Reflection seismic images from OBS profiling across External Hellenides

Seismic profiles were planned and acquired on the western offshore continental margin of Peloponnese within the framework of the EU-SEAHELLARC project. A large number Ocean Bottom Seismographs (OBSs) recorded shots from an airgun source on board of the R/V OGS-Explora and HCMR-Aegaeo for deep crustal prospecting. They were deployed in deep water and a rough sea bottom topography and lined up with four profiles. The technique was proposed to partially substitute a deep seismic reflection survey with a long streamer and large airgun sources. The processing of these data as standard reflection seismic data was performed utilizing only near vertical incidence after relocation of OBSs at the sea-surface. At the beginning we picked the first arrivals for a tomographic inversion to create a detailed near sea bottom velocity model by iterative ray-tracing and travel time calculations. The information was used to relocate shots and OBSs (receivers) to the same datum plane applying the Wave Equation Datuming (WED) and subsequently for dynamic corrections, migration and depth conversion. WED, operating on a common-source gathers, has the effect of extrapolating the receivers from one datum to another, and, because of reciprocity, operating on a common-receiver gather, changes the datum of the source. The WED process of upward or downward continuation of the wave-field between two arbitrarily shaped surfaces requires the datum surface totally below or above the sea bottom morphology. We utilized an intermediate datum plane first, at 5 to 6 km depth (downward continuation), then OBSs and shots were relocated to the sea surface (upward continuation). The intermediate depth datum solution is required to bypass near sea bottom tectonic complexities and to increase the number of traces (along the profiles there are more shots and closer to each other than OBSs), which contribute to Kirchhoff summation in the migration procedure. WED removed time shifts related to the water layer and to near sea bottom tectonic complexities and velocity variations. A conventional processing sequence was then applied to the corrected data obtaining images of the deep crustal interfaces with an upper and a lower plate separated by an intra-plate detachment reflecting interval at 10 to 20 km depth on the different profiles. The Moho seems to be gently dipping from north to south and from the Ionian Sea towards the Peloponnese to where the collision of the Ionian cold and brittle crust with the Hellenic crustal roots was recognized with a pronounced crustal flexure reaching depths of more than 35 km beneath the western Peloponnese coasts.