Numerical Example to Estimate Groundwater Level Changes using Time-lapse Microgravity Data

Abstract

The study using time-lapse microgravity (TLM) data can be helpfull to understand near surface variation caused by hidrological variation. Shallow groundwater variation can be highly correlated with density variation as time-lapse microgravity source, because the changes are very close to the surface. For several days (or weeks or months), we can observe density variation in the near surface caused by the changes of water saturated zones. The time-lapse of density variation for several days can be simplified as groundwater table changes. The purpose of this work is to study time-lapse microgravity application for groundwater level estimation. Using numerical example, we utilize lateral variation of time-lapse microgravity signal and also surface topography. In order to do the estimation groundwater level for second TLM survey, we need the groundwater level data from first survey (and vice versa). After numerical example, we applied the algorithm in the study area of (5 × 5) km square in the Blora regency. The shallow groundwater level (0 to 14 meters below the surface) are observed after the second (2016) microgravity survey. From this work, we constrained the model using second groundwater level data. After iterative calculation of using TLM anomaly (value in the range of -80 to +80 microGal) with surface topography variation (in the range 90 to 230 meters), we get the estimation the first (2014) groundwater level variation in the range of -7 to 7 meters from the second (2016) groundwater level data.