Deep Crustal Imaging Using Industry Seismic Data: The Case of the Focsani Basin-Romania

Mucuta, Dana M. (University of South Carolina, Department of Geological Sciences) | Knapp, Camelia C. (University of South Carolina, Department of Geological Sciences)


Summary 2 D industry seismic data acquired in the Focsani Basin of Romania prove ideal for imaging deep crustal structures related to the tectonic evolution of one of the youngest yet deepest basins in the world. Fundamental plate tectonics research blends with oil exploration techniques to provide an image of the upper 60 km of the basin. The purpose of this study was to identify the best processing techniques that would provide deep information while preserving the shallow data. Introduction Processing of 2D shallow (5s) seismic data is presently done routinely with software that was developed for this purpose. However no software is designed for the sole purpose of processing deep seismic data. Here we present processing steps undertaken for processing 2 seismic lines with recording time of 20s TWTT using Landmark ProMAX seismic processing package. The deep seismic data presented here were collected in the Focsani Basin of the SE Carpathian foreland as part of oil exploration activities. The acquisition parameters are exploration-type, only the record length being increased to 20s. Some of the main problems of processing the data were related to signal depth penetration and its recovery, attenuation of multiples and velocity analysis. Standard industry offsets did not provide sufficient NMO and the thickness of the basin further complicated removal of multiples. The evolution of the basin is at odds, spatially and temporally with the formation of the Eastern Carpathians and to this effect the study of the deeper crustal structure was envisioned as a way of providing constraints on the main tectonic processes involved in the development of the SE Carpathian region. Geologic Setting The foreland basin in front of the SE Carpathians, Romania, formed during and after the Alpine continental collision bears significance due to its (1) rich and extensively exploited oil fields, (2) vicinity to the intermediate depth Vrancea Seismogenic Zone, (3) thick sedimentary cover (~ 18 km Miocene-Quaternary), (4) ongoing subsidence (~ 2mm/year), (5) localized and unusually low topography, (6) crustal scale faults oriented NNW-SSE, (7) documented normal faults concentric to the Vrancea area and (8) wide spread, low magnitude shallow seismicity. One of the most debatable features associated with the SE Carpathians fold and thrust belt is the significant concentration of intermediate depth earthquakes (70-210 km) in an extremely confined and vertical volume, named Vrancea Seismogenic Zone. In the context of plate tectonics this seismicity was interpreted as being produced by the sinking of an oceanic slab. However, new interpretations envision an entirely different process, namely continental delamination of over thickened continental lithosphere (Knapp et al., 2005), also responsible for the active subsidence of the Focsani Basin. In a broader sense, this study examines the relationships between crustal foreland basin deformation and VSZ, foreland deformation at the crustal scale suggesting a geometric association with the Vrancea mantle source region and implying a mechanical coupling of the seismogenic body with the overlying crust. Part of the SE Carpathian foredeep, the Focsani Basin is made up of sands, shale and evaporites that thicken to the west.

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