Establishing And Analysis Of Basic Geomagnetic Field Model In The Offshore Area Of China

Geomagnetic field is a function of time and space location, it is originated in theEarth magnetic fluid process of the outer core and the current system of Sun-Earthsystems, mainly composed of the core field, crustal field and the external field. Inmodern times, Geomagnetism as a branch of geophysics has been developing rapidly;it occupies a very important position in major international joint observationgeophysical research program such as The International Geophysical Year, theinternational magnetospheres’ studies, and Sun-Earth Energy Programs. In recentyears, the struggle over marine resources increasingly fierce, the development ofocean also let various developed and developing countries make it as an importantnational policy. Marine magnetic measurement, calibration, imputation, andgeomagnetic field model is one of the main marine geophysical survey, and animportant part of the marine environment measurement. Marine magnetic survey cannot only understand the distribution of matter in the Earth’s interior, provide maps andbasic data for underwater navigation and underwater detection of magnetic material,but also provide basic data for field surveys and scientific research of marineresources.In order to strengthen the construction of marine geological and geophysicalfoundation database series, China will have a comprehensive sorting and handling ona measured magnetic data by a variety of boats for forty years. Based on existingmarine geological and geophysical area survey data, it will use advanced informationtechnology to compiled basis digital library which meets national needs and meetsmodern science and technology international standards, and construct the "DigitalOcean" basic framework. Make a marine geology and geophysics comprehensivedigital information system achieve the services for maritime delimitation consultingservices, military Haiphong, exploration and development of mineral resources, landand marine planning, marine environmental protection, marine construction, marinefishery resources development and maritime shipping.Magnetic modeling is a comprehensive research work; it involves the mainmagnetic field theory, the magnetic field measurement, data assimilation, modelexpression, model explanation, the use of the model and other aspects of research. Geomagnetic field model and geomagnetic map are the main tool to describe theEarth’s magnetic field, through them it can research space distribution and timevariation of the geomagnetic field effectively, understand the characteristics of themagnetic field origin, analysis the geodynamic processes, and it is an important toolfor geomagnetic navigation and related military applications.Geomagnetic field model is divided into global geomagnetic field model andlocal geomagnetic field model. Global geomagnetic field model is the InternationalGeomagnetic Reference Field (IGRF) given by International Association ofGeomagnetism and Aeronomy (IAGA) every five years; Local geomagnetic fieldmodel is the model self-developed by numbers of regional countries and regions.International Geomagnetic Reference Field (IGRF) has an error about100nT-200nT,which cannot meet the needs of sophisticated data. Many countries began to study theregion geomagnetic field model by themselves.At present, there are many ways to establish long-term changes geomagnetic fieldmodel, such as Taylor polynomial method, Legendre polynomial method; morecomplex functions such as spherical harmonic analysis method (SHA), spherical capharmonic analysis method (SCHA), rectangular harmonic analysis method (RHA),Spline method and so on. Spherical harmonic analysis method (SHA), spherical capharmonic analysis method (SCHA) and rectangular harmonic analysis method (RHA)are based on Gaussian moments magnetic potential theory, they are expand under theboundary conditions and the solution of the region, and establish three separatecomponents of the geomagnetic field uniform model; the physical meaning of thegeomagnetic model obtained by this method is explicitly, and it can be carried out todo theoretical analysis and research for the change process of the geomagnetic fieldon the three-dimensional structure. Other functions, such as Taylor polynomialmethod, Spline method and other methods, can fit the independent component of thegeomagnetic field which distributed in a certain region of space; these calculationmethods are relatively simple, and they have a more extensive application in practicaluse. Typically used Spherical harmonic analysis method to build a global model of thegeomagnetic field, like the International Geomagnetic Reference Field, and usedTaylor polynomial method to establish the geomagnetic secular variation model of agiven region commonly.This article based on the analysis and research of the main theoretical basis andmethods of the Earth magnetic field models and modeling in recent years, select atypical method to establish and analysis the geomagnetic field model according to the area data. Use Taylor polynomial method, Legendre polynomial method, sphericalharmonic analysis method and rectangular harmonic analysis method these typicalmodeling methods, built, compare and analysis the base model in the Offshore AreaOf China through the degree of fit for each model, and draw the relevant maps, makeit describe the China Offshore Area’s magnetic field better.