Results in Geophysical Sciences最新文献

This study calculated the dissolution rates of biogenic silica deposited on the seafloor and the silicic acid benthic flux for 22 Ocean Drilling Program sites. Simple models developed for two host sediment types—siliciclastic and carbonate—were used to explain the variability of biogenic silica dissolution and recycling under present-day low (−0.3 to 2.14 °C) bottom-water temperatures. The kinetic constants describing silicic acid release and silica saturation concentration increased systematically with increasing bottom-water temperatures. When these temperature effects were incorporated into the diagenetic models, the prediction of dissolution rates and diffusive fluxes was more robust. This demonstrates that temperature acts as a primary control that decreases the relative degree of pore-water saturation with biogenic opal while increasing the silica concentration. The correlation between the dissolution rate and benthic flux with temperature was pronounced at sites where biogenic silica is hosted in surficial sediments mostly composed of biogenic carbonates. This association is because the dissolution of carbonates provides the alkalinity necessary for both silica dissolution (also its reprecipitation as opal-CT) and clay formation; thus strongly reducing the retarding influence of clays on biogenic opal dissolution. Conversely, the silica exchange rates were modified by presence of aluminosilicates, which led to a higher burial efficiency for biogenic opal in detrital- than in carbonate-dominated benthic layers. Though model prediction of first-order silica early transformation suggests likely effects from surface temperatures (0–4 °C) on opal-CT precipitation over short geological times (< 4 Ma) near seabed in the Antarctic Site 751, the relationship between silica solubility and surface-area variability in time is a more critical control. Since silica solubility and surface area decrease with time, the < 4 Ma elapsed time aged opal-A to the point that changes in specific surface area caused minor effects on solubility, allowing for formation of opal-CT at low temperature settings near the seabed.

Seismic attributes derived from post stack seismic data are important tools to delineate and characterize the subsurface geological features and discontinuities such as fracture, fault and many more. In this paper we have used two seismic profiles (MH-38A and MH-38B) of 2D multi-channel data along north-south direction with seafloor depth ranging from 1450 m to 2175 m in the deep offshore Mahanadi basin. The gas hydrate indicator, bottom simulating reflector (BSR) is easily identified but the discontinuities such as fractures are not observed directly from the seismic data. Therefore, we have computed the total four most suitable attributes which are reflection strength, instantaneous frequency, instantaneous bandwidth and instantaneous quality factor. We have compared the relative changes of the attribute values along the layers in the four derived attributes and hence, have recognized total three and seven discontinuities for MH-38A and MH-38B respectively, at near and away from the well location. Most faults are passing in NNW-SSE and N-S direction either in the gas hydrate layer or below BSR. These discontinuities are called as natural fractures which are validated by the fractures observed from resistivity image log at well site NGHP-01–19 and NGHP-01–09 for CDP ranges 1561 to 1681 in line MH-38A and 1357 to 1458 in line MH-38B respectively. The discontinuities in the gas hydrate sediments are useful for anisotropy study and to map the chimney system through which the free gas can flow from below BSR to up direction.