Two fundamental problems in groundwater hydrology are the need to locate and track subsurface water, and to identify subsurface properties that facilitate model predictions. These needs have led to the development of a wide range of data collection and interpretation methods. Among all of these methods, only one-time lapse gravity monitoring provides direct measurements of changes in the mass of water in storage. Recent advances in gravimeters allow for collection of spatially distributed, temporally continuous gravity data. To make full use of the data that these instruments provide requires the development of new approaches to coupled hydrologic/gravity modeling. Under this project, an extensive pre-existing gravity data set, collected at a water storage and recovery facility, will be modeled to develop and improve hydrogravimetric modeling approaches. Then, a spatially extensive data collection effort will be conducted based on these analyses to refine our interpretations of subsurface water movement beneath the storage and recovery facility.
The proposed research is at the forefront of the continuing evolution of integrating geophysical data with models and has wide applications in fields needing to tracking and modeling subsurface fluid movement, such as hydrology, but also including oil and gas reservoir monitoring, carbon sequestration, and volcano monitoring. The project will demonstrate that a new, superconducting gravimeter, which is the most precise instrument in existence, can be moved without affecting instrument drift, allowing for mobile monitoring with this advanced instrument. The proposed research supports student training specifically to develop the expertise in the use of these instruments within the United States.
Groundwater constitutes a major, unseen natural resource. There have been significant advances in our understanding of the movement of water underground. These advances have been accompanied by improvements in hydrologic measurement methods. However, the primary obstacle that limits our ability to monitor groundwater flow remains; we have to drill relatively expensive wells to make measurements. The field of geophysics has developed to address this need to characterize the subsurface through non-invasive measurement. Much of the historical focus has been on oil and mineral exploration and development. But, more recently, geophysical methods have been used to monitor water resources. Geophysics used for hydrology (hydrogeophysics) allows us to characterize water resources without drilling wells. This promises to improve both our scientific understanding of hydrologic processes and our ability to quantify water resources for better management. This project focused on the gravity method, which has received much attention as part of NASA’s Gravity Recovery and Climate Experiment (GRACE) mission. GRACE offers global coverage at very coarse spatial scales, on the order of several hundred kilometers. We examined the use of ground based gravity methods to provide local information at much higher resolution resolution. Like GRACE, these ground based methods measure the change in gravitational attraction with location and time. Most of these changes are due to changes in the amount of water stored in the subsurface. Therefore, gravity offers a unique opportunity to account for changes in water storage without the need for wells. The specific contributions of this study are as follows. First, we tested and furthered the use of a "superconducting" gravimeter for field investigations. Previously, this instrument had only been used in carefully controlled laboratory conditions. Second, we developed a method to use multiple gravimeters to focus the measurement sensitivity of the gravity method in the depth interval of greatest interest for a specific problem. In doing so we demonstrated that this new method is better able to reduce the impacts of tides and other unwanted sources of gravity change. Third, we have demonstrated, for the first time, how all different types of gravity instruments can be used jointly for time-lapse hydrologic investigations in a way that takes full advantage of their different capabilities. Finally, we have demonstrated a practical, effective method for monitoring artificial recharge, which can lead to better utilization of increasingly-managed groundwater resources. There has already been a very positive response within the scientific community to our work. Researchers in several earth science fields continually enquire about applying the methods that we have pioneered in this study. As a result, we are confident the new discipline of hydrogravimetry will emerge as a routine method for monitoring and studying groundwater. With these advances, gravity methods can make direct contributions to improved stewardship of groundwater resources around the world. This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
