Ground-penetrating radar (GPR) technology was used to detect the abnormality of a gravel cushion, and the characteristics of GPR response to four abnormal defects, including gravel cushion shedding, gravel cushions with dense and loose interfaces, a gravel cushion with varied thickness, and a gravel cushion filled with loose rock, were studied. Through temporal profile evaluation, drilling data validation, and comparative analysis of a typical single-channel time-domain waveform and its frequency spectrum, the results show the following: the typical single- channel waveform for gravel cushion shedding is characterized by an absence of the event on the profile, the reverse phase of the weak reflection of the interlayer, spectral energy distributed over the whole frequency range, and strong signals appearing in the 550 to 800 MHz bands; the typical single-channel waveform for the gravel cushions with dense and loose interfaces presents the event on the profile, spectrum energy that concentrates in the low-frequency area, and more energy peaks; the typical single-channel waveform for a gravel cushion with varied thickness shows the time difference in reflected waves between the upper and lower interfaces (upward convex event or pull-down event); the typical single-channel waveform for a gravel cushion filled with loose rock shows strong-amplitude complex waves (a random event with a diffraction fringe), significant energy peak superposition, and a higher peak.