Cross wavelet analysis between layered monitoring data of stratum deformation and groundwater level regime

Land subsidence is usually caused by over-exploitation of groundwater, and its occurrence and development have a certain lag relative to the change in groundwater level. Obtaining the accurate lag time of land subsidence has always been an important topic in land subsidence research. Based on the layered monitoring data of the long-time series of land subsidence and groundwater level regime from 2008 to 2018 by Land Subsidence Monitoring Station in Pinggezhuang, Beijing, the lag effect between stratum deformation in layers at different depths and groundwater level regime is analyzed in this paper by adopting methods of Mann Kendall trend test, continuous wavelet transform and cross wavelet transform, which provides a new technical idea for building a refined land subsidence model, improving the prediction accuracy of land subsidence and studying more effective preventive measures for land subsidence. The results show that the main oscillation period of confined water of medium and deep layers is about 1 a, while phreatic and shallow confined water has no significant period in most time domains. The stratum with a buried depth of more than 63 m exhibits a serious deformation, with the main oscillation period of about 1 a. A significant resonance period is found between the groundwater level regime and deformation. The lag times of the stratum deformation to the groundwater level from shallow to deep are (16.58±8.91) d、(7.16±7.09) d and (9.66±6.62) d, respectively. The strata with a buried depth of less than 63 m exhibit weaker subsidence, of which strata with a buried depth between 32-63 m have a main oscillation period of about 1 a and a significant resonance period with the medium layer confined water. The lag time of deformation is (32.02±9.67) d. The deformation of other strata has no significant periodicity and relevance with the groundwater level.