Short‐Time Geodetic Determination of Aquifer Storage Coefficient in Taiwan

Abstract

Specific yield (Sy) and specific storage (Ss) are two hydrogeological parameters often estimated in a costly pumping test (hydraulic method). Pumping or a natural aquifer recession can cause gravity change and surface deformation, useful for Sy and Ss determination by the more economic geodetic method. From 2013 to 2017, over a few days we measured gravity changes at four sites and subsidence at one site in Taiwan by a FG5 gravimeter and a precision level to estimate Sy and Ss. Using short‐time gravity and level records avoids complicated logistic supports and temporal effects. The measured gravity changes are associated with the MODFLOW‐modeled groundwater depletion. We succeeded in estimating Sy at two sites and failed at the other two. One successful case uses rapid postrain declines of gravity and groundwater level. We did leveling only at one site, but here the 500‐min, continuous, submillimeter subsidence records result in a Ss consistent with the hydraulic result. Lessons on the failed cases of Sy are suggested to avoid disrupting factors in gravity surveys. A semiautomatic leveling procedure is proposed to maximize the chance for Ss determination. Our simulations show that gravity changes increase with Sy and decrease with initial hydraulic head and horizontal gravimeter‐pumping well distance, helping to predict if the gravity‐based method works at a site and to best plan a gravity survey. Drilling data show that the Sy values from the hydraulic and geodetic methods represent aquifer storage coefficients over different aquifer layers.

Measuring Aquifer Specific Yields With Absolute Gravimetry: Result in the Choushui River Alluvial Fan and Mingchu Basin, Central Taiwan

“…Note that this conclusion is under the condition that the studied aquifer (LHES in this example) has a radius of more than 200 m and the seasonal groundwater level variations are used for analysis. This conclusion is not applicable to the short‐term groundwater level variations in a pumping test (Chen et al, 2018).…”

“…224 and 231) in Taiwan. The measurement procedure and method for gravity uncertainty estimation are the same as those described in Chen et al (2018) and Kao et al (2017).…”

“…The polar motions were from International Earth Rotation and Reference system Service (IERS), and the gravity‐atmospheric admittance was −0.35 μgal/hPa. These models and parameters were also used in our previous works (Chen et al, 2018; Hwang et al, 2009). Furthermore, we corrected the gravity effects due to significant vertical displacements at two gravity sites with continuous GPS stations (SJES and ELPS).…”

“…Accordingly, gravity changes measured by AGs and relative gravimeters (RGs) over time spans of several months or years have been successfully used for estimating S y (Gehman et al, 2009; Howle et al, 2003; Pfeffer et al, 2011; Pool, 2008; Pool & Eychaner, 1995). In addition, a previous study by Chen et al (2018) shows that GBM can determine S y using short‐time gravity changes such as those happening during cross‐well pumping tests and those induced by post‐rain groundwater level recessions. Furthermore, superconducting gravimeters (SGs), which are the most precise RGs, can be used to measure gravity changes.…”

Gravity surveys using a mobile atom interferometer

“…The use of gravity reference maps to aid inertial marine navigation requires onboard gravimeters with at least milliGalileo accuracy ( 31 ). Seasonal aquifer fluctuations can be monitored by sensing microGalileo-scale gravity changes ( 32 ). These examples illustrate that atomic gravimeters must be not only sensitive but also mobile and reliable in field conditions.…”