Accounting for relaxation during pulse effects for long pulses and fast relaxation times in surface nuclear magnetic resonance

Abstract

2017). "Accounting for relaxation during pulse effects for long pulses and fast relaxation times in surface nuclear magnetic resonance." GEOPHYSICS, 82(6), JM23-JM36. https://doi. ABSTRACTSurface nuclear magnetic resonance (NMR) is a geophysical technique providing noninvasive insight into aquifer properties. To ensure that reliable water content estimates are produced, accurate modeling of the excitation process is necessary. This requires that relaxation during pulse (RDP) effects be accounted for because they may lead to biased water content estimates if neglected. In surface NMR, RDP is not directly included into the excitation modeling, rather it is accounted for by adjusting the time at which the initial amplitude of the signal is calculated. Previous work has demonstrated that estimating the initial amplitude of the signal as the value obtained by extrapolating the observed signal to the middle of the pulse can greatly improve performance for the on-resonance pulse. To better understand the reliability of these types of approaches (which do not directly include RDP in the modeling), the performance of these approaches is tested using numerical simulations for a broad range of conditions, including for multiple excitation pulse types. Hardware advances that now allow the routine measurement of much faster relaxation times (where these types of approaches may lead to poor water content estimates) and a recent desire to use alternative transmit schemes demand a flexible protocol to account for RDP effects in the presence of fast relaxation times for arbitrary excitation pulses. To facilitate such a protocol, an approach involving direct modeling of RDP effects using estimates of the subsurface relaxation times is presented to provide more robust and accurate water content estimates under conditions representative of surface NMR.

Gaining insight into the T2*-T2 relationship through complex inversion of surface NMR free-induction decay data

“…Traditionally, RDP is not accounted for by modifying the forward model but instead accounted for by adjusting the time at which the initial amplitude of the signal is calculated (Walbrecker et al, 2009). In this work we employ an alternative scheme that instead attempts to account for RDP by solving the Bloch equations with appropriately weighted relaxation terms present (Grombacher et al, 2017). One complication of this updated scheme is that it requires the relaxation times T2 and T1 to be estimated.…”

“…However, we hypothesize that the flexibility to modify the forward model to reflect different T2*-T2 relationships may offer the potential to gain insight the true value of T2. This hypothesis stems from a desire to exploit that RDP effects manifest differently depending on the true T2*-T2 scenario (Grombacher et al, 2017). Figure 1 highlights the sensitivity of the signal amplitude and phase to the T2*-T2 scenario.…”

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

“…An implicit assumption with the free induction decay measurement is that relaxation begins only after the transmitting pulse has ended, but with the longer pulses (20–40 ms) used in surface NMR measurements, relaxation occurs during the excitation pulse. If unaccounted for, the relaxation during the pulse can cause underestimation of the true water content (Walbrecker et al, 2009; Grombacher et al, 2017). We accounted for relaxation during the pulse by providing an additional 10 ms (half of the 20‐ms pulse length) prior to the shutdown pulse, following the methods of Walbrecker et al (2009).…”

Estimating T2 from surface NMR FID data using a forward model based on the full-Bloch equation

“…To extend the range of conditions where this simplification remains accurate the extrapolation to mid-pulse approach can be used, which can improve water content estimates in the presence of relaxation during pulse (RDP) effects (Walbrecker et al 2009). However, even the extrapolation to mid-pulse approach breaks down at fast relaxation times and can lead to challenges describing alternative pulse types, such as adiabatic half-passage pulses (Grombacher et al 2017a).…”