1. A method for producing an image representative of a pattern of leakage of a fluid from a structure buried in the ground, comprising the steps of: translating an antenna array over an area of said ground having said structure buried therein; transmitting and receiving a signal having a discrete frequency selected from a predetermined range of frequencies at each of a plurality of points in said area; storing complex reflectance values having phase and intensity components in a computer memory, each said reflectance value representative of said signal received at one of said points; Fourier transforming said stored complex reflectance values to form a received spatial spectrum; propagating said received spatial spectrum to a predetermined depth; inverse Fourier transforming the propagated spectrum to form a calculated image representative of said pattern of leakage of said fluid; and displaying said calculated image.