Mapping of gas charged sediments in Guanabara Bay: Seismic characteristics and sediment properties

摘要

Seismic surveys at frequency of 12 kHz in the NE side of the Guanabara Bay revealed a large area of shallow seismic signal penetration caused by gas charged sediments. The main acoustic signatures related to gas include black shadow and gas blanket. In addition, features related to seepages to the water column (acoustic plumes) and gas percolation structures within the sediments (intra-sedimentary plumes, turbidity pinnacles) were also identified. The top of the gas blanket region varies from a few centimeters up to 9 m below bay bottom. Usually there is a sharp acoustic boundary between gas free sediment and the turbidity pinnacles and intra-sedimentary plumes. The intra-sedimentary plumes appear at different levels of signal penetration and are related to the lithology of the overlying sediments. The most likely origin of the gas is biogenic, since the geology of the area would not allow thermogenic origin and sediment is rich in organic matter. The occurrence of superficial gas; black shadows, is related to the migration of gas originated in the lower layers, and also near surface, once the study area meets favorable conditions for gas production i.e. a high accumulation rates ranging from 0.14 to 0.25cm.y-1 and with a large input of organic matter. P-wave velocity (Vp) and Gamma density data were obtained with the Multi Sensor Core Logger in cores collected in areas without and with gas-charged sediments. Cores ranging from 150???240 cm long have predominantly soft to fluid muddy sediments at the top, to a very compacted mud at the bottom. The values of Vp in the gas-charged sediments varies from 1372 to 1505 ms-1 and the density ranges from 1.10 to 1.46 gcm-3. In gas free sediments Vp vary between 1441 and 1541 ms-1 and the density from 1.08 to 1.48 gcm-3. Variations in the Vp followed the changes in sediment density. These changes were controlled mainly by the presence of gas in the sediments, bioclasts concentration, textural properties and percentage of org- nic matter.