SNMR with short pulses: Optimizing the kernel calculation by considering the influence of pulse shape and phase

Abstract

In the past few years, efforts have been made to extend the sensitivity of surface nuclear magnetic resonance (SNMR) to short relaxation times, typical for strongly bound water, which, for example, occurs in partially saturated soils. The two limiting factors for the sensitivity are the dead time after the excitation pulse and the duration of the pulse itself. To enable short pulses, while also achieving proper depths of investigation, high pulse amplitudes are needed. This makes it necessary to consider the Bloch-Siegert effect, i.e. the counter-rotating component and the parallel component of the excitation field have significant influence on the excitation. If an untuned transmitter circuit is used, the pulse shape will also be non-sinusoidal. In this paper, we demonstrate that this influences SNMR measurements with short pulses in two ways: On one hand, the pulse shape influences the phase of the fundamental frequency oscillation. On the other, at very high pulse amplitudes, other frequency components of the excitation field start to influence the excitation. The behavior of the macroscopic magnetizations in the subsurface during the pulse is simulated by solving the Bloch equations, using the pulse shape as an input. Since these calculations are computational expensive, we propose a lookup scheme that allows a time efficient modeling of the obtained SNMR data.