One advantage of nuclear magnetic resonance (NMR) is that we can learn about the pore-scale environment of saturated samples – specifically by using the relaxation time parameter as a proxy for surface area to volume ratio (S/V). While S/V and the related void-size-distribution are relatively challenging to measure, grain size distribution can be easily obtained from a sieve set or other more complex methods. We expect that the grain size distribution of a sand sample will approximately represent the void-size-distribution by way of a linear or nonlinear transformation. In this dataset the NMR signal is provided from three samples of known grain size and one sample of unknown grain size.

Measured sample description:
• Mixed sand sample: Comercial leveling sand of various grain sizes.
• Three sieved samples: These samples have been retrieved from the mixed sand by collecting grains in a sieve set. The reported grain size value means “larger than,” and all particles in each sample will be smaller than the next sieve size up.
• Calibration blank: A container of the same dimensions as the samples filled with water to calibrate the instrument to 100% water content.

Procedure
1. Insert the calibration blank into the NMR sensor centered as best as possible in a reproducible manner. We want to be able to get the sand samples in exactly the same position. Use the meter stick to measure the distance from each opening to attain lateral centering, and you may use foam padding for vertical centering.
2. Using the acquisition software on the laptop, measure the water blank.
3. Go to the analysis software and perform an inversion on the data that you just measured. Record the value of water content (likely less than 100%).
4. Repeat the measurement process for each of the four sand samples.
5. Perform the inversion and use the calibrated water content value from the blank in each sand sample inversion.

Other information
Measurements were made on the Vista Clara Corona lab NMR core analyzer. The grains consist of: majority quartz, lesser feldspar, trace mica and lithic fragments.

Video of measurement process: https://www.youtube.com/watch?v=-vT1_ajimyk&t=14s

Example questions to consider if this data is to be used in the context of laboratory coursework:
-Based on your understanding of NMR relaxation processes, why is relaxation time most appropriately related to S/V?
-How do you expect the pore size distribution from NMR to relate to the measured grain sizes? How does this expectation compare with what you observe? Be careful to consider all aspects of the grain sorting and pore size distribution, including how they were measured.
-How do you expect the water content/porosity from NMR to relate to the gravimetrically measured water content? How does this expectation compare with what you observe? Be careful to consider all aspects of both measurements and results.
-How can you assess uncertainty on any/all of these measurements (both geophysical and ‘direct’ measurements)?