Preliminary results of a 3D-4C seismic processing are presented. Reasonable hydrophone and vertical element sections are obtained. The radial component data, after horizontal reorientation, asymptotic binning, and correction for large receiver statics, produced section with continuous events. Shooting direction apparently caused acquisition footprint. The three cable-deployment methods used (trench, tape, sandbag) produced similar results for the P-wave sections. Initial results indicate that the taped deployment (unburied with no sandbags) worked better for the radial. There appear to be some contamination of S-wave energy on the vertical channel, but little P-wave energy on the horizontal channel. The survey provides very promising results for P and P-S analysis.

ABSTRACT. Elastic parameters for shallow marine sediments were obtained from the literature (Hamilton (1976(Hamilton ( ,1979; Hovem et al. (1991); Esteves (1996)) and previously unpublished geotechnical data from offshore Brazil. The Brazilian data showed reasonable agreement with Hamilton's results except in the very shallow (above 10 m) sedimentary section. A second order equation to calculate VS as a function of depth in marine sediments is derived empirically down to a depth of 140 m.Analyses of transmission and reflection coefficients for compressional-and shear-wave energy mode conversion using Zoeppritz equations were performed for both the sea bottom and a typical hydrocarbon reservoir top of Tertiary age. It is concluded that most S-wave reflection data recorded on the ocean floor by OBC is related to upcoming energy converted at an interface at depth and not from a downgoing shear conversion at the ocean floor.It was also concluded that, using elastic assumptions, mode conversion (both P-to S-and S-to P-) of the up going energy is negligible in the shallow (above 160 m) sediments.Keywords: V S in marine sediments, mode conversion. (1976,1979); Hovem et al. (1991); Esteves (1996)) e dados geotécnicos marítimos inéditos do Brasil. Os dados brasileiros mostraram uma correlação razoável com os resultados de Hamilton exceto para a seção sedimentar muito rasa (acima de 10 m). Uma equação de segunda ordem para calcular V S como função de profundidade foi derivada empiricamente até uma profundidade de 140 m. RESUMO. Parâmetros elásticos para sedimentos marinhos rasos foram obtidos de literatura (HamiltonAnálises de coeficientes de transmissão e reflexão para conversão de modo em ondas compressionais e cisalhantes usando equações de Zoeppritz foram realizadas para o fundo do mar e um topo de reservatório de idade Terciária típico. Conclui-se que a maior parte das ondas-S refletidas e registradas no fundo do mar por cabos de fundo oceânico está relacionadaà energia ascendente convertida em uma interface profunda e não de uma onda-S descendente convertida no fundo do mar.Foi concluído também que, usando premissas elásticas, a conversão de modo (tanto P-para S-quanto S-para P-) da energia ascendenteé negligenciável nos sedimentos rasos (acima de 160 m).Palavras-chave: V S em sedimentos marinhos, conversões de modo.Petrobras / UN-Rio / Atp-Ro / Res,

Shear-wave velocities in shallow (0 to 130 m deep) marine sediments are obtained from offshore Brazil. The data, acquired in-situ, consisted of both direct velocity measurements and indirect (geotechnical) predictions. Expressions to correlate VS and depth for shallow marine sediments were derived empirically. We find that VS=48z 0.44 (z is in m and VS in m/s) fits the data closely. This equation describes the sediment shear velocity in the 130 m directly below the ocean floor. This expression predicts values similar to those by Hamilton (1976), but matches very shallow data more closely.

A study of wave-mode conversion that occurs at the sea floor for downgoing seismic energy and a comparison with reflected conversions at a representative interface of Tertiary sediments is presented. To investigate if the presence of S-waves on the vertical component (and Pwave in horizontal) could be due to mode conversion close to the receivers, conversion for up-going seismic energy in the shallow sediments is also analyzed. It is concluded that: 1) most P-to-S conversion occurs as reflection at sediment interfaces, not at the sea-bottom, and 2) negligible mode-conversion (both P-to-S and StoP) occurs for the up-going seismic energy in the sediments and at the sea bottom.

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