Research On Geomagnetic Diurnal Variation Patterns And The Calculation Method Of Diurnal Variation

In magnetic exploration,in order to obtain magnetic anomalies caused only by underground geological factors,it is necessary to eliminate the magnetic field changes caused by other factors in the observation data.The diurnal variation correction is to eliminate the geomagnetic diurnal variation caused by the earth's external current system.In ground magnetic survey,it is only necessary to arrange diurnal-varying base stations in the survey area to observe the diurnal variation,and the diurnal variation can be effectively corrected.However,in marine magnetic survey,if the survey area is a wide sea area,it is often difficult to set up diurnal variation stations.How to obtain accurate diurnal variation correction value has become a difficult problem in marine magnetic survey.This research starts from the measured data from the geomagnetic stations of the seismic network,studying the geomagnetic diurnal variation patterns of longitude and latitude.Based on these patterns,proposes a reasonable calculation method of diurnal variation value,thereby expanding the calculation range and reducing the calculation error,and lays a foundation for realizing the use of remote reference stations to realize the diurnal variation prediction of the marine magnetic survey.Firstly,the longitude patterns of the geomagnetic diurnal variation is studied by selecting the latitude chain distribution stations,and the following conclusions are obtained through analysis and research:(1)The solar quiet variation of the geomagnetic diurnal variation depends on the local time,its field source is the Sq equivalent current system.Due to the rotation of the earth,the relative position between the station and the current system changes,resulting in a phase difference.The disturbance variation depends on the universal time.Because the field source is the magnetospheric current system far away from the earth,the changes are globally synchronized;(2)The solar quiet variation not only has a phase difference,but also has a difference in amplitude and curve shape.This is because the position and current intensity of the Sq current system change with time;(3)When eliminating the phase difference,the traditional method of adjusting the onehour time difference every 15° longitude difference can't correct the time difference effectively,when the longitude difference between two stations is large.This indicates that the "time difference" of solar quiet variation is changing.According to the correlation coefficient method,making full use of the characteristics of the diurnal curve,the calculated time difference can better reflect the time difference effect between actual geomagnetic stations;(4)The extreme value of the solar quiet variation and the longitude of the station can be roughly fitted by a quadratic function.In the middle latitude area,this regularity is strong.Stations in high latitudes are less affected by the time-varying effect of the current system,and there is no obvious regularity.Secondly,by selecting the distribution stations of the longitude chain to study the latitude patterns of the geomagnetic diurnal variation,the following conclusions can be obtained through analysis: The magnitude of the geomagnetic diurnal variation is mainly affected by latitude.The solar quiet variation and disturbance variation have a strong correlation with latitude on local time and universal time respectively,and can be well fitted by the quadratic function.Based on the understanding of the above-mentioned diurnal variation patterns,the calculation method of the predicted diurnal variation value is divided into two steps: the first step is to calibrate the latitude,use the base station of the longitude chain to calculate the diurnal variation value of the latitude at the calculation point through the quadratic function fitting;The second step is to correct the longitude.First,use the correlation coefficient method to correct the time difference.Then,according to the quantitative relation between the extreme value of the diurnal variation and the longitude of the station,calculate the extreme value of the longitude at the calculation point.Finally,the points on both sides of the extreme point are used for fitting to obtain a complete diurnal variation curve at the calculated point.Compared with the simple translation when calibrating the longitude,through the quantitative analysis of the extreme value of diurnal variation and the longitude,the error is reduced when calculating the diurnal variation of remote stations.However,due to the complexity of the time-varying effect of the current system,a simple function or model cannot be used to accurately express its changing law,which makes the calculation results of some stations not very ideal,and further research is needed.