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 target 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.
- Seismic methods
- Velocity analysis