| The high-precision absolute measurement of the earth's gravitational acceleration can be used to study the internal structure of the earth,geodynamics,earthquake monitoring and other related scientific issues,and can also be used to know the distribution of the earth's mineral resources and to conduct navigation and guidance for aircraft.In the process of absolute gravity measurement based on the principle of laser interference,many factors affect the final measurement results,and the environmental influence mainly comes from the solid tide,ocean tide,atmosphere,hydrology,electromagnetic field,temperature and so on.In order to ensure the accuracy of the absolute gravity measurement,the influence of the background environment must be fully considered and analyzed.This paper systematically analyzes the influence mechanism of environmental factors on gravity measurement,such as tide,atmosphere,hydrology,earth rotation,electromagnetic field,temperature,gravity vertical gradient,etc.,and quantitatively studies the influence of various environmental factors by means of theoretical model,digital simulation and practical measurement.The main innovative achievements are as follows:Green's function method is used to calculate the influence of atmosphere on gravity measurement.The Green's function of atmospheric gravity is calculated,and the effects of different temperature models,ground temperature,humidity,station elevation and surrounding terrain on gravity are analyzed.The influence of station humidity on gas constant value is discussed for the first time.The calculation results show that the gravity variation caused by different humidity conditions(40%-80%)is about 0.1 ?Gal.It is considered that the specific gas constant has no effect on the results.In this paper,the Green's function based on the actual station is constructed.The Green's function method is used to calculate that the influence of atmospheric variation within the 0.5° range of the station on gravity measurement and the result is less than 10 ?Gal.The atmospheric gravity admittance calculated by linear regression is-0.3826 ?Gal/h Pa,which is 0.08 ?Gal/h Pa different from the average admittance value of-0.3 ?Gal/h Pa obtained by the time domain and frequency domain admittance method.When the variation of atmospheric pressure around the station is less than 10 h Pa,the error of atmospheric influence it is less than 1 ?Gal,which basically meets the requirement accuracy of absolute gravity measurement.The whole hydrological system includes surface water,groundwater and soil water.Previous studies have only studied the influence of single hydrological factors such as groundwater or soil on gravimetry.This paper has analyzed and quantitatively calculated the influence of the whole hydrological system on gravimetry for the first time.The gravity residual from 2013 to 2015 of superconducting gravimeter in Wuhan were used to verify the simulation results.There is a difference of 1 ?Gal between the simulated effect of the whole hydrological system and the measured long-term gravity residuals variation,which proves the effectiveness of the simulated calculation.The results show that the influences of unconfined and soil water,which have great influence on gravimetry,should be deducted when the absolute gravimetry values of each period are used for geodynamics studies.The influence of earth rotation on gravity measurement includes pole shift,day length variation and Coriolis force.The results show that the influence of the pole shift on the absolute gravity measurement should be considered,while the influence of the day length change and the Coriolis force can be ignored.Through the angle of error transmission,the influence of of the parameters error such as : tide factor,station longitude and latitude,earth radius and rotation rate in the correction on the pole shift is considered.The results show that the correction error of 1.16 or 1.18 for tide factor is 0.1?Gal,and the error of 1° longitude and latitude for station is 0.01 ?Gal.The earth radius error of 8km leads to the error of0.01 ?Gal,and the rotation rate error of 10-8rad/s has the error of 0.1 ?Gal.The pole shift correction can basically meet the absolute gravity measurement accuracy requirements when the above parameter value error exists.The influence of changing magnetic field and static electric field on gravity measurement is discussed through theoretical calculation and experimental verification,the influence mechanism and magnitude of changing magnetic field on gravity measurement are given,the influence of changing magnetic field on gravity measurement can be produced,while the influence of electrostatic field can be ignored.The influence of changing magnetic field on gravity measurement in real environment(under high voltage transmission lines)is discussed.The data of regional magnetic field variation in 220-1000 k V EHV transmission lines and substations in China are statistically analyzed.The magnetic field gradient under transmission lines is about 0.5 ?T/m at most,and the magnetic field gradient in substations is 50 ?T/m.The calculation results show that their influence on the absolute gravity measurement is less than 0.2?Gal.The influence of temperature on gravity measurement is considered,and the theoretical calculation method of temperature change on laser wavelength,temperature change on vacuum degree and then on gravity measurement is given.By conducting experiments on laser wavelength and vacuum with temperature change in Beijing laboratory,the following new understanding is obtained: when the temperature rises from 20? to 30?,the observed gravity value increases by 6.2?Gal due to the laser wavelength change,which is larger than the theoretical calculation result(2.8 ?Gal).The temperature change shows a linear trend with the gravity change.When the temperature rises from 25 ? to 35?,that is,10? higher,the gravity value decreases by 10 ?Gal due to the change of vacuum degree.This value is larger than theoretical calculation result,and the change of gravity and temperature are basically linear.Aiming at the accuracy problem of using gravity vertical gradient value to correct the height of different types of absolute gravimeters,this paper studies the actual gravity gradient data measured at different points and heights,and the results show that the actual gravity vertical gradient value should be used to correct the height.In addition,the measurement results show that the results of different height gradients may differ greatly,resulting in the maximum reduction difference about 2.5 ?Gal for the 50 cm height difference,which affects the accuracy of absolute gravity height correction.In gravity observation,it is necessary to measure the gravity vertical gradient values at different heights.Through the analysis of the measurement results of the vertical gradient at different points in the laboratory,the results show that the influence of gravity horizontal gradient should be considered in the area with complex terrain. |
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