AbstractMass changes underground can cause the change of nearby gravity field. This makes gravity field measurements a tool for finding mass changes underground. The gravitational field at a point on the earth's surface changes in two ways. One is the change in the magnitude of gravity, which can usually be measured by a gravimeter; the other is the change in the direction of gravity, reflected as a deflection of the plumb line. It can be measured by an astrometric instrument. Numerous studies have been conducted on the inversion of underground mass changes due to the change of gravity. However, it is less studied on the inversion of underground mass change by using gravity direction change or plumb-line variation. Related models and methods are yet developed. If there is a single mass change region at a site, the correlation between the mass center parameters and the deflection angle of the plumb line at the measurement point can be established by using the mass center to equate the whole mass change region. Thus, The longitude, latitude, depth and mass of the center of mass of the region can be obtained from the observations of two stations. However, when there are more than one gravity change regions underground, the existing method is no longer applicable. It is necessary to explore methods applicable for multiple underground mass change regions. We consider the case that there exist two underground mass changes in the framework of Newton's law of gravitation. The relationship between the longitude, latitude, depth and mass of the center of mass in the double underground mass change area and the amount of variation of the plumb line at the measurement point in the longitude and latitude directions is determined. Due to the temporary unavailability of actual observation data, seven virtual stations are set up in the study area spanning about 10 degrees in longitude and latitude. Simulation experiments are conducted for 3480 sets of center-of-mass combination examples. Based on the differential evolutionary algorithm with low influence of initial values, we design the solution process and develop a Python program to solve the location and mass parameters of the center of mass in the double underground mass change regions. Our results of the paper show that under the experimental conditions of the thesis, the mass center parameters of the two underground mass change zones in the relevant area are determined in higher accuracy in the absence of observation errors. The method is sensitive to the observation error, while the joint solution of multiple stations is helpful to suppress the influence of the observation error. In the thesis, we further study the inversion of underground mass changes using data on the variation of the plumb line. It contributes to the development of subsequent research in this direction.
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