Research On Geomagnetic Diurnal Variation Patterns And Marine Magnetic Survey Diurnal Variation Correction

Due to the difficulty in erecting effective diurnal variation stations in open sea areas,the diurnal variation correction in marine magnetic surveys has become a difficult and urgent problem to be solved.The existing methods estimate the diurnal variation value of marine area by the land diurnal variation,but rarely take the patterns of the diurnal variation itself into consideration.The purpose of this paper is to effectively investigate and utilize the diurnal variation patterns in magnetic survey diurnal variation correction,thereby providing an appropriate method of physical connotation for the diurnal variation correction to reduce the calculation error and expand the effective calculation range.The geomagnetic diurnal variation consists of solar quiet variation and disturbances.Solar quiet variation depends on the local time while disturbances depends on the universal time,so it is more reasonable to calculate the two respectively in different time coordinate systems.In the non-magnetic storm period,disturbance has small amplitude and high frequency,and it can be separated by wavelet transform.Based on the theoretical guidance of Sq current system,some conclusions are obtained from the researches on the variation of solar quiet variation and its changes with the latitude and longitude.(1)Difference in longitude causes difference in phase of diurnal variation curves,and can be corrected by adjusting 1 hour on phase every 15-degree longitude difference;(2)difference in latitude causes difference in amplitude of diurnal variation curves,and value of horizontal component increases from the focus of Sq current system to South or North.Moreover,it has been found by further quantifying the latitude distribution characteristics of Sq that there is a clear pattern between Sq value and magnetic latitude.The pattern can be approximately fitted by a quadratic function,and based on the fitting function,a latitude correction method is established.In this method,observatories similar in longitude and varying in latitude are selected as base stations,from which the data are collected to obtain a fitting function of the diurnal variation value and geomagnetic latitude for latitude correction.Then time difference is adjusted for longitude correction,thus obtaining diurnal variation values of the stations.The calculation results demonstrate that compared to the weighted average method,this method can still maintain a high correction accuracy(<4nT)for remote stations(with a longitude difference of 40°,so that it is more suitable for marine magnetic survey.Based on the Gaussian distribution law of the Sq variation curve,a Gaussian fitting method is established,where the southernmost station is selected as the base station,and the difference Sq variation curve of the base station and other stations can be fitted by Gauss functions,and the Gaussian fitting function coefficients of each station have a function relationship that can be fitted with latitudes and longitudes.Therefore,an empirical model of the Sq variation can be formed.Sq variation of the position can be calculated by putting Sq variation of base station and latitude and longitude of the position into the model.The test results show that the calculation error of the model is <3nT.During the magnetic storm,the storm time change can be separated using Dst index.Sq can be calculated at the local time using the existing calculation method,and storm time variation can be obtained by the storm time variation – latitude model.The test results show that the separation calculation can reduce the correction error by 30% compared to the calculation of diurnal variation of the magnetic storm period as a whole.