Abstract: Ground penetrating radar (GPR) has been widely used in tunnel karst detection as a main tunnel advance geological forecast mean. But its time profile obtained by the analysis software in GPR can only analyze karst qualitatively, and is difficult to determine the size, location, shape of karst cave. In this context, a single channel signal method is developed for calculating karst position and the size based on GPR data. First, analysis software in GPR are used to determine which signal channel a karst is located in, then the data of signal with regard to the karst is extracted by self-compiled program and is transformed using the Hilbert function so as to draw out the figure of instantaneous amplitude, instantaneous phase, instantaneous frequency of signal (denoted as three instantaneous figure). and then mutation point of "three instantaneous figure" of each signal is integrated to determine the starting point and the end point of a karst. At last, the caculated results of all channel signals are united to determine the karst position, size and shape. The proposed method is applied to judge the position, size and shape of the karst at the section of DK189 + 135, DK189 + 75 in Zhongfu tunnel of Gan-Long railway, which proved that the method is applicable.

Key words: ground penetrating radar (GPR), karst, position, single channel signal, Hilbert functional transformation