  Surface nuclear magnetic resonance (SNMR) is a well-established technique for
the hydrogeological characterization of the subsurface up to depths of about
150m. 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, i.e. 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
modeling. 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 1ms
exponential or linear switch-off ramp are reduced by 17% and 65%, respectively.
Disregarding this effect would therefore yield an underestimation of the
corresponding subsurface water content of similar magnitude.
