Miaomiao Zhang

Land subsidence in coastal reclamation areas has emerged as one of the critical hidden hazards for coastal cities. This paper presents an integrated InSAR and coastline-change framework for land subsidence monitoring and driving-factor analysis in coastal reclamation cities. Taking the Fangchenggang City, China, as the study area, we utilized 266 scenes of Sentinel-1A SAR images (2016–2025) from the European Space Agency (ESA) and applied time-series SBAS-InSAR to obtain a 10-year time-series monitoring result of land subsidence, with cross-validation against PS-InSAR showing an RMSE below 4 mm at four checkpoints. Furthermore, influencing factors were analyzed by integrating data on coastline changes, precipitation, and groundwater indicators. The conclusions of this paper are as follows: (1) Seven distinct land subsidence funnels in Fangchenggang City from 2016 to 2025 were identified for the first time. Spatially, land subsidence exhibits significant heterogeneity, primarily concentrated in the coastal reclamation areas of the Qisha Peninsula and the Yuwan Peninsula, the maximum subsidence rate reached as high as −163.53 ± 4.90 mm/yr. Temporally, the subsidence rate in the study area shows a gradual deceleration trend over time. (2) The coastline change results from 1964 to 2025 reveal a three-stage temporal characteristic: the land area increased by 2.23 ± 4.51 km², 14.28 ± 6.19 km², and 23.31 ± 4.51 km² during the periods of 1964–1995, 1995–2008, and 2008–2025, respectively. The time-series InSAR results and coastline change results demonstrate that coastal reclamation is the primary factor contributing to land subsidence in this study area. (3) Apart from land reclamation, driving factors such as precipitation and groundwater indicators also show a slight correlation with land subsidence.

To ensure the sustained stability of absolute gravity benchmark points from 2017 to 2024, observational records from superconducting gravimeters (SGs) and absolute gravimeters were comprehensively examined in this work, and the environmental effects on gravitational acceleration were quantitatively assessed. The annual fluctuation of the SG (iGrav-012k) scale factor reached 0.268 μGal/V, with a weighted average of (–92.8702 ± 0.0265) μGal/V (relative precision of 0.3‰), providing a precise scale factor for long-term SG monitoring. By removing step discontinuities in the SG data using FG5-X249 absolute gravimeter measurements, the residual fitting error decreased to 6.3 μGal. In addition, the SG drift was estimated as 1.0 μGal/year through international comparison datasets and FG5 measurements, substantially improving the consistency of the time series. Further investigation showed that the SG residuals exhibited clear seasonal oscillations, which were mainly attributed to local hydrological processes and ground deformation near the benchmark sites. By integrating groundwater level and deformation monitoring data and applying a neural network model to separate hydrological load components, the peak-to-peak residual amplitude was reduced from 13 μGal to 3.5 μGal. Quantitative analysis indicated that hydrological effects contributed about 9.5 μGal to the seasonal variation, whereas surface deformation had only a minor impact (<2 μGal). The findings confirm that careful data correction and isolation of environmental effects are effective for maintaining the long-term stability of gravity benchmarks. The developed workflow provides a reproducible framework for high-precision gravity site maintenance and supports future dynamic monitoring of regional environmental load responses.

Gravity gradiometers hold substantial value across various domains, including scientific research, and resource exploration. Currently, the performance metrics of these instruments predominantly depend on self-calibration conducted by their respective development entities, and the calibration procedures lack standardized protocols. To facilitate standardized testing, calibration, and verification of gravity gradiometers operating under diverse principles, this study proposes a vertical gravity gradient modulation method grounded in Newtonian gravitation. The proposed model features two large, vertically aligned, fixed-position masses with the gravity gradiometer positioned between them. As the sensor undergoes vertical displacement, its relative position to the masses changes, thereby modulating the vertical gravity gradient at the device’s sensitive point. The simulation results indicate that the adoption of two tungsten-molybdenum alloy masses with an individual weight of 2125 kg, which are vertically arranged on the lateral sides of the sensing device at a distance of 1 m, enables the modulation range to exceed 260.0 E with a resolution finer than 1.0 E. This high-performance modulation model offers a well-characterized gravity gradient field for gradiometer calibration, featuring a wide dynamic range and high resolution—capabilities essential for advancing instrument standardization and precision assessment. Moreover, the model enables modulation of gravitational acceleration at the sensor’s sensitive point, achieving a vertical gravitational acceleration modulation range exceeding 10 μGal and a resolution better than 0.1 μGal. These characteristics meet the calibration requirements for absolute gravimeters.

The noise level of gravity stations is an important indicator for measuring the operating status of a station and is a prerequisite for evaluating whether the station’s observations can be used to extract weak geodynamic signals. With the continuous expansion of areas of human activity, gravity stations originally located in the wild may become increasingly closer to cities. Whether their noise levels change is an important issue that is worthy of attention. Based on power spectrum analyses and probability density function methods, the noise level of the superconducting gravimeter (SG) at Jiufeng station in Wuhan in the seismic frequency band of 0.001–0.04 Hz was calculated, and its time-varying characteristics were analyzed. The noise level of Jiufeng station did not change significantly before and after the lockdown of Wuhan due to the COVID-19 epidemic in 2020. No significant changes in the noise level were found before and after the official operation of Wuhan Metro Line 19 at the end of 2023. From October 2016 to April 2017, the noise level showed an abnormal trend of suddenly rapidly rising and then slowly declining, which was found to be caused by a tilt problem in the gravity sensor. Overall, in the seismic frequency band of 0.001–0.04 Hz, the noise level at Jiufeng station showed seasonal variation characteristics, and the noise was stronger in winter than in summer, which is consistent with the characteristics of Earth’s hum. Since January 2022, the noise level has shown an increasing trend year by year. The results of this study can provide an important reference for the operation of gravity stations and the extraction of weak geodynamic signals.