GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.1 GNGTS 2024 ISTRICI - STRUCTURAL INVERSION OF COMMON IMAGE GATHERS U. Tinivella 1 , M. Giustniani 1 1 Natonal Insttute of Oceanography and Applied Geophysics – OGS, Trieste, Italy In this paper, we present a free package consistng of codes developed by us and Seismix Unix (SU; Cohen and Stockwell, 2008), an open-source sofware package for seismic data analysis, to determine the seismic velocity feld in diferent geologic setngs. This package, called ISTRICI, was developed to facilitate the use of Seismic Unix, a free sofware designed to process seismic data. Seismic Unix is a powerful tool for performing pre-stack depth migraton and using the residual parameters to update the seismic velocity model based on the pre-stack depth migraton results. What is missing are codes and scripts that allow practcal, interactve, and easy applicaton of the Seismic Unix algorithms. Therefore, ISTRICI was developed to fll this gap. ISTRICI consists of three workfows (INTER, CIG and TRAD; Tinivella and Giustniani, 2023) for interactvely performing residual velocity analysis and pre-stack depth migraton, switching from one workfow to the next depending on the characteristcs and type of seismic data. The PreStack depth migraton (PSDM) adopted in SU is based on the method of Liu and Bleistein (1995); the PSDM output consists of two migraton sectons characterised by the same phase, but diferent amplitudes. The algorithm performs the PSDM using the input and perturbed velocity models. The rato of the amplitudes of these two PSDM sectons is used to evaluate the residual velocity (i.e.,Vargas-Cordero et al., 2010). The result of the migraton can be organized into CIGs: if the migrated refectons in the CIGs are fat, it means that a correct migraton velocity was used to migrate the data (Yilmaz 2001). By contrast, the slope of the refectons in the CIGs indicates an error in the migraton velocity, which can be corrected by analyzing the residual energy and then updatng the velocity (code available in SU; Liu and Bleistein, 1995). The residual energy (called the r-parameter) is a measure of the fatness deviatons of the refectons along the ofset in each CIG (i.e., Tinivella et al., 2009). A zero value for the r-parameter means that the velocity is corrected at the corresponding refecton. If the r-parameter has a negatve/positve value, it means that the velocity must be increased/decreased. Then, using the theory of Liu and Bleistein (1995), the r- parameter is converted to a residual velocity used to update the input velocity. It is important to remember that, when the medium has strong lateral velocity variatons, the algorithm works adequately even with small velocity correctons. All steps, such as PSDM, CIG analysis, r-parameter evaluaton, velocity update, are performed iteratvely untl all refectons in the CIGs are reasonably fat, i.e. when the variaton of the depth of the refector versus ofset is sufciently small in each CIG (Vargas-Cordero et al., 2010).