GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 81 In this study, we have applied the wave equation datuming (WED) technique to remove the effects of topography, the near surface velocity variations and source generated noise on seismic data, moving shots and receivers to a given datum plane. Moreover, WED has contributed to attenuate ground roll, enhance higher frequencies, increase the resolution and improve the signal/noise ratio. After WED, the sections were completed with standard processing steps, such as predictive- and fx-deconvolution, trimmed mean-dynamic filtering and post-stack time migration. The re-processing of the CROP seismic data. The transect CROP-18A&B was acquired in 1995, mostly along strike of the Apennine structural trends. The total length of the reprocessed part is 123 km subdivided in two lines: CROP-18A long 48 km and CROP-18B with 74 km. CROP-03 was recorded in 1993; we processed of this profile only the Tuscany segment crossing the Inner Northern Apennines domain with a length of 63 km, from the Tyrrhenian Sea to the town of Pienza. The main acquisition parameters are reported, for example, in Accaino et al. (2005). The first step of the re-processing consisted of picking all first arrivals. Thetomographic inversion of first breaks, by an iterative method of ray-tracing and travel times calculations, created a near surface velocity model ��� ��� ����� ���������� ����� ����������� ������������ for the upper sequences, where significant complexities in the structural setting characterised by variable lithologies and velocities are present�. To performthe inversion, an initialmodel was built with square cells 30mwide, corresponding to half receivers distance. We adopted the commercial software TomoPlus ® (GeoTomo LLC: www.geotomo.com ), which is based on the minimization of the difference between calculated and observed travel times. The forward ray-tracing modeling was carried out attaining the final representation after 50 iterations. This first output was re-entered in the tomography process as a new initial model, submitted again to 25 iterations before the final result. A pre-processing was applied to the seismic data to increase the signal to noise ratio with amplitude recovery and to balance the shot and receivers responses. The trace editing removed the noisy parts. Then, we proceeded with the wave equation datuming application in order to move all shots and receivers at the same surface, removing time shifts related to topography and near-surface velocity variations, and to improve the signal/noise ratio. The Kirchhoff integral solution to the scalar wave equation [using both near-field and far-field terms; Berryhill (1979, 1984)] can provide a basis computation to deal with irregular surfaces (Wiggins, 1984; Bevc, 1997) and variable velocities. In the pre-stack domain, WED is applied in two steps: (i) common-source and (ii) common-receiver domains. Operating on a common-source gather has the effect of extrapolating the receivers from one datum to another, and, because of reciprocity, operating on a common-receiver gather changes the datum of the source. Recalling the main principles of the theory, it is important to distinguish between migra- tion and WED (e.g., Barison et al. , 2011). Migration involves computing the wave-field at all depths from the wave-field at the surface, i.e. it can be intuitively explained as a succession of downward continuation steps, with the elevation moving ever deeper into the sub-surface. In addition to downward continuation, migration requires imaging principle. WED produces an unmigrated time section at a specified datum plane (Yilmaz and Lucas, 1986). In this respect, WED is an ingredient of migration, when migration is applied as a downward-continuation process. Basically, WED is the process of upward or downward continuation of the wave-field between two arbitrarily-shaped surfaces. For details, see Barison et al. (2011) and references therein. The application of WED requires a regular geometry, i.e., constant distance (equal to 60 m in our case) between receivers in each common shot gather and constant distance (equal to 120 m) between shots in each common receiver gather. So, we added shots with zero traces and the missed traces near the shots by developing home codes. For CROP-18A and CROP-18B, the chosen datum was equal to 0 m.s.l., while for CROP-03 the chosen datum was equal -50 m b.s.l., because the elevations of the shots charges were below 0 m.s.l. in the proximity of the coastline.