GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 3.1 37 4270 m and the thickness of sediments above the BSR ranges from 500-600 m. This value seems to be higher respect the common depth of the GHSZ. AVO seismic attributes analysis. By the analysis of the P and S wave reflectivity, of the velocity values variation and of the components of the seismic data (amplitude, phase and frequency), we can obtain the information that allow to ascribe or not this seismic features to the presence of gas. Nowadays, theAVO analysis has become a commercial tool for the oil industry; in particular, it lets geophysicists to better locate and classify gas or oil reservoirs, reducing the possibility of unproductive drills. The AVO techniques could be considered a good discriminator even in the case of gas-hydrate. Theoffset-dependent reflectivityhas itsbasis in the theoretical relationshipsbetweenreflection coefficient, angle of incidence and variation of the compressional P wave velocity, shear S wave velocity and density across an interface; these contrasts are in turn dependent on rock properties variations. In presence of gas, brine-saturated and water-saturated unconsolidated sediments, these parameters have distinctive relationships; thus, using AVO analysis, bright spots caused by gas accumulation can be distinguished by mislead anomalous amplitude reflectors that could be caused by coal, cineritic or other low impedance units. The AVO analysis is based on the observation and interpretation of the curves describing the variation of the Incident reflectivity with the angle of incidence (Castagna and Swan, 1997). Different AVO attributes have been extracted to apply multivariate analysis: the two most important are the zero offset reflectivity, defined Intercept, and the AVO Gradient, based on Shuey’s approximation. In presence of gas hydrates, the analysis of P- and S-wave reflectivity, which are related to the Poisson’s ratio, gives important information on the fluid content and on rigidity of the solid matrix (Carcione and Tinivella, 2000). S-waves do not cross the fluid, so they are independent by the fluid contents showing changes of the solid matrix, which is due to the hydrocarbon hydrate compaction. We analyze the P and S reflectivity by software Focus (Paradigm), which extract 9 AVO attribute sections from the generation of common angle gathers. For instance, the zero-offset extrapolated amplitude section is created on a sample-by-sample basis by transforming the data to the form of amplitude versus the trigonometric sin of the reflectance angle squared. A linear regression analysis is then performed and the fitted line is extrapolated to zero-offset, which forms the zero-offset section. Fig. 3 – Application of AVO seismic attributes using Focus software (Paradigm) after true amplitude recovery by spherical divergence and generation of common angle gathers. In a), the result of AVO Intercept/P wave reflectivity at zero offset enhances the acoustic impedance in correspondence of the BSR, characterized by positive values; in b), the result of S wave reflectivity that shows a weak reflection in correspondence of the black arrow, denotes change of density across the interface.