PreviousNext No AccessSEG Technical Program Expanded Abstracts 2011Time‐lapse gravity as a water resource management tool — Applications to unconfined aquifersAuthors: Dennis L. HarryWilliam E. SanfordCarter GehmanJoshua WoodworthBrian N. DamiataDennis L. HarryColorado State UniversitySearch for more papers by this author, William E. SanfordColorado State UniversitySearch for more papers by this author, Carter GehmanHess Corp.Search for more papers by this author, Joshua WoodworthShell Oil Co.Search for more papers by this author, and Brian N. DamiataUniversity of California at Los AngelesSearch for more papers by this authorhttps://doi.org/10.1190/1.3627978 SectionsSupplemental MaterialAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Time‐lapse gravity surveys involve gravity campaigns conducted at different times. Differences in gravity measured at given locations between the surveys, after corrections for barometric pressure changes, Earth Tides, and instrument drift, reflect changes in subsurface mass. In this study, we associated these mass changes to changes in groundwater storage. We demonstrate that, in unconfined surficial aquifers, water table elevation changes of ca. 70 cm can be detected, and, if drawdown data are available, specific yield, storativity, and specific storage can be estimated (although specific storage is poorly constrained). We present two case studies, one involving an artificial recharge project in which water table changes are mapped via time‐lapse gravity over a ca. 10 km2 region and a second in which estimates of specific yield, transmissivity, and specific storage are obtained at a single location during a pumping test.Permalink: https://doi.org/10.1190/1.3627978FiguresReferencesRelatedDetails SEG Technical Program Expanded Abstracts 2011ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2011 Pages: 4424 Publisher:Society of Exploration Geophysicists HistoryPublished: 25 May 2012 CITATION INFORMATION Dennis L. Harry, William E. Sanford, Carter Gehman, Joshua Woodworth, and Brian N. Damiata, (2011), "Time‐lapse gravity as a water resource management tool — Applications to unconfined aquifers," SEG Technical Program Expanded Abstracts : 3718-3721. https://doi.org/10.1190/1.3627978 Plain-Language Summary PDF DownloadLoading ...

Two high‐precision gravity surveys were conducted to determine groundwater mass changes at a managed groundwater recharge site in northeastern Colorado. Gravity data were collected during pumping and two months after pumping ceased. During pumping, gravity was lower by as much as 46 μGal near the pumping wells and higher by as much as 90 μGal near the recharge ponds in comparison to data collected after pumping had ceased. These differences are attributed to changes in groundwater mass associated with drawdown and infiltration. Inverse modeling of the gravity data indicates a 5.1 × 105 m3 decrease in storage beneath the recharge ponds between the two surveys, which we attribute to dissipation of the groundwater mound created by recharge during pumping. This estimate of the change in groundwater storage is made independently of assumptions of physical properties of the aquifer. Dividing the change in water volume per unit area determined from the gravity modeling by the change in water level measured in wells provides an estimate of specific yield (Sy) of 0.21 ± 0.03, which is within the range of specific yield estimates derived from aquifer tests at the site. Water level changes predicted from the gravity data agree on average to within ±0.45 m of those measured, which we take to be an estimate of the uncertainty in water table depth predictions that can be obtained from gravimetric data in unconfined aquifers. The study covers a 3.2 km2 area, providing a prototype for similar studies at larger scales.

Over 120 microGal (μGal) gravity measurements were collected at a stream augmentation site in northeastern Colorado to correlate water table fluctuations with temporal gravity data. Up to 8 m changes in water table elevation occur at this site as a result of controlled groundwater pumping and subsequent infiltration at recharge ponds. Temporal gravity data were collected in two phases: 1) During pumping of the alluvial aquifer and infiltration at the recharge ponds, and 2) after pumping had ceased and the recharge ponds were dry. Gravity data were collected with a Scintrex CG-5 Autograv meter at 250 meter spacings over a 3.2 km2 area, except around pumping wells where station spacing was roughly 25 meters. The difference between gravity measurements taken during each phase of the study reveals distinct gravity lows and highs at the pumping wells and recharge ponds, respectively. During phase 1, gravity was lower by as much as 106 μGal near the pumping wells and higher by as much as 98 μGal at the recharge ponds. Decreases in gravity correspond to water table drawdown at the pumping wells and increases correspond to groundwater mounding at the recharge ponds. Specific yield was estimated at seven well locations and values ranged from 0.07–0.41 with a mean specific yield of 0.23. This mean value is within the range of specific yield values (0.1–0.3) determined by aquifer tests in the area. Based on these results, the temporal gravity technique has the potential to estimate regional changes in groundwater storage and contribute critical data to water management plans.