GNGTS 2013 - Atti del 32° Convegno Nazionale

positioned at two different depths. As an additional issue, there is a complex salt structure found within the model that causes illumination problems when migrating the data. In Fig. 1 a part of the migrated section is shown in the image at left. The 3 arrows on top of the image indicate the locations of the three dip-angle CIGs shown in the images at right. In particular panel 1c is located exactly above two isolated diffractor points. The diffractions are clearly visible in panel 1c as two strong horizontal events embedded in a pattern of concave events. Being this gather located on a sedimentary sequence, it is noticeable how the apex of the concave events are positioned approximately at the same lateral position in the gather. This is what we expected for a sequence of approximately parallel events. Images 2b and 2d show two dip-angle gathers located at the right and at the left of the same point diffractors. Also in these gathers diffractions are easy to distinguish from the dominant pattern of concave reflections. Here they appear as dipping events whose slopes are determined by the horizontal distance between the location of the gather and the location of the diffractor. The images of Fig. 2 illustrates, from left to right, the effect of three steps of the proposed procedure in the dip-angle gathers. First the original dip-angle CIG, on a location exactly above two point diffractors, is shown. The diffractors are quite noticeable as the strong horizontal events. Also the concave events corresponding to reflections are easily identifiable. The second image shows a CIG correspond to the same location, this time after dip-filtering on the common dip images: the concave events corresponding to reflections have been correctly removed while the diffractors have been properly preserved. However, several other events with no easy interpretation are present in addition to the two evident diffractions. In the third image from left the estimated local coherency computed on the previous CIG is shown. It is quite evident that the two strong diffractors are clearly identified as large zones of high coherency. Finally in the last image the regions of high coherency have been emphasized leading to an enhancement of the actual diffraction events. In Fig. 3 we show a depth migrated section of the Sigsbee 2A dataset in the zone of the point diffractors. In the left image the result of the depth migration is shown. In the image at right the migrated image of enhanced diffractions is shown (i.e. the stack over dip angles of the enhanced CIGs). It is noticeable that most of the reflectors have been properly removed and that diffractor points have been highlighted. Conclusions and future works. Diffractions are reliable indicators of small structural and lithological elements of the subsurface. However, since they are order of magnitudes less Fig. 3 – Migrated section of the Sigsbee 2A dataset. Zoom of the zone of the point diffractors, before (left) and after (right) separation and enhancement of diffractions. The arrow refers to the position where the common image gather in Fig. 2 is placed. 75 GNGTS 2013 S essione 3.1