I. Introduction
One of the tantalizing exploration problems in South Korea has been the detection of an intrusive man-made tunnel about 2 m in height. Several researchers had investigated methods to enhance the detectability of such a deeply located tunnel in the 1980s [1]–[5]. Two types of cross-borehole radar systems have been operated on site. One type is cross-borehole continuous wave (CW) radar, the most typical feature of which on an empty tunnel is two significant signal drops at the positions corresponding to the top and bottom boundaries of the tunnel at a specific frequency [6]. It should be noted that the fourth intrusive tunnel in South Korea was detected by employing the cross-borehole CW radar system in 1990 [7]. Since then, some investigations on the cross-borehole CW radar system have been performed [8]–[10]. The other type is cross-borehole pulse radar, the detection principle of which is based on the relatively fast propagation of electromagnetic pulse through an empty tunnel. The pulse radar system has been widely used due to its high exploring speed and relatively compact transmitting and receiving modules [11]–[14]. Underground rock is highly weathered, jointed, and fractured, so the detection of the time-of-peak (TOP) variation according to the empty tunnels is very difficult because of the noisy profile generated by scattering by faults, joints, lodes, and underground water [15]. Olhoeft [3] investigated the first and second peaks of the pulse signatures to extract not only the TOP profile as an equivalent velocity profile in depth but also the time delay between two peaks as a measure of pulse dispersion rate.