[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 "NMR") 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.