 dB; this means that a new EM method with fixed source is feasible. However, in their calculations, the DC which plays a very important role for large off sets was neglected. This paper pays much attention to derive the formulae of the ‘earth–ionosphere’ mode with a horizontal electric dipole source, and the DC is not neglected.We present some three layers modeling results to illustrate the basic EM field characteristics under the ‘earth–ionosphere’ mode. As the offset increases, the contribution of the conduction current decreases, DC and ionosphere were taken into account, and the EM field attenuation decreases. We also quantitatively compare the predicted and observed data. The comparison of these results with the data reveal the excellent agreement between the experimental and theoretical results. The DC and ionosphere affects the EM fields, however impedances (ratio of E to H) are unaffected, and this means we need to include ionosphere and
DC effects to accurately model the EM field amplitudes for optimal setting of measurement parameters, but we do not need to include these complications for the interpretation of the data for the Earth conductivity.