Subsurface cavity imaging by crosshole borehole radar measurements

Travel time tomography uses direct waves of crosshole radar data. A great deal of information concerning the media is contained in later arrivals, such as reflections and diffractions. More reliable results can be obtained by incorporating both direct waves and later arrivals. In this paper, the combination of travel time tomography and migration is introduced. A smoothed velocity model obtained by travel time tomography is used as the basis of migration. Then, a reverse-time migration technique is applied to direct-wave-suppressed crosshole data. The result of tomography indicates that there is an abnormal area, and the result of migration gives a more accurate position of the abnormal area, which is a cavity. The real example shows that the cooperation of tomography and migration is an effective technique for subsurface-cavity imaging. The synthesis of all information, including parallel crosshole measurement and forward simulation, assists in the interpretation of the results of tomography and migration.