Application of the Vertical Gravity Gradient and Time Lapse Microgravity Method for Identifying Groundwater Vulnerability Boundaries and Land Subsidence Zones in the Gedebage District, West Java

Abstract

Gedebage District in the Bandung Basin experiences land subsidence at a rate of 9– 14 cm per year, primarily due to excessive deep-aquifer groundwater extraction, exacerbated by geological conditions and human activities. This study employs the Vertical Gravity Gradient (VGG) and Time-Lapse Microgravity (TLM) methods to map groundwater vulnerability zones and detect subsidence-prone areas. The survey was conducted from February to june 2025 at 54 measurement points (46 valid after selection), using a Scintrex CG-5 gravimeter and VGG measurements at two elevations, with temporal corrections applied for instrumental drift and Earth tides. Results show VGG anomalies ranging from −326 to −273 μGal/m. Positive values correlate with high-density lithology and areas already affected by subsidence, whereas negative values indicate porous, water-saturated formations with recharge potential. Three-month TLM anomalies range from −3 to +13 μGal, where positive values are associated with subsidence and mass gain due to compaction or recharge, while negative values reflect mass loss from intensive groundwater extraction. Simulations indicate that 1 cm of subsidence increases gravity by +3.1 μGal, whereas a 1 cm drop in groundwater level decreases gravity by −0.1 μGal (25% porosity). Spatial analysis reveals that high VGG anomalies and positive TLM values consistently mark critical zones of land subsidence and groundwater vulnerability, particularly in the northwest, west, and central parts of the study area. Negative VGG values, though generally representing recharge zones, may still be susceptible to subsidence under overextraction. The integration of VGG and TLM significantly improves the accuracy of detecting and interpreting subsurface mass changes, providing an effective framework for groundwater management and subsidence mitigation in urban alluvial basins.