Adiabatic pulses enhance surface nuclear magnetic resonance measurement and survey speed for groundwater investigations

Abstract

Surface nuclear magnetic resonance (surface NMR) is an extremely powerful tool for groundwater resource investigations. However, the technique suffers from an inherently low signal-to-noise ratio (S/N), which commonly necessitates extensive signal averaging, resulting in very long measurement times. Previous approaches to improve S/N and measurement efficiency have focused primarily on reducing noise, through hardware and processing advancements. We introduce a new and divergent approach to actually increase the signal amplitude by modifying the form of the transmitted pulse used to excite the groundwater signals. An on-resonance pulse, the only form of excitation pulse previously used in surface NMR, has a fixed frequency and induces coherent excitation over a narrow range of transmit field strengths. Given spatially inhomogeneous fields underlying the surface coil, an on-resonance pulse excites water, a limited volume of water, producing a similarly limited signal amplitude. An adiabatic pulse, one of many pulse forms used for medical imaging and chemical spectroscopy, modulates pulse frequency and provides excitation over a much larger range of transmit field amplitudes. Numerical simulations of surface NMR with adiabatic pulses demonstrate almost a factor of three improvement in the peak signal amplitude compared to an on-resonance pulse. Simulations also show that a single measurement using an adiabatic pulse with high transmit current provides sensitivity to water over a wide range of depth. In contrast, multiple on-resonance measurements using a range of transmit currents are required to span sensitivity over a similar range of depths. Numerical simulation results are validated by the first field experiments comparing on-resonance and adiabatic pulses. We have considered how improvements in S/N can be used for dramatically improved measurement speed and how other advantages of adiabatic pulses may more generally be used to enhance surface NMR measurements.

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

“…Summarizing the above discussion, we conclude that the Torus-NMR should be used as a fast interpolating tool between boreholes and conventional surface NMR measurements. In this respect, a very promising option is the future combination of Torus-NMR with adiabatic excitation, a novel technique in surface NMR for improving resolution properties and S/N, especially at shallow depths (Grunewald et al, 2016).…”

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

“…The m y -component following the chirp pulse (bottom row) is described by an initial peak at a small B 1 followed by strong oscillations at a large B 1 . The oscillations are not a result of modeling instability, but they rather are a consequence of the adiabatic condition not being well-satisfied at a large B 1 given the initial 100 Hz offset of the investigated chirp pulse (Grunewald et al, 2016). Larger initial offsets will reduce the magnitude of the oscillation and can help extend the right side of the main peak in the m x profile to larger B 1 .…”

“…However, for many types of pulses, this is not the case. For example, adiabatic pulses, which are the subject of recent surface NMR research (Grunewald et al, 2016), vary the transmit frequency throughout the pulse resulting in a dynamic B eff orientation. As a result, the trajectory of the magnetization is very different than the trajectory during an on-resonance pulse, potentially leading to different sensitivities to RDP.…”

“…This is done to approximate surface NMR transmit conditions, in which the transmit coils are tuned and will result in similar B 1 modulation during the adiabatic pulse. This particular chirp pulse is chosen for its simplicity and its similarity to the adiabatic pulse previously demonstrated in a surface NMR field test by Grunewald et al (2016). This pulse is not optimized for surface NMR conditions; it merely functions to contrast the behavior of an adiabatic pulse against the standard on-resonance case.…”

“…Reliable results in the presence of fast relaxation times are essential for surface NMR investigations of vadose zone processes Yaramanci, 2011a, 2011b), magnetic settings (Roy et al, 2008;Grunewald and Knight, 2011), and fine sands/clayey environments (Sen et al, 1990;Schirov et al, 1991). In addition, growing interest in the use of alternative transmit schemes in surface NMR, such as composite pulses (Grombacher et al, 2014) or adiabatic pulses (Grunewald et al, 2016), requiring longer pulse durations also necessitates the development of a scheme to handle RDP for a broader range of relaxation times that is effective for an arbitrary excitation pulse type.…”