Crosshole radar tomographic inversion without ray tracing

We implemented crosshole tomographic inversion algorithm without ray tracing. We presented three types of regularization method for traveltime inversion of crosshole ground penetrating radar (GPR) first-arrival data to estimate velocity structure. FMM algorithm was used to expand wavefront. Vidale's method was used to calculate traveltimes by solving the eikonal equation for the position of the wavefront and thus avoided tracing rays. Velocity estimation was carried out by the iterative linearized inversion approach. Matrix inversion was implemented by using the LSQR algorithm. Two assumed physical models with abnormal velocity areas were built and we tested the suggested method on them. By comparing the results of three types of weighting operators, we can see that the proposed technique is an efficient traveltime inversion scheme for crosshole GPR data.