Improved water distribution estimation in merokarst system using surface and borehole nuclear magnetic resonance
Abstract
Nuclear magnetic resonance (NMR) has been extensively applied in hydrogeological exploration and aquifer characterization due to its sensitivity to water content. The non-invasive surface NMR sounding (sNMR) is one preferred form of NMR following the successes of borehole NMR (bNMR) in groundwater detection. However, the application of sNMR is limited in complicated geology with the mixed lithology since the long 'dead time' (e.g., 10ms in our case) of sNMR cannot capture signals from smaller pores. This study is aiming to better estimate water distribution from sNMR by enhancing relaxation signal with bNMR data and lithology information. To achieve this goal, both borehole and sNMR data were collected in a merokarst (carbonate aquifers interbedded with shale) watershed of Konza Prairie, Kansas, where hydrologic records and boreholes are available. Shale and limestone display distinct pore attributes and consequently different NMR relaxation distribution. In different lithologies, characteristic sNMR T2 relaxation signals below 10ms are extracted from adjacent borehole NMR measurements which have a shorter 'dead time' (0.5 ms) and added to the recorded surface NMR relaxation signals. Comparing with limestone, shale layers have higher fraction of water signal below 10ms. The modified sNMR signals then are used to conduct the inversion that accounts the multi-exponential decay of NMR relaxation. The water content estimated from the modified sNMR signals matches borehole NMR data better than which was originally estimated. Meanwhile, the inversion results elucidate the differences between shale and limestone layers when the sNMR signals are less than 10ms. Our results indicate that modifying initial sNMR signals based on lithology and borehole NMR data is practical and desirable in order to overcome the instrumental limitation of long 'dead time', therefore to improve water content estimation which is helpful for the application of surface NMR in complex geologic settings.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H21H1808Z
- Keywords:
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- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1835 Hydrogeophysics;
- HYDROLOGY;
- 1865 Soils;
- HYDROLOGY;
- 1899 General or miscellaneous;
- HYDROLOGY