GNGTS 2013 - Atti del 32° Convegno Nazionale

Synthetic example. To test our nostretch routine, we apply the NMOPC to a synthetic gather where the near-offset traces are completely missing, in fact, the minimum offset in the CMP of Fig. 1 is 1000 m. In this example the receiver interval is 50 m and the maximum offset is 5000 m. This synthetic gather is built using the earth layered model of Tab. 1. In this model at far offset the traveltimes deviate from the hyperbolic trajectories particularly for the second reflection. Note that the ray-tracing algorithm we used (Cerveny, 1985) properly computes the amplitude and phase variations with offset of the reflected waveforms. Six reflections with offset variations are present, and the two deeper events are nearly overlapped due to the small thickness of the sixth layer (50 m only). Moreover, the noise level is such that the fourth reflection is almost buried in the noise. Tab. 1 – Earth layered model used for the computation of the synthetic CMP gather of Fig. 1. Thickness (m) Velocity (m/s) 400 1500 200 1700 700 1600 500 2000 1000 2100 50 2400 ∞ 3600 Fig. 2a shows the NMO corrected synthetic data without any mute applied, obtained employing the conventional NMO technique. Several well known artifacts can be observed in this image. First, the two shallower reflections are affected by significant over-stretching and distortions in the interfering part. Second, the traveltime curves which do not correspond to Fig. 1 – Synthetic gather with random noise obtained using a ray-tracing routine (Cerveny, 1985) on the six layers earth model of Tab. 1. To demonstrate the efficacy of the NMOPC the traces up to 1000 m offset have been removed. 19 GNGTS 2013 S essione 3.1