Field Experiments to Investigate Cross-polarization Characteristics of a Dipole Array Antenna in a Water-filled Borehole

Summary We investigated cross-polarization characteristics of a dipole array antenna in a water-filled borehole. We theoretically determined the electromagnetic fields when a plane wave is incident to multiple cylindrical layers. According to our analysis, the dipole array antenna may be characterized by a spatial frequency spectrum of the dipole array signals. If a cross-polarized wave is incident on the antenna, the absolute value of the spectrum is maximized to the first order. Furthermore, there is a phase difference of 90 deg. between the co-polarization and cross-polarization incidence angles at the first order spatial frequency. Numerical simulations confirmed that the cross-polarization characteristics in the spatial frequency spectrum may be observed as suggested in the theoretical analysis. Field experiments in soil were carried out at a test site. Data equivalent to signals of a dipole array in a borehole were obtained using a transmitting loop in another borehole. In the experimental data, cross-polarization characteristics were observed similar to those deduced from the theoretical and numerical analyses.