Jan O. Walbrecker

Accounting for relaxation processes during the pulse in surface NMR data

Under favorable conditions, the surface nuclear-magnetic-resonance (NMR) technique can provide direct quantitative estimates of subsurface water variations and indirect information on pore size and hydraulic conductivity. The technique is based on various exponential relaxation processes that become measurable after the intrinsic spin magnetic moments of groundwater protons have been rotated out of equilibrium by a pulse of alternating electromagnetic (EM) field generated at the surface. An implicit assumption in previous surface NMR studies is that relaxation processes need to be considered only after the EM pulse has been extinguished. Although this approximation is valid for short EM pulses, neglecting relaxation during the pulse (RDP) can result in significant errors for the generally long pulses used in surface NMR investigations. Because the influence of RDP cannot be isolated and quantified using field-scale approaches, this study is based on sample-scale NMR experiments and numerical simulations (Bloch equations) that mimic field practices and conditions. The results demonstrate that standard surface NMR methods that ignore the effects of RDP may yield significantly erroneous estimates of water volume (RDP-related errors of 25% are possible) and the key relaxation parameter (RDP-related errors of 50% are possible) that supplies information on pore size and hydraulic conductivity. Fortunately, the study also demonstrates that relatively simple interpretational approaches can reduce the RDP-related errors to less than 2%.

Off-resonance effects in surface nuclear magnetic resonance

Surface nuclear magnetic resonance (NMR) is a noninvasive geophysical tool used to investigate groundwater reservoirs. The relevant physical process in surface NMR is the nuclear spin of hydrogen protons in liquid water. Standard single-pulse surface NMR experiments provide estimates of water content in the shallow subsurface. Under favorable conditions, pore-structure and even hydraulic-conductivity information can be extracted from double-pulse surface NMR data. One crucial issue in surface NMR experiments is the resonance condition: the frequency of the excitation field should closely match the Larmor frequency of the protons, which is controlled by the local magnitude of the earth’s magnetic field. Although the earth’s field can be measured accurately by an on-site magnetometer, several effects impede perfect matching of the frequencies. These include temporal variations of the earth’s field, instrumental imperfections, and the magnetic susceptibility of the underlying rocks. We assess the impact of v...

Estimating the longitudinal relaxation time T1 in surface NMR

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 th...

Direct measurement of internal magnetic fields in natural sands using scanning SQUID microscopy

NMR experiments are ideally carried out in well-controlled magnetic fields. When samples of natural porous materials are studied, the situation can be complicated if the sample itself contains magnetic components, giving rise to internal magnetic fields in the pore space that modulate the externally applied fields. If not properly accounted for, the internal fields can lead to misinterpretation of relaxation, diffusion, or imaging data. To predict the potential effect of internal fields, and develop effective mitigation strategies, it is important to develop a quantitative understanding of the magnitude and distribution of internal fields occurring in natural porous media. To develop such understanding, we employ scanning SQUID microscopy, a technique that can detect magnetic field variations very accurately at high spatial resolution (∼3μm). We prepared samples from natural unconsolidated aquifer material, and scanned areas of about 200×200μm in a very low background magnetic field of ∼2μT. We found large amplitude variations with a magnitude of about 2mT, across a relatively long spatial scale of about 200μm, that are associated with a large magnetic grain (>50μm radius) with a strong magnetic remanence. We also detected substantial variations exceeding 60μT on small spatial scales of about ∼10μm. We attribute these small-scale variations to very fine-grained magnetic material. Because we made our measurements at very low background field, the observed variations are not induced by the background field but due to magnetic remanence. Consequently, the observed internal fields will affect even low-field NMR experiments.

NMR Image Reconstruction in Nonlinearly Varying Magnetic Fields: A Numerical Algorithm

We propose a numerical algorithm for nuclear magnetic resonance (NMR) imaging in low magnetic fields that vary spatially in a nonlinear way. Since we operate at low frequencies ≤3 MHz, moderate electrically conductive materials in large sample volumes can be accessed. Frequency swept spin-echo pulse sequences are employed to control the excitation bandwidth. The resolution and sensitivity of this nonconventional imaging approach is studied. To localize the origin of the NMR signal in the sample volume we do not rely on linear magnetic field gradients as in modern high-field imaging devices, but utilize the generic spatial inhomogeneity of RF and DC fields generated by relatively large surface and volume coils. A numerical forward and inverse modeling example related to applications in the water and oil research is described. The algorithm may be applied to single-sided and borehole imaging systems in the future.

Imaging in electrically conductive porous media without frequency encoding.

Understanding multi-phase fluid flow and transport processes under various pressure, temperature, and salinity conditions is a key feature in many remote monitoring applications, such as long-term storage of carbon dioxide (CO(2)) or nuclear waste in geological formations. We propose a low-field NMR tomographic method to non-invasively image the water-content distribution in electrically conductive formations in relatively large-scale experiments (∼1 m(3) sample volumes). Operating in the weak magnetic field of Earth entails low Larmor frequencies at which electromagnetic fields can penetrate electrically conductive material. The low signal strengths associated with NMR in Earths field are enhanced by pre-polarization before signal recording. To localize the origin of the NMR signal in the sample region we do not employ magnetic field gradients, as is done in conventional NMR imaging, because they can be difficult to control in the large sample volumes that we are concerned with, and may be biased by magnetic materials in the sample. Instead, we utilize the spatially dependent inhomogeneity of fields generated by surface coils that are installed around the sample volume. This relatively simple setup makes the instrument inexpensive and mobile (it can be potentially installed in remote locations outside of a laboratory), while allowing spatial resolution of the order of 10 cm. We demonstrate the general feasibility of our approach in a simulated CO(2) injection experiment, where we locate and quantify the drop in water content following gas injection into a water-saturated cylindrical sample of 0.45 m radius and 0.9 m height. Our setup comprises four surface coils and an array consisting of three volume coils surrounding the sample. The proposed tomographic NMR methodology provides a more direct estimate of fluid content and properties than can be achieved with acoustic or electromagnetic methods alone. Therefore, we expect that our proposed method is relevant for geophysical applications, such as for monitoring CO(2) injections in saline aquifers or detecting water leakage into nuclear waste deposit sites installed in electrically conductive formations.