Mode conversion noise attenuation, modelling and removal: case studies from Cyprus and Egypt

Abstract

The wavefront from a source that strikes an acoustic impedance contrast separates into four variables: transmitted and reflected compressional waves (P-wave) and transmitted and reflected shear waves (S-wave). Converted waves are those whose mode changes at the interface and can be recorded in a marine environment in the presence of large velocity contrasts. The difference in acoustic velocities at the boundary means that in high contrast media the distinction between compressional and shear velocities across the boundary is small; shear waves are not as refracted as compressional waves and can be recorded as partially converted reflected energy. The eastern Mediterranean contains two major sedimentary basins, the Levantine and Herodotus (see Figure 1). They have shared depositional regimes and therefore contain similar sedimentary sequences. The Levantine Basin sits offshore Cyprus, Israel, Palestine and Lebanon, and was formed by prolonged phased rifting, with subsequent subsidence and loading by deltaic sediments (Gradmann et al., 2005; Gardosh and Druckman, 2006). It forms a northeast-oriented depression about 2 km deep and contains more than 14 km of Mesozoic and Cenozoic sequences that include up to 2 km of Messinian salt (Druckman et al., 1995; Ben-Avraham et al., 2002; Netzeband et al., 2006). The local reservoirs include both Cretaceous and Jurassic sandstones and limestones, with older potential reservoirs in the Triassic sandstones. Shales and marls of Paleogene, Neogene, Cretaceous and Jurassic, along with Messinian salt and Triassic evaporites all form local seals (Roberts and Peace, 2007) in plays of a both stratigraphic and structural nature. The Herodotus Basin is a northeast trending 3 km deep depression, and represents a slab of the Early Mesozoic Neo-Tethys Ocean (see Figures 1 and 2). The basin is bounded on the north by the Mediterranean Ridge; on the northeast by the west portion of the Cyprus Arc and on the south by the Nile Delta cone. The Eratosthenes Seamount (ESM), a fragment of continental crust (Robertson, 1998), forms the eastern limit of the Herodotus Basin, and additionally delineates the boundary between the cone of the Nile Delta, the Herodotus Basin, and the Levant Basin. The Herodotus Basin itself contains up to 7 km of Mesozoic–Cenozoic sediment (Voogd and Truffert, 1992; Garfunkel, 1998), and is overlain by up to 3 km of Messinian salt (Garfunkel, 1998; Aal et al., 2000; Loncke et al., 2006).