MF band compact helical antenna design and imaging techniques for hydraulic fractures detection

摘要

In this paper we present medium frequency (MF) band imaging systems 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 λ/1400 × λ/1400 × λ/35 while maintaining a relatively high efficiency of about 10%. The feeding network is designed to match the antenna input impedance with the frequency tunability. Based on the proposed antenna, radio-wave techniques for detecting fractured regions in subsurface rock layers are developed. Compared with conventional high frequency mapping system, the penetration distance has increased to kilometers range due to the low propagation loss at MF band. Multi-hole and single-hole setups with different imaging algorithms are studied in the paper. Many methods such as beam-forming, adaptive null-generating and time-gating are performed to minimize the direct-link. Post-processing maps the conductivity contrast in the target area with a two-dimensional resolution.