Abstract: An approximate formula is derived to calculate the echo from a buried target in the sediment by the modified Kirchhoff approximation of the scattering in two liquid mediums. The echo characteristics of buried rigid targets with arbitrary shapes can be calculated by means of the planar element method and geoacoustic model of the sediment. The effects of sediment are mainly the loss of the sound propagation in the sediment and the effects of the interface are the loss generated by refraction and transmission when the wave through the interface two times. The calculation results show the echo-to-reverberation ratio of a buried target is always very low so that the detection by conventional methods is quite difficult.