Borehole Nuclear Magnetic Resonance for Environmental Site Management

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Detailed Description

Borehole nuclear magnetic resonance (NMR) is an emerging geophysical method being applied to hydrogeology investigations. NMR is a quantitative geophysical method that can be used to make in situ assessments of porosity, water content, mobile and immobile water fraction, and estimates of permeability. While borehole NMR is commonly used in the oil and gas industry, it is only recently that NMR tools have been designed for use in small-diameter boreholes that are typically used in groundwater studies. This video presents an overview of borehole NMR and example applications for environmental site management.

https://doi.org/10.5066/F73J3BW0

Development of this video was supported by the Environmental Security Technology Certification Program (Project #ER-201567-T2) and the U.S. Geological Survey.

Imagery:

  • U.S. Geological Survey
  • Department of Earth and Environmental Sciences, Rutgers University Newark
  • Medical MRI image courtesy of U.S. Department of Health and Human Services, National Institutes of Health

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Details

Image Dimensions: 1280 x 720

Date Taken:

Length: 00:04:14

Location Taken: US

Video Credits

Written and Produced by: C.B. Dawson, F.D. Day-Lewis, C.D. Johnson, and J.W. Lane, Jr., U.S. Geological Survey Office of Groundwater, Branch of Geophysics
Written and Produced by: J.L. Robinson and L.D. Slater, Department of Earth and Environmental Sciences, Rutgers University Newark

Transcript

[Narrator]

The goal of hydrogeophysics is to use geophysical methods to find, detect, and assess properties relevant to hydrologists. Accurate porosity, saturation, and hydraulic conductivity estimates are critical for understanding 
 - aquifer heterogeneity, 
 - fate and transport of contaminants in the environment, and 
 - water availability. 

Whereas most geophysical methods are only indirectly sensitive to hydrologic properties, Nuclear Magnetic Resonance (or ìNMRî) is unique in that it directly senses the presence of water. As a result, borehole NMR can provide in-situ estimates of hydrologic parameters that scientists and project managers need for environmental site and water resources assessment. 

Borehole NMR contains information on the overall water content as well as the distribution of the water in porous material. Using borehole NMR logs, practitioners can estimate 
 - vertical distributions of water content, 
 - porosity, and 
 - even pore size distribution. 

Using established textbook formulas, it is possible to estimate hydraulic conductivity from borehole NMR measurements. Furthermore, the sensitivity to pore size distribution may allow discrimination between bound and unbound water, which controls back diffusion. Project managers can use this information to develop site conceptual models and construct predictive models to improve environmental management. 

Borehole NMR is extensively used in oil and gas reservoir characterization and can replace traditional logging methods for estimating porosity that use radioactive sources that have safety concerns. 

Recent technological advancements have led to cost-effective and smaller borehole tools suitable for environmental applications. With safer, more economical, and more portable tools, borehole NMR logs produced by these tools can be easily repeated to monitor changes in porosity and hydraulic conductivity over time, such as those caused by biogeochemical processes associated with contaminant transformations.

NMR is based on the same physical principles as MRI in medicine. 

In the NMR measurement, hydrogen atoms in water molecules preferentially align with strong magnets inside the tool establishing a known background state. Then, the tool pulses a radio frequency, pushing the water molecules out of alignment with this background state. This perturbation causes a measurable magnetization, as the atoms precess and decay back to the background state. The total amount of water present is derived from the magnetization strength. The pore-size distribution is calculated from the timing of the decay. 

Borehole NMR tools use these principles to generate logs presented in terms of water content and pore size distribution. 

Although borehole NMR can provide valuable site data, site conditions and other factors may limit its use. For example, because of the strong magnet, the tool cannot log in steel casings. 

Different borehole NMR tools are sensitive to different radial distances from the tool, therefore different diameter boreholes require different tools.

Each tool is sensitive to a specific distance from the center of the tool. Before you install your borehole, you have the undisturbed subsurface formation. When a borehole is drilled, the area immediately surrounding the borehole is also disturbed. When we conduct borehole NMR logging, each tool is sensitive to a different radial distance in order to take measurements of the undisturbed formation.

Relative to conventional logging tools, the measurements may be more time consuming. As a result, cost and time constraints may necessitate discrete measurements rather than continuous logging.

Like all geophysical tools, borehole NMR is not a silver bullet. Rather, borehole NMR should be viewed as one component of a toolbox of integrated geophysical methods for environmental site management. 

To learn more about borehole NMR, visit our web site for a list of references for this video or explore the USGS Fractured Rock Geophysical Toolbox Method Selection Tool.