GNGTS 2015 - Atti del 34° Convegno Nazionale

36 GNGTS 2015 S essione 3.1 (PNRA) during the project SCOTIA (March 1995). The MCS data where acquired using an airgun source (combined chamber capacity of 80 l) and 120-channel seismic streamer with maximum offset of 3000 m. The recording parameters are: shot spacing 50 m, group interval spacing 25 m, CMP spacing 12.5 m and fold 30%. The total recording length is 15-s TWT and sample interval 4 ms; in this work, we considered the time window of 8-s TWT depth. Along the analyzed seismic profile, the presence of a high amplitude reflector, which is almost parallel to the sea floor and cuts the reflections related to the sedimentary succession overlying the basement, is recognized at a depth of about 6.75-s TWT. It extends over about 600 CMP, covering a length of almost 7 km (Fig. 2a). These characteristics allow to define the recognized reflector as a BSR. A BSR can be ascribed to: 1) a density increase across the interface between the two forms of opal, producing a strong reflection with a positive polarity; 2) the base of the stability zone (pressure and temperature conditions) of gas hydrates. Above a BSR, a solid phase of methane combined with water is present within a clastic matrix and this horizon produces an increment of the P-wave velocity. Below the BSR, free gas is present in the porous space of the rocks causing a drop of P-wave velocity. For this reason, the interface between the two gas phases is generally highlighted by a strong impedance contrast and high amplitude reflectors with negative polarity. To analyze peculiar seismic features along the reflector, we re-processed the seismic profile focusing on the velocity analysis, depth migration and seismic attribute analysis. Re-processing and pre-stack depth migration. We re-processed the portion of seismic profile using Focus (Paradigm) software, focusing on the velocity analysis of the acoustic wave across the interface; the first results of this analysis doesn’t evidence clear velocity inversions along the entire length of the considered reflector. A preliminary seismic attribute analysis of this reflector has been carried on to highlight the amplitude-phase and frequency characteristics. The main objective of the use of attributes is to provide detailed information to the interpreter on structural, stratigraphic and lithological parameters of the seismic prospect. The attributes extraction allows to detect more easily some features that might not be directly observed on the original data and which are typically expressed by the attribute “amplitude”. The benefit is to obtain the measured attributes at the same scale as the original seismic data. Ageneral definition of seismic attribute has been given by Chopra and Marfurt (2005). In the most general sense, they consider as seismic attributes all the quantities derived from a seismic data, including interval velocity, inversion of acoustic impedance, pore pressure prediction, reflectors termination and Amplitude Variation with Offset (AVO). This approach is useful in hydrocarbon exploration, because it contributes to improve the reservoir analysis and some attributes can be used as specific Direct Hydrocarbon Indicators. Instantaneous amplitude, phase, cosine of phase and frequency attributes are the most effective to highlight the features of the observed BSR and could give a qualitative approach for a first ascription to one of the different hypothesis. We analyzed theApparent Polarity attribute to recognize possible phase inversion of acoustic wave at the base of BSR, due to solid/gas conversion below the Gas Hydrates Stability Zone (GHSZ). This attribute (Fig. 2b) shows in red-blue-red the phase behavior of the sea-bottom; the BSR shows the same passage of phase. Together with the velocity analysis, this represent a further result that reduces the possibility of a gas hydrate BSR. Using Geodepth (Paradigm) software, we performed a further velocity analysis in order to convert root mean square values into interval velocity values using Dix equations. This let to create Common Depth Gather and to obtain a depth migrated sections (Fig. 2c). This processing phase allowed to reconstruct the real geometry of the reflectors and to measure the thickness of the sedimentary sequence above the BSR. The water depth is at about