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 - 10.48550/arXiv.2005.03960

DO - 10.48550/arXiv.2005.03960

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 -