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.