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In recent years, several methods for calculating the seismic wave’s travel time have been presented (Moser 1989, Vidale 1988). Among them, the finite difference ray tracing has given reliable results. In this study in order to calculate the travel time in a cell model, Vidal’s finite difference eikonal solver was used. To evaluate, different velocity models were tested and in all cases the results were compatible with the theoretical existing basis.
Then by the damped least square method and conjugate gradient algorithm, which was introduced by Scales (1987), the inversion of the travel time residuals for producing a velocity section of the subsurface media was performed.
In order to qualify the final section, statistics parameters such as resolution matrix, covariance matrix, uncertainty matrix and rms, were used.
This method has been applied on the real data from a weighted concrete dam. At the bottom of the final tomogram a low velocity zone is detected which was attributed to poor quality of the existing rock in that zone. The back projection method was used on the real data to evaluate the method that has been applied in this article. The result obtained from the back projection method compared to the conjugate gradient method was not satisfactory