GNGTS 2015 - Atti del 34° Convegno Nazionale

30 GNGTS 2015 S essione 3.1 with amplitude values much smaller than those of seismofacies A and B. In Fig. 2 is shown an example of the result obtained through the application of the Reflection Strength attribute for the inline1560 and the limits between the seismofacies A, B and C and the amplitude anomalies L, M and N are highlighted. The Instantaneous Phase attribute is applied in order to emphasize the lateral continuity of strong as well as weak events, to estimate the geometrical properties of the amplitude anomalies and to highlight the discontinuities, such as discontinuities. The Instantaneous Frequency attribute is useful to discriminate the different range of frequency that characterized the data. In particular, the three seismofacies exhibit distinct Instantaneous Frequencies trends: the seismofaciesAand B are characterized by medium to high frequency reflectors, while the seismofacies C shows a low frequency content. Furthermore, the amplitude anomalies highlighted by the Reflection Strength attribute correspond to low Instantaneous Frequency anomalies. The Sweetness attribute is calculated by the combination of Instantaneous Frequency and Reflection Strength (Hart, 2008; Riedel, 2010; Yushun, 2011) and it is useful to better distinguish the events characterized by high values of acoustic impedance. This attribute clearly highlights the limits of the three seismofacies as well as the anomalies. The seismofacies A presents dominant Sweetness values around zero and, within it, few reflectors with higher Sweetness values are observed. The seismofacies B is made up of packages of laterally continuous reflectors with medium-high Sweetness values. Below, the seismofacies C is characterized by the presence of less continuous reflections, compared with the seismofacies A and B. They exhibit quite high Sweetness values and are enclose in a background that presents Sweetness values comprised between 0 and 1000. Therefore, the detection of the anomalies related to the igneous bodies is performed through the application of the seismic attributes. In particular, the Reflection Strength and the Sweetness attributes permit to isolate the high amplitude value anomalies from the low reflective seismofacies C. The Instantaneous Phase allows the identification of some sills within the seismofacies B masked by chaotic reflections. These igneous bodies present antiformal junctions, T- and F-shaped morphologies and saucer-shaped geometries. The Instantaneous Frequency permits to characterize the sills as bodies with a frequency range of 25-30 Hz. The amplitude anomalies within the seismofacies C show values between 10 and 25 Hz, while the anomalies in the seismofacies A show values up to 40 Hz. Regarding to those within the seismofacies B, they are not resolved due to the chaotic facies with too high frequencies. The Continuous Wavelet Transform (CWT). In order to improve the geometry interpretation, the Continuous Wavelet Transform (CWT) is applied (Debauchies, 1988; Mallat, 1989; Zhang et al. , 2006). The CWT provides a method for displaying and analysing signals as a function of time and scale. The scale and the frequency are related to an inverse relationship of proportionality, which means that the higher the scale the lower the frequency. Therefore, the wavelet transform allows to decompose the seismic data into a time versus scale (frequency band) domain and subsequently, if needed, to reconstruct them in the scales of interest. The analysis of the reconstructed common scale volumes permits to obtain a representation of the frequency information, which is contained in a seismic section. In particular, the CWT is defined as follows (Debauchies, 1988; Zhang et al. , 2006): It can be considered the cross-correlation between the seismic trace and the dilated and scaled versions of an user defined wavelet. The mother wavelet is the analysis wavelet function, a is the location parameter of the wavelet, so that (t- a ) defines the shift of the wavelet along the trace, and b is the scaling (dilation) parameter. By shifting and scaling the mother wavelet, the Wavelet Transform is able to capture information of short duration (high frequency) or information of long duration (low frequency), at the same time.