Natural gas hydrates can be directly identified by means of sediment sampling, drilling sampling, and deep dive inspections. They can also be used to simulate sea floor reflection (BSR), anomalous structures of velocity and seismic amplitude, geochemical anomalies, multi-velocity soundings, and ocean bottoms. Television cameras and other methods indirectly identify. Here are some indirect signs.
Earthquake sign
The main seismic signatures of ocean natural gas hydrates include sea floor reflection layer (BSR), amplitude deformation (blank reflection), velocity inversion, and velocity-amplitude abnormal structure (VAMP). Large-scale accumulation of methane hydrate can be directly interpreted by high resistivity (>100 ohm) acoustic velocity, low volume density equal sign.
The BSR is a reflection interface on the seismic section that is parallel or substantially parallel to the seabed and can be cut through all levels or faults. A large amount of free methane gas is also trapped under the natural gas hydrate stability zone, resulting in seismic reflection profiles. Produce BSR. It has been confirmed that BSR represents the base of the gas hydrate stability zone, which is a solid hydrate zone with high sound wave velocity, and below it is free gas or only pore water filled sediment, with low acoustic wave velocity. Therefore, a strong negative impedance reflection interface is formed on the seismic section. Therefore, BSR is caused by a large difference in acoustic impedance (or acoustic wave propagation velocity) between a low permeability hydrate layer and a large amount of free natural gas and saturated water sediments underneath it. Since the bottom interface of the hydrate layer is mainly controlled by the geothermal gradient in the sea area where it is located, it is often located at a certain depth below the seabed. Therefore, the BSR is basically parallel to the sea floor and is called the “quasi-sea floor reflection layerâ€. In addition to the use of BSR to identify the presence of natural gas hydrates and the preparation of hydrate distribution maps, BSR was also used to determine the top and bottom boundaries and the occurrence of gas hydrate formations, and to calculate the depth, thickness, and volume of hydrate formations.
However, BSR is not present in all hydrates. In the gentle seabed, BSR is not easily identified even in the presence of natural gas hydrates. BSRs often appear in slopes or undulating sea areas. In addition, not all BSRs have natural gas hydrates. In rare cases, other factors may also lead to BSR. It should also be noted that although most of the hydrate layers are located above the BSR, not all hydrate layers are located above the BSR, which has been deep-sea Drilling certification. Therefore, BSR can not be used as the only sign of natural gas hydrate and should be combined with other methods to make a comprehensive judgment. In recent years, analyzing and studying the speed structure of earthquakes has become the frontier of this discipline. The hydrate layer is a high velocity layer where the saturated or saturated layer is a low velocity layer. On the speed curve, the speed at the BSR interface suddenly decreases, showing an apparent speed anomaly. In addition, analyzing the amplitude structure can also identify natural gas hydrates. In contrast, the hydrate layer is a rigid layer, in which the saturated gas or saturated layer is a plastic layer. On the amplitude curve, the amplitude at the BSR interface suddenly decreases, showing a significant amplitude anomaly. These methods are particularly important for the gentle sea bottom.
Geochemical signs
The methane concentration in the shallow sediments and the bottom seawater is abnormally high, and the Cl − content (or salinity) and δ18O of the pore water in the shallow sediments are abnormally high, and the siderite rich in heavy oxygen appears. These gases can be used as natural gas hydrates. Geochemistry sign.
Sea floor topography
In the marine environment, the permeation of hydrocarbon gases in the hydrate-enriched areas can form a special environment and special microtopography on the sea floor. The geomorphological signs of natural gas hydrates include vent windows, methane gas seedlings, mud volcanoes, punctiform terrain, carbonate shells, and chemically synthesized biota. In recent years, the Geomar Institute at the University of Kiel, Germany, has discovered many discontinuous hydrated bleed windows with a size of about 5 cm2 on the Cascadia hydrate subsea terrace on the continental margin of western Oregon in the United States. The seedlings exuded in one strand and the infiltration rate was 5 liters per minute. There are microorganisms, helium, and carbonate shells around the permeate stream.