Karst cavities beneath bored cast in situ piles are hazardous to the stability of infrastructure projects. Therefore, it is important to detect karst cavities during the construction of piles. Downward-looking sonar deployed at the bottom of a pile hole can be used to detect cavities; however, interference of multiple reflected surface waves from the walls of the pile hole masks the weak cavity reflections. We have developed a sonar method that exploits the instantaneous phase difference between signals recorded at two receivers to detect karst cavities beneath piles. The receiver separation is set to half the dominant wavelength of surface waves propagating along the pile hole. We define the instantaneous phase difference intensity (IPDI) as an index that measures the similarity of instantaneous phase between the signals at the two receivers. Higher IPDI values signify that the two signals have a similar instantaneous phase at that time, which implies the arrival of a reflection from a cavity. Reflected surface-wave arrivals exhibit low IPDI by design of the receiver geometry. Thus, the first break of reflected P-waves from the roof and floor of a cavity can be identified. We evaluate the effectiveness of the IPDI-based analysis method using numerical tests simulating the varying depth, azimuth, and size of karst cavities. A prototype using the IPDI analysis demonstrates the application of the new pile hole sonar method at two field investigations. Advance drilling, borehole optical image logs, and crosshole tomography verify the IPDI detection results. We conclude that the two-receiver sonar instrumentation along with the IPDI analysis are effective for detecting cavities beneath piles.