4.2 First-break traveltime tomography

A first-break traveltime tomography model was computed using the picked first breaks for the refraction static corrections using a diving-wave finite-difference based code (Trygvasson et al., 2002). Although the code calculates ray paths and traveltimes in 3D space, the model was forced to be 2D by taking a large cell size perpendicular to the profile. The starting model was set to take into consideration the topography. After considering the characteristics of the picked first-breaks and some tests with different starting models, a model that uses 1000 m/s below the topography with a velocity gradient reaching 5500 m/s at 100 m depth was chosen. A 10 m by 10 m cell size in inline and depth directions were adopted for the computation. After 8 iterations the resulting RMS value was 4.011 ms.