| As an important part of geomagnetic field,lithospheric magnetic field reflects the distribution characteristics of magnetic rocks in the crust and upper mantle.The horizontal wavelength in the lithospheric magnetic field can be divided by the model degrees,which can reflect the distribution scale of the lithospheric magnetic field feature.For studies of lithospheric field,one of the problems has always been correct calculation of the long wavelengths of the lithospheric field–from perhaps 200km wavelength up to 3000km(above which the core field becomes dominant).High-precision lithospheric magnetic field model plays a very important role in geophysical exploration,research of geological structure,geophysical origin and earth evolution,space weather and other geopherical research,which has important value both on theoretical and economic.This research aim to the lithospheric magnetic field in continent China(18°N-54°N,73°E-136°E).On the basis of summarizing the existing research results of geomagnetic field,we combine the aeromagnetic data,Swarm satellite data and its derived models,focus on the study of geomagnetic data fusion,lithospheric magnetic field modeling,model evaluation and analysis.The main research contents and results are as follows:(1)We investigate the use of satellite geomagnetic data and its derived field models to study the crustal field.We focus on China to investigate whether the crustal field of regional extent is fully explained by field models,and so whether high-quality satellite contain additional information,particularly at small scales,which might enhance crustal field studies and improve model resolution.We use Swarm satellite data(2014-2015)and the CHAOS-6model,and examine along track differences of one degree in latitude bin averages of satellite data to seek additional small-scale structure.The results show that the CHAOS-6 model does a very good job of explaining the data.The mean square deviation is about 3n T.The model also broadly explains secular variation(SV)with some misfit,but not systematic between tracks.We find an anomalous unmodeled feature in the data,but as this is located near the magnetic equator(0°N-10°N),,it is possible that this may be a result of an external field source.We conclude that crustal field studies should focus primarily on models and not data.(2)We investigate the data fusion between Chinese aeromagnetic data and satellite models such as LCS-1,MF7,CHAOS-6 and NGDC720.We focus on the distribution features of the two data sets in continent China and the consistent between the data sets,which might enhance crustal field studies and provide a method selection in the regional geomagnetic field modeling combine different data sets.We compare and analyze the distribution of the lithospheric magnetic field from both aeromagnetic data and satellite models in continent China.As an example,aeromagnetic data and LCS-1 model are used to study the consistency and compatibility by residual distribution,running average and Fourier analysis.The results show that Chinese aeromagnetic data and satellite models are completely different,they are not compatible.Whether in the time domain or the frequency domain,the satellite models can not fully describe the trend of aeromagnetic data.There are big differences in the Sichuan Basin,Tibet,Northeast and North China.We conclude that regional lithospheric magnetic field modeling combine aeromagnetic data and satellite models should focuse on a depleted basis of global spherical harmonic functions and not spherical cap harmonic analysis or complex Slepian functions.(3)We study the application and simplification of a depleted basis of global spherical harmonic functions.We construct a uniform‘data set'around a polar cap(0°-30°)of vector field measurements.Using these data locations,we obtain a normal equations matrix,and from this eigenvalues and eigenvectors of linear combinations of spherical harmonics which provide the natural basis of the solution.Using such a basis provides two reductions in complexity of the system.Subsequently,this basis rotate to China by Euler's rotation theorem and theorem of spherical harmonic addition.Finally,we certain the damping term by calculate the RMS of model with 97994 Chinese aeromagnetic data and 28662 Swarm satellite data,respectively.The results show that in the research area,a depleted basis of global spherical harmonic functions has the same modeling effect as the global spherical harmonic functions.Model accuracy will not be weakened by the reduction of computational complexity.The damping term keep the balance between inconsistent data sets,consider both model feature(damping too large)and boundary effect(damping too small).The damping term should be set to 10~7 when we combine Chinese aeromagnetic data and global satellite models.(4)We present new regional models,denoted CLAS(Chinese Lithospheric field model combine Aeromagnetic data and Satellite model),of the Chinese lithospheric field,combining the long wavelength information provided by satellite-derived models:CHAOS-6,MF7,LCS-1 and NGDC720,and an extremely high-quality compilation of 97994 aeromagnetic survey data with 10km×10km resolution for shorter wavelength.The models are estimated using a depleted basis of global spherical harmonic functions centred on China.CLAS models are determined include harmonic degrees up to 400.Although some accuracy of aeromagnetic data is lost in order to balance the consistent of two data sets,the results show that CLAS models have a high correlation with the satellite models at low degree terms(degree correlation>0.9)but with more power at high degree terms,reflecting more features of the lithospheric field in continental China.Examples of improvement include Changbai mountains,Sichuan Basin and Qinghai-Tibet Plateau.At satellite altitude(about 500km),the addition of aeromagnetic data imposes some features on the long-wavelength part of the model,such as North China.CLAS models have good agreement(coherence>0.9)with Chinese aeromagnetic data at wavelength down to about 100km(corresponding to spherical harmonic degree n=400),filling the usual gap between satellite models and aeromagnetic data.Comparison with aeromagnetic data filtered at 100 km gives good agreement(correlation>0.95).The residuals between CLAS models and aeromagnetic data are still large(RMS>70n T),but with most of misfits arising from shorter wavelength fields that the model cannot fit at degree up to 400;such misfit could be reduced by increasing the model degree.We provide a geological example of how the inclusion of satellite data can change the geological conclusions that can be drawn from the magnetic information.However,the two data sets are not completely consistent,future models should start from a reanalysis of the aeromagnetic data and its line leveling to ensure consistency with the satellite model.Since the existing high degree magnetic models do not consider Chinese aeromagnetic data,This data set will effectively fill the gap in the World Digital Magnetic Anomaly Map(WDMAM). |
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