Fault detection from back-scattered energy in MONA LISA wide-angle seismic sections from the south-eastern North Sea

Back-scattered energy from faults associated with the formation of deep sedimentary basins has been observed in the MONA LISA seismic wide-angle data. The back-scattered arrivals constrain the geometrical shape of these faults which reach deep into the crystalline basement. One of the modelled faults forms the western boundary of a deep sedimentary basin in the basement high between the Central Graben and the Horn Graben within the Mid North Sea/Ringk˘bing-Fyn system of basement highs (MN-RFH). This fault corresponds to a zone with steeply dipping reflections in coincident normalincidence reflection data. Back-scattered arrivals from salt domes were recognized in reflection seismic sections already in the 1940s (Swartz & Lindsey 1942; Robinson 1945). Nevertheless, such arrivals are not included in standard processing algorithms, although some scientists have used back-scattered energy in migration processes for detailed imaging of the steep sides and overhang of salt domes (Hale et al. 1992). More recently, Hole et al. (1996) modelled the geometry of the San Andreas Fault using phases which were reflected fromthe fault plane. The back-scattered phases observed in the MONA LISA wide-angle data are reflected from fault zones partly within the crystalline crust. Various factors may cause faults to be reflective. Clearly, a velocity discontinuity across a fault plane can result in reflected seismic energy. However, other reasons exist why fault zones in the crystalline crust should be reflective. Based on borehole and seismic data together with laboratory experiments Mooney & Ginzburg (1986) discuss how such fault zones may be detectable by seismic methods because of densely fractured rocks, lowvelocity fault gouge, compositional layering, retrograde metamorphism and foliated mylonites. In a geological model of faults in the continental crust Sibson (1977) suggests that cohesive, foliated mylonites may exist in the ductile part of the crust. Cartwright (1990) identifies internal crustal reflections within the basement high of the MN-RFH, which he interprets to originate from mylonitic shear zones formed along or adjacent to bounding faults of Palaeozoic sedimentary basins. The MONA LISA (Marine and Onshore North Sea Acquisition for Lithospheric Seismic Analysis) data set from the south-eastern North Sea (MONA LISA Working Group 1997) consists of four deep seismic normal incidence reflection profiles recorded to 26s and coincident wide-angle reflection/ refraction data recorded on 29 offshore ocean bottom hydrophones (OBH's) along three of the profiles (1, 2 and 3) and 26 onshore mobile seismometers. The EW trending MONA LISA profile 3 traverses the basement high of the MNRFH and intersects the N-S striking Central Graben and Horn Graben (Fig. 1). Velocity modelling of the MONA LISA wide-angle data in the MN-RFH area reveals sedimentary basins with more than 4 km thick Palaeozoic strata (Fig. 2). Modelling of wide-angle refractions and reflections images the base of these Palaeozoic basins where the P-wave velocity increases from &5.1 km s71 to around 6.0 km s71 which is believed to be representative for crystalline basement. Interpretation of the spatial extent of sedimentary sequences in the North Sea area is traditionally based on conventional reflection seismic data. This paper focuses on back-scattered energy from steep fault planes recorded in seismic wide-angle data collected to large offsets, and on how these arrivals may be used for detailed studies of fault location and geometry. Modelling of these arrivals delineates the geometry of faults associated with the formation of pre-Zechstein basins within the basement high. This helps to estimate the thickness and lateral extent of the down-faulted Palaeozoic strata. Integrated interpretation of wide-angle refraction and reflection data together with normal-incidence reflection data is used to constrain detailed images of the spatial position of steep faults and deep, hitherto unknown, sedimentary sequences.