Practical limitations and applications of short dead time surface NMR

Abstract

There is increasing interest in the unique measurement capabilities of nuclear magnetic resonance (NMR) for hydrologic applications. In particular, the ability to quantify water content (both bound and free) and to infer the permeability distribution are critical to hydrologists. As the method has gained in acceptance, there has been growing interest in extending its range to near‐surface and vadose zone applications and to measurement in finer grained and magnetic soils. All of these applications require improved resolution of early‐time signals, which requires shorter measurement dead times. This paper analyses three physical/electrical processes that limit the minimum achievable measurement dead times in surface NMR applications: 1) inherent characteristics of electromechanical and semiconductor switching devices, 2) the effective bandwidth of the receiver and signal processing chain, 3) transient signals associated with induced eddy currents in the ground. We then describe two applications of surface NMR that rely on reduced measurement dead time: detection and characterization of fast decaying NMR signals in silt and clay and the detection of fast decaying NMR signals in magnetic geology.

Nonexponential decay of the surface-NMR signal and implications for water content estimation

“…When the dead-time is reduced from 30 to 10 ms, water content errors are significantly reduced but are still large when magnetic susceptibility is high. Recent advancements in SNMR hardware have achieved instrument dead-times as short as 4 ms (Walsh et al, 2011). The influence of decreasing the deadtime can be viewed as a downward shift of the previously described diagonal trajectory across Figure 3, leading to a reduction in water content estimation errors.…”

Torus-nuclear magnetic resonance: Quasicontinuous airborne magnetic resonance profiling by using a helium-filled balloon

“…Moreover, the ongoing technical and methodological improvement of the surface NMR method opens new and expands existing application fields. In this regard, prominent milestones within the past decade were, for instance, the spatial separation of transmitter and receiver loops for improved resolution of 2D (Hertrich et al, 2005(Hertrich et al, , 2009Dlugosch et al, 2014) and 3D (Jiang et al, 2015b) applications, the introduction of multichannel instrumentation for remote reference noise cancellation (Walsh, 2008;, the decrease of instrumental dead time for investigations in silty environment and in the unsaturated zone (Walsh et al, 2011, the application of advanced pulse sequences for improved estimation of hydraulic conductivity Grunewald and Walsh, 2013;Müller-Petke et al, 2013) and for investigation in environments with significant magnetic heterogeneity (Legchenko et al, 2010; Manuscript received by the Editor 4 September 2015; revised manuscript received 15 April 2016; published online 10 June 2016. 1 2014), and the development of joint inversion strategies with resistivity methods for investigations of saltwater intrusion (Behroozmand et al, 2012;Günther and Müller-Petke, 2012).…”

Processing of surface-nuclear magnetic resonance data from sites with high noise levels

“…Complex rotated amplitude data were used for inversion . The first data point represents the measurement effective dead time consisting of half of the transmitter pulse (to consider relaxation during the pulse; Walbrecker et al, 2009), the instrument dead time, and the processing dead time Walsh et al, 2011). The inversion started from a homogeneous half-space model for all parameters, and we used the stretch-exponential relaxation-time distribution model within the layers (Behroozmand et al, 2012b).…”

“…The method can provide depth-resolved estimates of the water content and the hydrologic properties of the geologic layers (Hertrich, 2008;Legchenko, 2013;Behroozmand et al, 2015a). Despite the vast hardware and software developments (Trushkin et al, 1994;Hertrich et al, 2005;Radic, 2006;Walsh, 2008;Jiang et al, 2011;Walsh et al, 2011;Dalgaard et al, 2012;Costabel and Müller-Petke, 2014;Larsen et al, 2014;Müller-Petke and Costabel, 2014), the applicability of the method is, however, often limited by the signal-tonoise ratio (S/N), which is often very poor due to electromagnetic noise (Legchenko, 2007;Chalikakis et al, 2008). A one-size-fits-all noise reduction strategy has yet not been obtained, and the detrimental effect of electromagnetic noise is a hurdle that must be overcome before widespread use of surface NMR becomes feasible.…”