# A Seabed Real-Time Sensing System for In-Situ Long-Term Multi-Parameter Observation Applications
## 1 Introduction  
The deep seabed is closest to the Earth's interior and is an important area for ocean observation [1]. In deep-sea bottom observation, the construction of a seabed real-time observation system is both a new stage and a need of marine scientific research and marine engineering applications. So far, the ocean observation system has developed into a three-dimensional observation network, including satellite remote sensing, land-based ocean observation stations, sea surface buoy arrays, scientificresearch vessels, submarine buoy arrays, underwater profile buoys, sea bottom observation networks, etc. It can provide basic information and data services in real-time or quasi-real-time in the world [2; 3]. The prototype of the sea bed observation system can be traced back to the US Navy's acoustic monitoring system [4; 5]. At present, the sea bed observation system mainly includes self-contained submarine observation instruments, ship-borne real-time observation instruments, and sea bottom observation networks. The self-contained submarine observation instrument can realize long-term in-situ observation, and its observation data is processed and analyzed after the instrument is recovered. It has been widely used in ocean long-term in-situ observation applications. For example, the French NKE company and the Korean Aeronautical University developed a sea bottom self-contained acoustic wave measurement instrument, which is fixed on a seabed support structure and measures the elevation of the sea bottom by acoustic wave propagation time [6; 7]. The University of Washington developed a self-contained multi-parameter seabed observation instrument observing the ocean current velocity and the hazy layer [8]. However, self-contained submarine observation instruments cannot meet the needs of real-time or quasi-real-time observation in marine science or engineering applications. The ship-borne real-time observation instrument can observe ocean parameters in real time [9; 10]. An example is ROSON, developed by Vandenberg in the Netherlands, which is used to collect soil mechanical parameters of seabed sediments [11]. However, the ship-borne real-time observation and measurement period of the marine parameters is short, and it is impossible to get the long-term dynamic changes of the marine parameters. Sea bottom observation networks can meet the needs of long-term in-situ real-time observation. The construction of sea bottom observation networks has become the consensus of major marine countries. In 2009, the world's first large-scale sea bottom observation network, the North East Pacific Time Integrated Undersea Networked Experiment in Canada (NEPTUNE-Canada), was built and realized long-term multi-parameter observation on the seabed [12]. In 2015, Japan completed the Dense Ocean-floor Network system for Earthquakes and Tsunamis (DONET) installation, which was mainly used for earthquake and tsunami observation [13; 14]. In 2016, a larger submarine observation system designed by the United States, the Ocean Observatories Initiative (OOI), was officially launched, and all-weather and long-term observation of specific sea areas can be realized through the internet [15]. The European Union's European submarine observation program set up the European Seas Observatory Network (ESONET) for the Atlantic, Arctic Ocean, Black Sea and Mediterranean Sea observation [16]. ESONET was developed to EMSO (European Multidisciplinary Sea-floor and water column Observatory) for sea bed observation reliability and cost reduction [17]. As for China, in 2006, the key technology project of the submarine observation network test node was launched. In 2009, the construction of the Xiaoqushan experimental station for the East China Sea submarine observation was completed. In 2011, Tongji University and other institutions formally connected the self-developed cable-based seabed observation network with the MARS deep-sea observation network of the United States.