GNGTS 2019 - Atti del 38° Convegno Nazionale

536 GNGTS 2019 S essione 3.1 includes sub-bottom profiles (CHIRP) and multi-beam data. Some of them have been acquired along the same transect (e.g. MS29 seismic line, OCSS3 seismic line and CHIRP profile). This means that we can image the same target with different kind of information and details, that represents a great advantage, generally rare. The seismic data acquired during OCSS_15 survey was raw, so processing was needed before their interpretation. Conversely, other data (e.g. CHIRP), were ready to be interpreted. Processing. Raw seismic data are composed by a lot of seismograms and their relative shots and receivers’ positions. Applying several algorithms - that organize and “clean” the signals - we are able to display the geometries of the geological formation at depth. The processing was made using Paradigm suite software. We processed the new dataset acquired in the Otranto Channel in the 2015 (OCSS_15 project), and we conducted a re-processing of the MS29 seismic line. We applied timemigration algorithmat every analyzed line and a depthmigration at theMS29 seismic line. Migration of a seismic line is mandatory to get the real position of the reflectors, correcting diffractions and re-positioning the reflectors. A time migration is made in the time domain and the resulting section has the vertical scale in seconds. Conversely, depth migration needs a more accurate velocity model to handle lateral velocity variations and to obtain the real position and shape of the reflectors in depth, with vertical scale in meters-unit. The OCSS_15 seismic lines involved principally standard processing steps, but we focused the attention on multiple suppression/attenuation in order to conduct the velocity analysis based on move-out hyperboles. These data were acquired using a short cable so, the arrivals delay at the geophones (Normal Move Out - NMO) were not so appreciable and the velocity analysis was difficult to conduct. Having a multiple-cleaned section, this operation is easier to do. New processing flow on the MS29 vintage data has been applied, as a robust despiking procedure, useful to mitigate the analog-to-digital-conversion undesired effects and the SRME procedure (Surface Related Multiple Elimination, by Verschuur et al. , 1992), to attenuate multiple reflections. The MS29 lines was acquired using a long streamer (2400 m). Therefore, the NMO was appreciable and we conducted a precise velocity analysis in an iterative way to get the best-as- possible velocity field to pre-stack time migrate this section. In order to pre-stack depth migrate the MS29 seismic line we used two different tomographic inversion procedures to handle the best-as-possible velocity-depth model: 2D Grid Tomography and Horizon-Based Tomography. These two tomographic inversions were conducted to fit the real wave-fronts travel in depth, not to use hyperbolic approximation as previously done for time migration. Interpretation. The interpretation part consists in the identification of seismic facies and their geometric and chronologic relationship. We also made seismic-attribute section to clarify some doubts about the possible gas presence in the subsoil. Usually seismic sections display the amplitude of the signals, instead attribute sections focalize the attention on a specific property. For examples, we produced frequency section (instead of the classical amplitude), chaos sections (describing the continuity of a signal) and polarity sections (showing the wave-phase). We also made time-structural maps and isopach maps of the geological formation recognized in the Otranto Channel. These maps allow a 3D visualization of the interpreted seismostratigraphic units: time-structural map illustrate the framework in depth of each seismostratigraphic units meanwhile isopach maps show the thickness of the formations. To correlate deep and shallow structures we linked seismic profiles (deep information) and CHIRP ones (shallow information). Results. Processing. A time migrated seismic section of each line and a depth migrated seismic section of the MS29 profile have been obtained. The re-processing of the MS29 line brings to an improved display of this profile compared to older ones. After two tomographic inversions we got a very precise velocity field, useful to apply a depth migration at the MS29 seismic section and to improve the visualization of the depth migrated section. The OCSS_15 lines show a detail of seismic facies that cannot find analogous in literature. It was not easy to match such a different resolution hence, some doubts arose about previous interpretation, based