Experiments with a dipole array and loop antenna in a cylindrical water layer

This paper describes subsurface measurements using a singlehole directional borehole radar system that revealed the presence of a kind of guided wave propagating along the borehole. The radar system used dipole arrays for both the transmitter and receiver. In this case, all the antenna elements were located off the center axis of the borehole. Computer simulations confirmed that the off-center dipole would excite the HE
₁₁
mode above about 200 MHz. The characteristics of the mode are that there is much phase difference among the array signals, and waveforms in the time domain are shaped like a damped oscillatory wave. To test these predictions, a borehole radar system was designed and built with a dipole array and a loop antenna for both the transmitter and receiver; the system was used for field experiments in soil. We observed the HE
₁₁
mode waves at frequencies above 200 MHz, as predicted in the simulation. When the center frequency was 100 MHz, there was no damped oscillatory wave in the time domain in the experimental data, and we confirmed that there was no guided wave, as expected from the computer simulation.