The index definition, interpretation and visualization of discrete time-variable gravity data

The campaigned gravity survey is a sort of time-lapse terrestrial gravity survey, which generally means the gravity observed repeatedly at a fixed station with the same routes and similar time schedules. The direct output of campaigned gravity survey is discrete time-variable gravity data. Each station in the network is visited with a fixed time interval (half or one years), in order to measure the time-variable gravity change, which is at a scale few tens of microgals, at each station during the period between these two observations. Because the shape of gravity network is very irregular and the gravity changes is not significant compared with the observation error, the general gridding method is not suitable to visualize this sort of gravity data. In this paper, we proposed a new approach for the visualization of time-variable gravity data according to the data measured by the campaigned gravity survey. According to this approach, the patterns of gravity changes and uncertainty associated with the measurement can be visualized in the same figure with the gravity difference between two adjacent stations together. Furthermore, for the purpose of evaluating the magnitude of regional gravity change, we also defined two indexes, G and C, to evaluate the significant levels of the regional gravity changes. On the basis of this approach, we analyzed the real campaigned gravity data in the capital area. The result show that the approaches are different from the classical contour map in expressing the regional gravity changes derived by the campaigned gravity survey, and can visualize and locate where the gravity changes happen, the reliable gravity changes and the level of significance. The approaches can provide more quantitative basis for the study of the gravity change potentially associated with the earthquake preparation and occurrence.