## 4 Quality Assessment of Actual Gravity Datasets
### Drift Estimation of the Different Relative Gravimeters  
In static and field observations, instruments exhibit temporal variation in the display of the zero position, which is called instrumental drift and represents one of the inherent features of change of relative gravimeters with time. On the basis of July 2020 gravity data, the daily drift rate of the different spring-based relative gravimeters is shown in Figure 3, fitted using the BGA method. Here, the drift rate is an average value calculated daily and expressed as the drift rate change per hour. It is evident that all the instruments show nonlinear drift characteristics, although the amplitude of the drift rate between each gravimeter presents large deviation. The drift rate of the CG-5 gravimeters (up to approximately 37 uGal/h) is much higher than that of the LCR-G instruments (broadly below 1.5 uGal/h). It is also evident that the drift rate of the CG-5 instruments shows asynchronous difference, indicating that the CG-169 gravimeter is more sensitive to the observation route or the observation environment, and that the drift rate exhibits substantial nonlinear characteristics. The deviation between the maximum and minimum values of drift rate of CG-169 is approximately 10 uGal/h, i.e., the maximum drift difference will reach up to 240 uGal in a single day, which far exceeds the observational error in the measurement campaign. Figure 4 shows the variation of GD residuals of the survey time series obtained using the CLS (Figure 4a) and BGA (Figure 4b) methods for different relative gravimeters. The amplitude of the GD residuals of the CG-5 gravimeters varies markedly, fluctuating in the range of \(\pm\)30 and \(\pm\)10 uGal for CG-169 and CG-170, respectively. The two CG-5 instruments also present obvious nonrandom variation characteristics when using the CLS method, and the variation trend is consistent with the trend of change of drift rate shown in Figure 3a. However, the GD residuals obtained using the BGA method present as a random white noise signal, which means that the variation of GD residuals has no obvious correlation with drift rate. In Figure 4e-h, the GD residuals of the LCR-G instruments are constrained to \(\pm\)20 uGal, and because the drift rate amplitude of LCR-G instruments is negligibly small, there is no obvious difference between the GD residuals of the two LCR-G gravimeters. Nevertheless, we have found that the number of GD residuals (<5 uGal) from BGA method is slightly larger than that from CLS method in Figure 5. In addition, the SD of GD residuals decreased from 8.0 to 7.8 uGal and from 7.7 to 6.9 uGal for CG-5 and LCR-G gravimeter respectively, which means that the BGA method is more effective than the CLS method in removing errors caused by nonlinear drift. Figure 3: Nonlinear drift rate variation of different relative gravimeters obtained using the BGA method: (**a**) CG-5 gravimeters and (**b**) LCR-G gravimeters. The nonlinear instrument drift rate of the different relative gravimeters obtained using the BGA method during 2018-2020 is shown in Figure 6. All instruments were obviously affected by irregular nonlinear drift, and the general trends of the drift rate variation appear broadly consistent for the two instrument pairs in the same observation period. The nonlinear drift rates of the two LCR-G gravimeters change only slightly with low amplitude of approximately 1 \(\upmu\)Gal/h. However, the drift rate of the CG-5 instruments increases with observation time during the survey campaigns, while on the long term, the drift rate decreases gradually with increasing age and use of the instruments (Figure 6a,b). The mean drift rate of CG-169 and CG-170 stabilized at approximately 27 and 33 \(\upmu\)Gal, respectively, whereas the nonlinear drift rate of CG-169 is unstable and its amplitude changed drastically in comparison with that of CG-170.