Detection of shallow objects using refracted and diffracted seismic waves

The detection of targets such as tunnels, karst, mines and other local heterogeneities is an important but difficult task in subsurface studies. In this paper a new methodology for determining a near-surface velocity-depth model is described. Common shot gathers and first breaks of refracted seismic waves are used as input data. Low-frequency components of the model are constructed by the intercept time method and coherency 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 both 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.