Basic principles and measuring techniques of land son ar method, i.e., the land minimum offset distance high-frequ ency elastic wave -reflection continuous profile method, are presented, and an application exampl e of karst detection for a highway foundation is given. Compared with ordinary d etection method, the method has many merits such as high resolution ratio, avoid ing lots of disturbed waves, continuous collection for single trace, high energy of reflection waves and minimum offset distance. And it can receive good effect for shallow*.layer seismic wave exploration and can be used in narrow area and complicated environment.