GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.1 577 reflectors continuity/discontinuity and different local frequency contents. The use of multi- attribute display on the new attribute images enhances the possibility to detect different geophysical signatures reflecting variations of geological features. Data and Methods. The dataset used in this work is an offshore Pre-Stack-Time Migration (PSTM) 3D seismic volume. The seismic attributes selected and computed were the Root Mean Square (RMS), Spectral Decomposition and Coherence. The RMS is an amplitude-based attribute that provides a scaled estimate of the trace envelope and so it measures the reflectivity, therefore highlighting variations in acoustic impedance. It is computed in a sliding tapered window of N samples as the square root of the sum of all the trace values divided by the number of N samples. Generally, the higher are the acoustic impedance variation, the higher are the RMS values. High RMS values in channels may result from a high acoustic impedance contrast of channel fill with the surrounding lithologies. The Spectral Decomposition is a frequency-based attribute that transforms the seismic data into the frequency domain via the discrete Fourier transform; the amplitude spectra delineate temporal bed thickness variability, while the phase spectra indicate lateral geologic discontinuities (Partika et al. , 1999; Puryear et al. , 2012). It is a useful tool to push the seismic interpretation to a “below resolution” level, commonly used for sand thickness estimation, and particularly useful to enhance channelized structures. The Coherence attribute is a measure of the similarity between seismic traces in a specified window that gives an indication of the traces continuity for example along a picked horizon (Gersztenkorn and Marfurt, 1999). It gives information on lateral changes in waveform. It can be used to map the lateral extent of a formation, as well as used to improve faults visualization, channels boundaries or other discontinuous features. In our workflow, these attributes were computed along seven horizons, each one over an area of 375 km 2 and 60 milliseconds (TWT) of interdistance, to investigate about 550 m of the 3D volume (using a mean velocity of 2800 m/s). The attributes were mostly applied on such horizons but, in some cases, also on the seismic inlines/crosslines and time-slices in order to compare the geometries and the relationships of complex reflections. Results. The results obtained after the application of the three attributes are related to the specific characteristic of each attribute computed. RMS amplitude allowed inferring variation of lithologies along the channels in accordance with the acoustic impedance contrasts, in particular from the channel fill and the levees (Fig.1). Coherence attribute enhanced the lateral Fig. 1 - Crossline with RMS attribute applied on horizon. The blue arrows indicates the areas of the horizon and seismic line where there is an high value of acoustic impedance contrast.