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

Müller‐Petke, Mike; Walbrecker, Jan O.; Knight, Rosemary

doi:10.1190/geo2013-0035.1

Cited by 23 publications

(10 citation statements)

References 28 publications

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“…The method can be used for conventional relaxation time measurements that are available for in situ experimental techniques, such as for surface NMR and shallow groundwater NMR logging. Recent developments have shown that reliable relaxation-time measurements of T 1 or T 2 are feasible for surface NMR measurements (Grunewald and Walsh, 2013;Müller-Petke et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

Nuclear magnetic resonance average pore-size estimations outside the fast-diffusion regime

et al. 2015

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The pore-size distribution (PSD) of geologic materials is an important rock parameter to understand the flow of water in the subsurface. PSDs can be obtained from sieving analyses, mercury porosimetry measurements, and imaging techniques, but none of these methods is available for in situ measurements. Nuclear magnetic resonance (NMR) measurements are controlled by rock parameters such as the surface-area to porevolume ratio. NMR is available for in situ measurements. State-of-the-art NMR relaxation time measurements need a calibration of the surface relaxivity ρ to extract pore-size information. State-of-the-art NMR diffusion measurements avoid the calibration of ρ but are limited to small pores. We developed an approach that estimates the average pore size without calibrating ρ by means of incorporating higher order modes into the signal interpretation of NMR relaxation times. We conducted forward-modeling studies using an analytic solution for cylindrical tubes, 2D finite-element simulations to incorporate fractal pore spaces, and laboratory experiments on synthetic and natural samples. Our experimental data indicated that relaxation can occur outside the fast-diffusion regime not only for coarse-grained materials, but also for fine-to medium-grained unconsolidated sandy materials due to high surface relaxivities. We found that the rock-fluid interface's roughness had a significant impact on the diffusion regime and led to an apparent increase in ρ, which may cause intermediate or slow diffusion. The methodology was limited to materials with a narrow PSD and uniform distribution of ρ because we assumed multiexponential decay due to diffusion in single isolated pores.

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Section: Introductionmentioning

confidence: 99%

Nuclear magnetic resonance average pore-size estimations outside the fast-diffusion regime

et al. 2015

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“…Integration of the MRDP scheme into the surface NMR workflow will likely require a protocol similar to surface NMR T 1 inversions (Müller-Petke et al, 2013), where the current estimate of T 1 distribution is used to form an updated kernel and improve the results iteratively. In this case, the current T Ã 2 estimates would be used to improve the ability of the forward model to more robustly reproduce the initial amplitudes estimates given the correct water.…”

Section: Discussionmentioning

confidence: 99%

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

2017

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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.

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“…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).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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The application of surface nuclear magnetic resonance (NMR) measurements, although proven to be valuable for various hydrogeological issues, is in practice often limited to single 1D surveys because measurement progress is slow. First, large stacking rates are necessary to overcome the low signal-to-noise ratio and, second, time and effort are required to move the equipment and to place the measurement loops in the field. We have evaluated a novel approach to cope with the latter. We have assessed and tested a data acquisition scheme based on a torus-shaped helium-filled balloon carrying the loop and moving it rapidly from one to the next measurement position along the profile lines. We have referred to this system as Torus-NMR. We have showed the feasibility of the system to deliver reliable results using common processing and inversion. Surface NMR field data are acquired at successively overlapping positions along a test profile using the Torus-NMR system. To take advantage of the faster loop setup, the overall measurement time of the single NMR soundings is reduced by reducing the number of stacks, whereas the number of soundings is increased to obtain highly overlapping coverage. Regarding a single measurement position, a significant loss of vertical resolution must be accepted that can, however, be compensated to some extent by inverting the complete profile data set simultaneously using a laterally constrained inversion. We have found that for simple subsurface models, e.g., a layered earth with up to three layers, the proposed scheme provided reasonable results, which were demonstrated using synthetic and real field data. We do not expect the Torus-NMR concept to replace 2D data acquisition schemes if high spatial resolution is required. However, we expect fields of application that aim at rapid lateral overview of how specific hydrogeological parameters of shallow aquifers are distributed over large catchment-scale areas.

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

Cited by 23 publications

References 28 publications

Nuclear magnetic resonance average pore-size estimations outside the fast-diffusion regime

Nuclear magnetic resonance average pore-size estimations outside the fast-diffusion regime

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

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

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