The Sonoran Desert cities of Phoenix and Tucson face long-term challenges for maintaining sustainable water supplies for existing populations and anticipated future growth. Phoenix and Tucson rely on groundwater to meet a significant portion of their water needs, and both cities are expanding their aquifer storage and recovery operations. Optimized management of these critical (in same cases sole-source) groundwater resources requires a more detailed understanding of aquifer hydrologic properties, including hydraulic conductivity and effective porosity, at the sub-basin scale. In some areas the aquifers utilized for groundwater storage and production extend to depths of thousands of feet, making traditional low-resolution hydrological measurements impractical and/or very expensive.
NMR logging measurements were acquired in existing PVC-cased monitoring wells using a Javelin® Wireline NMR logging tool with 4-frequency, 3.5 inch diameter NMR logging probe. One well was located on private industrial property in Chandler, AZ, and another was located in a municipal production wellfield near Oro Valley, AZ. Each well was drilled with a nominal 10 inch diameter and was completed with centralized PVC with screened intervals for production and appropriate filter and grout intervals. This enabled collection of useful data in three of the four Javelin tool sensitive shells, ranging from 12 inches to 15 inches in diameter. High-resolution NMR logs were acquired at a rate of 30 m/hr.
Javelin® NMR Wireline Logging Tool
The NMR log acquired at Chandler, AZ (top) shows highly variable, layered, and generally productive aquifer sediments in this part of the valley. Large contrasts in mobile porosity, bound porosity, and hydraulic conductivity are evident at sub-meter vertical scales.
The NMR log acquired near Oro Valley, AZ (bottom) indicates lower overall porosity with more subtle and gradual changes in pore size and permeability, consistent with the poorly sorted sediments at this location near the foothills of the Santa Catalina Mountains.
Importantly, the NMR log indicates that the most permeable and porous segment of the aquifer lies above 400 feet, and was inadvertently excluded from the screened interval.
These two NMR logs demonstrate the value of NMR logging for characterization and management of groundwater resources from disparate sedimentary deposits in critical desert urban areas. The results also illustrate how the high resolution information return from a relatively low cost Javelin Wireline NMR log can positively impact the yield from an individual well, through improved and optimized placement of well screens.
Acknowledgements: USGS, Bob Crowder