Estimating the longitudinal relaxation time T₁ in surface NMR

Abstract

Surface nuclear magnetic resonance (NMR) is a noninvasive geophysical method that is primarily used in hydrological investigations of shallow aquifers. An important parameter in surface-NMR experiments is the relaxation time T1. Information on pore structure and even hydraulic permeability/conductivity may be inferred from accurate estimates of this parameter. Estimates of T1 are usually obtained by evaluating the spin response of groundwater molecules to excitation by two sequential electromagnetic pulses, the second of which is delayed and phase-shifted by π relative to the first. We have discovered that variations of the excitation field with distance from the transmitter and common imperfections in the transmitted pulses introduce considerable bias in estimates of T1 (e.g., errors as large as 50%). We assess the significance of these problems via numerical simulations based on the Bloch equation. As a result of this assessment, we propose a novel yet simple modification to the T1 acquisition method that resolves the identified problems. Our new scheme involves applying two types of double-pulse sequence, one in which the second pulse is phase-shifted by π relative to the first (i.e., the current procedure) and one in which the second pulse is in phase with the first. Subtracting the voltage responses measured after each of the two double-pulse experiments eliminates the bias, thus allowing T1 to be reliably estimated under general surface-NMR conditions.

Hydraulic properties at the North Sea island of Borkum derived from joint inversion of magnetic resonance and electrical resistivity soundings

“…These off-resonance effects can lead to significantly wrong parameters. Moreover, Walbrecker et al (2011b) showed that the inversion scheme for T 1 measurements is not generally valid. Since an appropriate measuring scheme was not available at the time of our study, we did not use T 1 , but T * 2 measurements.…”

The inversion of surface-NMR T1 data for improved aquifer characterization

“…In surface NMR, T 1 data are acquired using a scheme referred to as pcPSR that involves repeated double-pulse sequences (Walbrecker et al, 2011). The two on-resonance pulses P1 and P2, of duration τ p1 and τ p2 , are separated by the delay time τ (Figure 1).…”

“…The first published method to acquire surface-NMR T 1 data was initially termed "saturation recovery" (Legchenko et al, 2004). It was later renamed "pseudosaturation recovery" (PSR) (Hertrich, 2008), to stress the important differences to the saturation recovery scheme that is used in laboratory NMR (Walbrecker et al, 2011). It was found that the PSR method was prone to T Ã 2 components leaking into T 1 data, potentially leading to biased T 1 estimates.…”

“…It was found that the PSR method was prone to T Ã 2 components leaking into T 1 data, potentially leading to biased T 1 estimates. This issue has been addressed through the development of a modified acquisition scheme named phase-cycled pseudosaturation recovery (pcPSR) (Walbrecker et al, 2011). The importance of the pcPSR acquisition scheme was demonstrated in a region shown by core samples to be composed of a single lithologic unit.…”

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

“…The difficulty is that given only FID measurements it is extremely difficult to determine which mechanism controls T2*, and therefore, if T2* can be reliably used to estimate permeability. To improve surface NMR's links to pore geometry much work has focused on developing surface NMR experimental protocols capable of directly measuring alternative relaxation times, called T2 and T1, that are less/not influenced by dephasing (Legchenko et al, 2010;Walbrecker et al, 2011;Grunewald and Walsh, 2013;). Despite the successes of these techniques the FID remains the standard measurement due to its reduced collection times and increased penetration depths.…”