Study On Key Techniques For Marine Calamity Forecast System Based On Laser Interferometry

The 21st century is a century for sea and ocean. The sea economy takes more and more proportions of national economy in many countries. However, marine calamity happens frequently in China and causes giant loss of life and property. The marine calamity forecast can effectively promote economy development and safeguard life and property safety.This article depends on a research project from the International Science and Technology Corporation Plan organized by the Ministry of Science and Technology of the People′s Republic of China: Marine Calamity Forecast System Based On Laser Interferometry (project serial number: 2008DFR20230). Firstly, the current situation of China's sea power disaster and the measuring technology for geodynamics in different countries are summarized; then the laser interferometer adopted in this project and the processing techniques for interference pattern are introduced; Finally, the relation between the Earth power and the micro deformation of Earth's crust is experimentally studied at two 250-kilometer-seperated experimental stations constructed in Russia; the empirical data are scientifically analyzed, and related conclusions are drawn.Using the laser interference technology to measure the infinitesimal deformation of the Earth's crust is innovatively proposed. Common vibration with low-frequency of the Earth's crust can be measured with a displacement measuring accuracy around 10-11m. This good response to low-frequency vibration is a unique characteristic of laser interference technology.The prime tasks of this article are as follows:1,The measuring accuracy and the stability are completely decided by the frequency stability and the coherence of the laser source. Therefore, choosing He-Ne laser with extremely stable frequency as light source can guarantee an extremely good coherent characteristic in a long optical path. 2,In order to increase the sensitivity, a Michaelson laser interferometer with a 52.5-meter-long arm is adopted. The moving mirror is the only unceasing moving part in the laser interferometer. So the stability of moving mirror guarantees the quality of interference pattern and the spectrum restoration. A cat-eye moving mirror is used to overcome the influence from the tilt of moving mirror. The working principle and the selecting principle of cat-eye moving mirror are also analyzed simultaneously. The reference arm of laser interferometer was constituted of PZT groups with modulation and compensation function. The optimal combination of PZT groups and the optical path changes induced by modulation signal and feedback compensation signal are also analyzed. The above methods solve the key technological problem in measuring low-frequency small vibration by laser interferometer.3,From the measurements of spaced laser strainmeters, the horizontal crustal microstrains caused by surface and internal sea waves are observed. The possibility of earthquake forecast is experimentally revealed.4,The conversion from low-frequency hydroacoustic source to seismoacoustic signals is studied. When a hydroacoustic source is located on shelf or behind slope, seismoacoustic signals are modulated by various low-frequency signals. By using a low-frequency hydroacoustic source in a shallow or deep sea to generate hydroacoustic oscillations, the variations of signals received by a laser deformograph reveal that the hydroacoustic wave interacts with the bottom at around critical angles and transforms into a seismoacoustic wave.5,We testified that the energy of internal sea waves is transformed into the energy of elastic vibrations of the seafloor, instead of the energy of small-scale turbulence. During the interaction of internal and surface sea waves with the seafloor, the energy is mostly absorbed by the lithosphere in the form of Rayleigh waves.6,The data of low-frequency lithospheric deformations produced by marine internal gravitational waves (IGW) propagating in the continental shelf zone of the Sea of Japan are analyzed. A conclusion is deduced that wave period may increase as a result of additional pressure associated with an increase in the IGW amplitude during the waves propagate from the shelf edge toward the coast of the Sea of Japan. The above-mentioned research results indicated the laser interferometry technology may provide scientific data for marine calamity forecast.