GNGTS 2014 - Atti del 33° Convegno Nazionale

82 GNGTS 2014 S essione 3.1 In fact, as mentioned before, WED is the process of upward or downward continuation and the datum surface must be totally below or above the topography. WED was carried out using Seismic Unix package (SU), free software developed at the Colorado School of Mines and home codes which were developed to integrate SU. The code (written by Salinas and based on Liu (1995)) adopts a far field approximation of the original formula formulated by Berryhill for post-stack and pre-stack corrections (Berryhill, 1984). The velocity field used to evaluate the travel time table was the tomographic velocity; the datuming aperture was considered equal to 3500 m. It is worth to mention that heavy computations are called for WED application and long-lasting times are required by using PC based work stations. After WED, the zero traces introduced to have a regular distance between receivers and shots were removed. The rapid variation of the topography and velocity in the CROP-18B was a problem in order to complete WED along the whole line. In a part of the line, the problems were mainly related to ray tracing computation that miss. In fact, in several common shot and receiver gathers the topography variation is too steep to be modeled and we carried out several tests in order to avoid these errors. For example, we designed a floating datum instead of a flat datum, we smoothed the velocities and the recording surface, but the ray tracing miss still remained. Finally, we considered each trace as a common shot and receiver gather constituted by a single receiver and shot, respectively. The result was a re-datuming supposing a narrow aperture. A shot gather and its amplitude spectrum extracted along the CROP-18A is shown in Fig. 1 in order to demonstrate that WED enhances higher frequencies, increasing resolution, and improves the signal/noise ratio. Fig. 1 – Shot gather from the line CROP-18A and its amplitude spectrum computed in the time window of 3800 ms: before (left) and after WED (right). The time shift of 500 ms (right) was applied before WED computation to preserve the data. The smiles in proximity of the source point are due to WED migration and were muted before stack.