Details
| Original language | English |
|---|---|
| Pages (from-to) | 815-826 |
| Number of pages | 12 |
| Journal | Geophysical journal international |
| Volume | 222 |
| Issue number | 2 |
| Publication status | Published - 13 May 2020 |
| Externally published | Yes |
Abstract
Surface nuclear magnetic resonance (SNMR) is a well-established technique for the hydrogeological characterization of the subsurface up to depths of about 150 m. Recently, SNMR has been adapted to investigate also the shallow unsaturated zone with small surface loop setups. Due to the decreased volume, a pre-polarization (PP) field prior to the classical spin excitation is applied to enhance the measured response signal. Depending on the strength and orientation of the applied PP-field, the enhancement can often reach several orders of magnitude in the vicinity of the PP-loop. The theoretically achievable enhancement depends on the assumption of an adiabatic, that is perfect, switch-off of the corresponding PP-field. To study the effect of imperfect switch-off, we incorporate full spin dynamics simulations into the SNMR forward modelling. The affected subsurface volume strongly depends on the chosen PP switch-off ramp and the geometry of the loop setup. Due to the imperfect switch-off, the resulting SNMR sounding curves can have significantly decreased signal amplitudes. For comparison, the signal amplitudes of either a 1 ms exponential or linear switch-off ramp are reduced by 17 and 65 per cent, respectively. Disregarding this effect would therefore yield an underestimation of the corresponding subsurface water content of similar magnitude.
Keywords
- Electromagnetic theory, Hydrogeophysics, Numerical modelling
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geophysics
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Geophysical journal international, Vol. 222, No. 2, 13.05.2020, p. 815-826.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Utilizing pre-polarization to enhance SNMR signals - Effect of imperfect switch-off
AU - Hiller, Thomas
AU - Dlugosch, Raphael
AU - Müller-Petke, Mike
N1 - Funding information: We would like to thank two anonymous reviewers for their comments and suggestions. Furthermore, we would like to thank Stephan Costabel and Eiichi Fukushima for fruitful discussions and constructive comments on the application of SNMR-PP. This work was supported by the German Research Foundation under the grant MU 3318/4-1.
PY - 2020/5/13
Y1 - 2020/5/13
N2 - Surface nuclear magnetic resonance (SNMR) is a well-established technique for the hydrogeological characterization of the subsurface up to depths of about 150 m. Recently, SNMR has been adapted to investigate also the shallow unsaturated zone with small surface loop setups. Due to the decreased volume, a pre-polarization (PP) field prior to the classical spin excitation is applied to enhance the measured response signal. Depending on the strength and orientation of the applied PP-field, the enhancement can often reach several orders of magnitude in the vicinity of the PP-loop. The theoretically achievable enhancement depends on the assumption of an adiabatic, that is perfect, switch-off of the corresponding PP-field. To study the effect of imperfect switch-off, we incorporate full spin dynamics simulations into the SNMR forward modelling. The affected subsurface volume strongly depends on the chosen PP switch-off ramp and the geometry of the loop setup. Due to the imperfect switch-off, the resulting SNMR sounding curves can have significantly decreased signal amplitudes. For comparison, the signal amplitudes of either a 1 ms exponential or linear switch-off ramp are reduced by 17 and 65 per cent, respectively. Disregarding this effect would therefore yield an underestimation of the corresponding subsurface water content of similar magnitude.
AB - Surface nuclear magnetic resonance (SNMR) is a well-established technique for the hydrogeological characterization of the subsurface up to depths of about 150 m. Recently, SNMR has been adapted to investigate also the shallow unsaturated zone with small surface loop setups. Due to the decreased volume, a pre-polarization (PP) field prior to the classical spin excitation is applied to enhance the measured response signal. Depending on the strength and orientation of the applied PP-field, the enhancement can often reach several orders of magnitude in the vicinity of the PP-loop. The theoretically achievable enhancement depends on the assumption of an adiabatic, that is perfect, switch-off of the corresponding PP-field. To study the effect of imperfect switch-off, we incorporate full spin dynamics simulations into the SNMR forward modelling. The affected subsurface volume strongly depends on the chosen PP switch-off ramp and the geometry of the loop setup. Due to the imperfect switch-off, the resulting SNMR sounding curves can have significantly decreased signal amplitudes. For comparison, the signal amplitudes of either a 1 ms exponential or linear switch-off ramp are reduced by 17 and 65 per cent, respectively. Disregarding this effect would therefore yield an underestimation of the corresponding subsurface water content of similar magnitude.
KW - Electromagnetic theory
KW - Hydrogeophysics
KW - Numerical modelling
UR - http://www.scopus.com/inward/record.url?scp=85086073257&partnerID=8YFLogxK
U2 - 10.1093/gji/ggaa216
DO - 10.1093/gji/ggaa216
M3 - Article
AN - SCOPUS:85086073257
VL - 222
SP - 815
EP - 826
JO - Geophysical journal international
JF - Geophysical journal international
SN - 0956-540X
IS - 2
ER -