Review of time-lapse microgravity correction due to shallow groundwater variation

Microgravity surveys can be applied for reservoir monitoring, such as monitoring of hydrocarbon or geothermal reservoirs. One factor that needs to be corrected from gravity data in reservoir monitoring is the gravity signal due to shallow groundwater variation. The correction due to shallow groundwater effect shall be calculated from groundwater change, in this case can be measured from groundwater table change. So far, correction due to shallow groundwater variation in microgravity data is calculated against the influence of groundwater mass change in the saturated zone only, while the change of mass of groundwater in the unsaturated zone is ignored, thus giving an over-estimated correction result. The purpose of this paper is to review time-lapse microgravity correction due to shallow groundwater variation by including the influence of groundwater mass variation in the unsaturated zone. This review includes a case study using three-point piezometer groundwater data. The correction due to groundwater mass variation in the unsaturated zone is obtained by calculating the difference between the total water content at two measurements over a certain time interval. The calculation of total moisture content in the unsaturated zone is based on a soil-water retention curve that describes the relationship between moisture content and pore pressure. Considering the effect of water mass in the unsaturated zone, the total correction due to shallow groundwater variation has a smaller range, about 35% to 50% compared to the correction that only considers the effect of water mass in the saturated zone alone. (paper)