GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.1 GNGTS 2024 The novelty of the ISTRICI package is to propose three diferent ways to organize the CIGs to improve the seismic velocity analysis depending on diferent elements, such as targets and characteristcs of the data, optmizing the result quality and reducing the tme needed to analyze the r-parameters in the residual semblances. ISTRICI has been developed on the basis of the most common cases: (i) the contnuity of some refectons along the seismic line, such as the seafoor, suggests that the picking procedure can be performed semi-automatcally in the residual semblance; (ii) the availability of the interpretaton of the seismic secton suggests that the velocity analysis can be performed along the interpreted refectors; (iii) in complex geological setngs and/ or in the presence of non-contnuous refectors, the velocity analysis can be performed as a functon of depth rather than in a layer-stripping approach. The user can choose and move from one workfow to another in order to beter resolve the targets and/or scientfc questons on the basis of the characteristcs of the data. Moreover, for the same purposes, the user can update and modify the codes related to the velocity analysis. The STEP 1 of all workfows consists of performing PSDM obtaining two outputs: the standard migraton and the additonal migraton with extra amplitude (see details in Liu and Bleistein, 1995). Before this step, an inital velocity model has to be created, which is input into the workfow along with the seismic data for the frst step. In the INTER workfow, the r-parameter analysis is performed along a selected refector. The procedure of INTER consists of four steps (Fig. 1). First, afer PSDM, the migrated secton is interpreted by picking the selected refector; then, the picks are interpolated to obtain one pick for each migrated trace (STEP 2). Then, in a selected range of CIGs, residual propagaton analysis is calculated over the depth of the selected refector with a defned window based on the wavelength of the refecton.Then, the operator simply selects the r-parameter along the refector; the selectons are interpolated to obtain an r-parameter value for each CIG (STEP 3). STEP 4 consists of evaluatng the velocity and updatng the refector depth for each CIG based on the r- parameter value and migraton results. The procedure is repeated from STEP 1 to STEP 4 untl the diference between two successive iteratons is less than a certain threshold. The fnal velocity model is composed of the selected layers. In each layer, the velocity changes in horizontal directon, but is constant in depth. In the CIG workfow, the analysis of the r-parameters is performed along a selected refector, but unlike the INTER workfow, the picking is performed by analysing the total semblance at each selected CIG (see also Vargas-Cordero et al., 2010). This workfow also requires four STEPS, as described in Fig. 2. In STEP 2, the migrated secton is interpreted and the picking of a refecton is performed. As in the INTER workfow, the picks are interpolated along the seismic secton. STEP 3 consists of picking the r-parameter at depth in proximity of the picked refector at selected CIGs. To help the user, the code specifes the depth of the selected refector at the semblance to pick the r- parameter where the energy is higher. In this way, the interpretaton of the seismic line (i. e., the depth of the target interface) is changed according to the maximum coherence of the semblance. STEP 4 is the same as the one in the INTER workfow as well as the fnal result. Also in this case, STEPs 1-4 are repeated untl the diference between successive iteratons is less than a certain threshold.