Analysis of a directional borehole radar signal with a time-frequency distribution

The authors developed a directional borehole radar which is equipped with conformal micro array antennas on a conducting cylinder. We estimate the incident angle of incoming reflected wave by measuring the induced surface current at several points on the cylinder. Field measurement was carried out in Kamaish mine, Japan. Measurement was made with a network analyzer based system to obtain broad-band information (O.3MHz - 3GHz). We calculated transient reflection signal from the measured data in the frequency domain by FFT algorithm. However, we found that the measured signal suffers from the direct coupling between transmitting and receiving antennas. Direct coupling is one of the most common problems in ground penetrating radar, it sometimes shades the reflected wave from the surrounding material. Applying time-frequency analysis to the measured signal, we obtained a time-frequency distribution (Choi-Williams distribution). Phase velocity of guided waves along this structure was theoretically analyzed and we could identify them in this time-frequency distribution of the received signal. Consequently, we could extract reflected waves, which contain information inside the surrounding rock.