The detection of targets such as tunnels, karst, mines and other local
heterogeneities is an important but difficult task in subsurface stud
ies. In this paper a new methodology for determining a near-surface ve
locity-depth model is described. Common shot gathers and first breaks
of refracted seismic waves are used as input data. Low-frequency compo
nents of the model are constructed by the intercept time method and co
herency inversion. Detection of high-frequency velocity variations is
carried out by refraction tomography using the low-frequency model as
background. Diffracted waves contain valuable information regarding bo
th the structure and composition of seismic media, especially in cases
where tar et size is comparable to seismic wavelength. A diffraction
stack serves as an additional tool for delineation of local scattering
objects. The methodology was applied to define tunnel position and to
search for the karst formations. (C) 1998 Elsevier Science B.V.