Neutron Probe Data Processing: Calibration and Conversion to Water Content using Nuclear Magnetic Resonance, and Uncertainty Analysis

Throughout our field studies, we encountered several challenges with interpreting the neutron probe data. First, there were many dates where the neutron probe did not pass an in-field chi-squared test indicating that the probe was not producing accurate estimates of neutron counts. Second, in several instances, we encountered anomalous increases or decreases in neutron counts across surveys between time periods where there were no precipitation inputs or evapotranspiration, thus, no significant change in moisture content and neutron count was expected. This indicates that the instrument was likely experiencing drift which has been observed in several previous studies (e.g., Hoedlmoser et al. 2012; Vachaud, Royer, and Cooper 1977; Yao et al. 2004; Zhang 2017). To account for these issues, we eliminated survey dates that failed the chi-squared test or produced anomalous count values. We then quantified the amount of instrument drift by averaging measurements across multiple survey periods at depths in the borehole where no change in neutron count was expected to apply a drift correction. Data, scripts, and further explanation for neutron probe data processing is available as part of this Hydroshare resource.

Data_Processing_Description.pdf offers a throrough description of the processing workflow, providing explanation for why and how data was processed. 

The Neutron_Data folder contains Raw_Neutron_Data.csv in addition to the Corrected_Data folder, which contains corrected neutron data for each borehole: N4W1, N4W2, N4P1, N4P2, N4P3, N4P4, N3W1, N1P2, S3P1, S3P2, S2P1, and S1P2. 

The NMR_Data folder contains all NMR data: 1Dvectors.csv, logMetadata.csv, SEdecay.csv, T2bins.csv, and T2dist.csv. 

The Scripts folder contains all .pynb files necessary for the data processing and figure creation. The intial neutron probe data correction and inspection is done by slope, equator-facing and pole-facing. Each folder, Equator-Facing Boreholes and Pole-Facing Boreholes, contains the scripts 0_inspect_uncorrected, 1_correct_neutron_probe_visual, 2_apply_correction, and 3_inspect_corrected. These scripts should be run in this order. 
4_PlotCounts_wet_dry_survey_diff.pynb plots the corrected neutron data, and 5_Calibration_nmr_np.pynb investigates different relationships between the NMR and nuetron probe data in order to relate neutron counts to volumetric water content.