Borehole Antenna Considerations in the EMRE System: Frequency Band 312.5-2500 kHz

ABSTRACT

The ElectroMagnetic Radiofrequency Echoing (EMRE) system is utilized in radiofrequency imaging (RIM) measurements between two deep boreholes. It is based on radiowave attenuation, making it possible to reconstruct the attenuation distribution of the borehole section (tomogram). The main parts of the EMRE system are a continuous wave (CW) transmitter and superheterodyne-type receiver. The insulated dipole antennas are the transducers between the device and environment, or together they form a transmission line. The system is bistatic because the antennas are placed in separate boreholes.

When an antenna is situated in a dissipative medium, e.g. in a deep borehole, its performance becomes highly environmentally sensitive. Direct measurements of the feed-point current and radiative characteristics are impossible, and numerical models must therefore be used to investigate the borehole effects. A generalized transmission line model of an insulated antenna is a simple and useful way to examine the antenna-related properties. A drawback is that the borehole water must be excluded from the models due to the high wave number of water. The commercial simulation package FEKO is a numerical modelling package and a powerful tool when constructing more realistic antenna and borehole models. FEKO models and transmission line models (WKG, CHEN) were compared and the results coincided in the frequency band of 312.5-2500 kHz. According to the FEKO studies, the influence of borehole water and the location of the antenna in the borehole, i.e. centric or eccentric, did not appear to have a significant effect. The scattering parameter s11, the ratio of forward and returned power, is a useful antenna parameter to estimate the power level that is radiated from the antenna to the surrounding rock. According to the FEKO models, the highest measurement frequency (2500 kHz) of the EMRE system seems to suffer the most under the borehole conditions due to the low values of s11 (~-2dB). The performance level of the system determines the maximum distances (translumination distance) at which the signals are still detectable. According to the results using a simplified measurement geometry, the EMRE system can operate at distances of < 1000 m when the host rock has resistivities of >10 000 Ωm and the lowest frequency of 312.5 kHz is used. At minor distances of 400-600 m, the system operates well at all frequencies.

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