This article mainly presents the mechanism of a new method of seabed cone penetration test (CPT) in acquiring soil properties for Natural Gas Hydrate (up to 30 m depth). This new method of seabed CPT system can realize the automatic docking and dismantling of the probe rods underwater, which greatly reduces the workload of manual docking and improves the survey efficiency.
Natural gas hydrate (NGH), commonly known as "combustible ice", is characterized by wide distribution, abundant reserves, and high energy density (Ren et al., 2020). However, the sediments in which gas hydrates are stored may be subject to explosive decomposition due to certain factors, or the free gas may cause changes in the soil properties of the sediments, such as the compressibility, shear strength and permeability of the sediments, which may lead to secondary engineering geological hazards such as drilling safety, methane leakage, ground subsidence and submarine landslides (Lunne et al, 2009; Koh et al, 2012).
Assessment of the risk of engineering geological and environmental hazards require a good knowledge of the mechanical behaviour of soils and of their spatial variability. In-situ survey techniques can detect different physical properties of sediments with different probes, which can assess risk of engineering geological and environmental hazards (Seifert et al,2008). Such information can be best obtained from a properly planned program of both laboratory and in situ tests. The two methodologies are very much complementary rather than competitive. However, in situ tests can often be preferable to laboratory tests because of important advantages such as cost—time effectiveness, the ability to assess the soil in its natural environment and the possibility to estimate the spatial variability of the deposit (Lunne et al 2007; Ron et al, 2011).
The interpretation of test data can be applied to survey of NGH. Different data can reflect sediment type, temperature gradient, acoustic properties, presence of natural gas, state of emplacement, influence factors affecting seismic data discrimination, chemical anomalies, etc.
