Super-miniaturized borehole antenna design and radio-wave estimation of sub-surface hydraulic fractures at MF band

In this paper we present a medium frequency (MF) band imaging system to estimate the sub-surface hydraulic fractures. We propose a helical dipole antenna for the borehole imaging application. The proposed antenna is loaded with a ferrite-bundle and is optimized to achieve an ultra-compact size to fit within a volume of λ₀/1400 × λ₀/1400 × λ₀/35 while maintaining a relatively high efficiency of about 20%. The feeding network is designed to match the antenna input impedance with the frequency tunability. Based on the proposed antenna, a radio-wave technique for detecting fractured regions in subsurface rock layers is developed. Compared with conventional high frequency mapping system, the penetrating distance has increased to kilometers range due to the low propagation loss at MF band. The method is based on single-hole setup with separate transmitter and receiver antennas. The time-gating is performed to minimize the direct-link signals. Post-processing maps the conductivity contrast in the target area with a good resolution.