the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Technical note: High density mapping of regional groundwater tables with steady-state surface nuclear magnetic resonance – three Danish case studies
Denys Grombacher
Matthew Peter Griffiths
Lichao Liu
Jakob Juul Larsen
Abstract. Groundwater is an essential part of the water supply worldwide and the demands on this water source can be expected to increase in the future. To satisfy the needs and ensure sustainable use of resources, increasingly detailed knowledge of groundwater systems is necessary. However, it is difficult to directly map groundwater with well-established geophysical methods, as these are sensitive to both lithology and pore fluid. Surface nuclear magnetic resonance (SNMR) is the only method with direct sensitivity to water and it is capable of non-invasively quantifying water content and porosity in the subsurface. Despite these attractive features SNMR has not been widely adopted in hydrological research, the main reason being an often-poor signal-to-noise ratio, which leads to long acquisition time and high uncertainty on results. Recent advances in SNMR acquisition protocols based on a novel steady-state approach has demonstrated the capability of acquiring high quality data much faster than previously possible. In turn, this have enabled high-density groundwater mapping with SNMR. We demonstrate the applicability of the new steady-state scheme in three field campaigns in Denmark where more than 100 SNMR soundings with approximately 30 m depth of investigation were conducted. We show how the SNMR soundings enables us to track water level variations at the regional scale and we demonstrate a high correlation between water levels obtained from SNMR data and water levels measured in boreholes. We also interpret the SNMR results jointly with independent transient electromagnetics (TEM) data, which allows us to identify regions with water bound in small pores. Field practice and SNMR acquisition protocols where optimized during the campaigns, and we now routinely measure high-quality data on eight to ten sites per day with a two-person field crew. Together, the results from the three surveys demonstrate that with steady-state SNMR it is now possible to map regional variations in water levels with high quality data and short acquisition times.
Mathias Vang et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2022-356', Anonymous Referee #1, 16 Dec 2022
Please find my comments and assessment in the file attached.
- AC1: 'Reply on RC1', Mathias Vang, 14 Feb 2023
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RC2: 'Comment on hess-2022-356', Anonymous Referee #2, 15 Jan 2023
Review of the manuscript «Technical note: High density mapping of regional groundwater tables with steady-state surface nuclear magnetic resonance – three Danish case studies»
By Mathias Vang et al.
General
The team around Aarhus University and Aarhus Centre for Water Technology have further developed the technique of Surface Nuclear Magnetic Resonance and have introduced the approach of steady state pulse sequences. In the near past they have published a series of scientific articles on this topic where they systematically investigate different aspects of the fundamentals, measurements, and processing and inversion. The present manuscript describes case studies that are based on the previous work.
The present “technical note” presents data from such measurements at three different sites and bring them in context with complementary information from boreholes and 2D-profiles of electromagnetic measurements.
In general, the manuscript provides data of quality, relevant information for the scientific community and a good standard of scientific conduct.
Structure and Content
Even though the shown data is of good quality and relevance, the manuscript is lacking clear statements. For example:
- The application of a new measurement device and the new sounding approach. Is that relevant for the paper? Do the authors want to present the advantage of SS-NMR over FID-NMR. In this case the presented data and corresponding discussion do not reveal this.
- Correlation with local and regional groundwater regimes. For each site piezometric data from nearby boreholes are presented. Yet, the message is not clear. Is NMR complementary data to piezometric data or does it replace these? Or is it the combination like for confined/artesic aquifers as explained in the text but not supported by data.
- The relevance of the TEM resistivity sections is also not obvious. Water contents and resistivities correlate in different fashion for the different sites and partially no correlation is visible or even contradictive. What exactly is the intention of the authors to demonstrate?
NMR
The presentation of real NMR surveys with the novel approach is relevant and the data is in general useful for this demonstration. Yet, the authors miss to outline some relevant statements. The aspects of i) frequency offset, ii) tau, iii) pulse moments, iv) regular/alternating pulses, are addressed in section 2.3. Yet, from the description of the data collection at the different sites it is not clear which of the aspects is key to the success of the measurements. Is there anything special in choosing or varying these parameters or is their choice arbitrary?
One important information remains largely unclear to me in the application of the novel SS-SNMR approach compared to conventional FID-SNMR. In FID-SNMR the spatial sensitivity is varied be increasing the pulse-moment. This results from the effect that successively deeper parts of the subsurface are excited by ~90° angles of excitation while shallower parts mutually cancel out at multiple revolutions. In this manuscript and the cited literature, I did not find a conclusive explanation how the spatial sensitivity varies for SS-SNMR and how then the water content profile with depth is inverted. I apologize if I have missed this information, the authors were quite productive in publishing papers on this topic and the review of the present manuscript required a bit of reading supporting material. The cited article of Griffith 2022 is still not published and was not available for this review.
For the Aars field site in Table 2 the authors list 4 different tau at 16 Q each. In Table 1, they list 64 as no. of Q’s. I conclude that the total number of Qs is the number of different currents times the number of different tau, is that correct? (Does not apply for the Sound site). And do the different resulting Ernst angles lead to different spatial sensitivities? This needs to be explained or referenced in more detail.
Presentation of field results
A major shortcoming of the manuscript is the compilation of the figures. The authors mix between “Elevation” (above sea level?) for the resistivity cross sections and groundwater tables, and “depth below surface” for the single SNR soundings and the maps. This is largely confusing. In the maps the “depth below surface” is shown for NMR derived water tables and boreholes, but no information about topography is given (only partially qualitatively in the text). The major information about the applicability of SNMR for local groundwater table estimates is therefore missing.
In various cases, the authors refer to a good agreement of the water content profiles and resistivity sections with the geology observed in the boreholes. But no geologic profiles from boreholes is shown.
Conclusion
The information presented in the present manuscript is very relevant to the scientific and technical community. Yet, the scope of the study is not well presented. With moderate effort to rearrange parts of the text, sharpen the scope of the study and improve the figures, the manuscript can be brought to an acceptable standard.
Citation: https://doi.org/10.5194/hess-2022-356-RC2 - AC2: 'Reply on RC2', Mathias Vang, 14 Feb 2023
Mathias Vang et al.
Mathias Vang et al.
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