A method of determining a location (x,z) of a subsurface boundary (38) of a body (30), comprising providing a wave source (34) and a receiver (36) on a surface (32) of body (30) measuring a travel time of a wave from source (34) reflected to receiver (36) via the boundary (38) and determining horizontal slowness of the wave at the receiver (36) and/or the source (34), as well as the curvature of the wave at the receiver or the source. The location (x,z) is determined on the basis of the measured travel time and the determined horizontal slowness(es) and/or curvature of the wave. The wave may be a seismic wave for geophysical surveying purposes, or an ultrasound wave for medical diagnostic purposes.
