Sea-bottom shear-wave velocities and mode conversions

Abstract

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 (1991); Esteves (1996)) and previously unpublished geotechnical data from offshore Brazil.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,

The Effect of the 3‐D Structure on Strain Accumulation and the Interseismic Behavior Along the North Anatolian Fault in the Sea of Marmara

“…In previous studies, similar sets of interseismic GNSS data were interpreted both in terms of the interseismic behavior of the MMF (Bulut et al, 2019;Ergintav et al, 2014;Klein et al, 2017;Meade et al, 2002;Özbey et al, 2021) and asymmetric strain accumulation due to deep basins (Le Pichon et al, 2003, 2005. The two offshore geodetic experiments showed direct evidence of interseismic heterogeneity on the surface.…”

“…For the sedimentary basins, we used the average elastic parameters for sediments in the Sea of Marmara from Hergert and Heidbach (2010). In order to inspect the effect of this choice, we also tried several more values based on the classifications of sedimentary structures for soft sediments, hard sediments, and overburden (Table 1; Bassin et al., 2000; Rodriguez‐Suarez, 2005).…”

“…Since these two segments are known to be locked, we use this simple approach to fix their locking depths in order to decrease the number of parameters to optimize when performing the grid search for the segments beneath the Sea of Marmara (segments B to E in Figure 2c). Material Properties (Bassin et al, 2000;Hergert & Heidbach, 2010;Hergert et al, 2011;Rodriguez-Suarez, 2005) Used for the Homogeneous and Various 3-D Heterogeneous Elastic Models…”

A Seismic Facies Analysis to Determine the Relative Age and History of the Al Idrissi Mud Volcano from Offshore Larache Located in the NW Moroccan Atlantic Margin

“…The latter corresponds exactly to the formation period of the buried bicone. Besides, this tectonic regime has never affected the morphology of the buried bicone which is in a similar case as the Late-Messinian Early-Pliocene sedimentary facies [24]. Nevertheless, the short dormant period between the buried bicone and the superficial bicone could be related to the rapid decrease in the subsidence rates in the Early Pliocene [16].…”