Borehole Nuclear Magnetic Resonance Research Articles

Core Analysis Using Nuclear Magnetic Resonance

Core analysis is critical for oil and gas exploration and recovery. Nuclear magnetic resonance (NMR) has been widely applied to the petroleum industry for many years, and NMR core analysis plays an important role in supporting borehole NMR formation evaluation and understanding the mechanisms of petrophysics and petrochemistry. It’s unfortunate, however, that there has been a lack of commented descriptions of the basic procedures for NMR rock core analysis in the literature, which has led to issues of comparability and repeatability of NMR measurements between different laboratories. In this paper, we present a brief guideline for NMR rock core analysis of conventional T2, T1, T1-T2, and D-T2 measurements. As an established procedure in daily laboratory work, NMR core analysis includes three sophisticated steps: rock sample preparation, an NMR experiment, and data analysis. The acquisition and saturation of rock samples are described, and corresponding solutions are also considered. Detailed NMR experimental measurements, data setting, and calibration are addressed. Laplace inversion for different NMR experiments and models for estimation of porosity, irreducible water, permeability, pore structure, and oil saturation are included. The aim of this paper is to try to standardize general NMR experiments for rock core analysis.

Borehole Nuclear Magnetic Resonance Estimation of Specific Yield in a Fractured Granite Aquifer

AbstractIn this study, we introduce a novel field‐based method to estimate specific yield (Sy) in fractured, low‐porosity granite aquifers using borehole nuclear magnetic resonance (bNMR). This method requires collecting a bNMR survey immediately following a pump test, which dewaters the near‐borehole fractures. The residual water content measured from bNMR is interpreted as “bound” and represents the specific retention (Sr) while the water drained by the pump is the Sy. The transverse relaxation cutoff time (T2C) is the length of time that partitions the total porosity measured by bNMR into Sr and Sy. When applying a calibrated T2C, Sy equals the bNMR total porosity minus Sr; thus, a calibrated T2C is required to determine Sy directly from NMR results. Based on laboratory experiments on sandstone cores, the default T2C is 33 ms; however, its applicability to fractured granite aquifers is uncertain. The optimal T2C based on our pumping test is 110 ± 25 ms. Applying this calibrated T2C on a saturated, A‐type granite at our field site, we estimate the Sy to be 0.012 ± 0.005 m3 m−3 which is significantly different from the Sy (0.021 ± 0.005 m3 m−3) estimate using the default T2C of 33 ms. This Sy estimate falls within a range determined using traditional hydraulic testing at the same site. Using the conventional T2C (33 ms) for fractured granite leads to an inaccurate Sy; therefore, it is essential to calibrate the bNMR T2C for the local site conditions prior to estimating Sy.

Permeability evaluation based on Nuclear Magnetic Resonance analysis for gas hydrate reservoir

Permeability (or hydraulic conductivity) is one of the most important parameters for analyzing hydraulic behavior of underground. For example, in the geotechnical engineering, permeability is necessary to calculate water inflow and draw down of water level during excavation, to predict the deformation time due to the consolidation, and to evaluate the barrier of the ground for radioactive waste disposal. In the petroleum industry, permeability is considered an essential parameter which is directly linked to the productivity of oil and natural gas. Therefore, predicting permeability using wireline logging or logging while drilling (LWD) is required during the assessment of the reservoir. Nuclear Magnetic Resonance (NMR) logging is performed at this point. Low-field NMR measurements are sensitive to the hydrogen proton, 1H, in liquid water but not to the 1 H bond in solid form, making it possible to obtain the bulk water volume. In addition, the hydrogen proton NMR- T2 (transverse or spin-spin) relaxation time distribution is used to infer pore-size information and to estimate permeability [1, 2]. The major methods used for estimating permeability, k, are the Schlumberger Doll Research (SDR) model and Timur-Coates (TC) model [3, 4]:
 Where fNMR is the NMR-estimated (effective) porosity. SDR model uses the averaged value of the T2 distribution plotted on log-scale of time and matches it to the experimental reference permeability with the fitting parameter a [mD/(ms)2]. The TC model seeks to differentiate between the fraction of free (mobile) fluid volume (FFV) and bound fluid volume (BFV) and estimates permeability with FFV/BFV ratio and the material constant c [m2/s2]. The permeability in the research and development of methane hydrate, which is expected to be a next-generation resource, has been estimated using above methods. Methane hydrate is an ice-like solid crystalline structure of water and methane molecules. Because hydrate is solid, NMR is not sensitive, the initial (effective) permeability can be estimated by NMR analyzer. On the other hand, A strong disconnect (up to 2 orders of magnitude) between NMR-based estimates of permeability with those determined from a laboratory study of pressure cores from the same reservoir has further cast uncertainty in the application of borehole NMR data in hydrate evaluation [5, 6]. Recently, authors provided comparisons between NMR-derived values and direct fluid flow estimates of permeability in naturally occurring hydrate-bearing sediment [7, 8]. On the basis of the NMR measurement for hydrate-bearing pressure core sediments, which were recovered from the Gulf of Mexico and Alaska North Slope, the following new hydraulic radius model was proposed [7].
 This equation estimates the specific surface based on the NMR signal and has been shown to successfully estimate the initial permeability of a hydrate reservoir by expressing a hydraulic radius model based on Kozeny's equation [9]. However, the proposed equation is not universal and has been found to overestimate for clayey and silty hydrate free sediment. This study summarizes the methods and parameters used to estimate permeability using NMR signals from hydrate-bearing sediment obtained from logging and laboratory tests for the Gulf of Mexico and Alaska North Slope. In addition, we reports parameters for predicting model for muddy sediments which was recovered from overburden of hydrate-reservoir in the Hyuganada sea in 2022 Japan [10, 11]. When using the hydraulic radius model to predict permeability of hydrate-bearing sediment, reasonable estimates can be obtained by setting the shape factor for grains Cs = 1/2 (for a circular grain) and the shape factor for the flow path cn = 4 (for the cylinder network), respectively. Although, for hydrate free muddy sediment, Cs = 1/3 for parallel plate grain shape and cn = 2 for the fracture network model matched its permeability. Furthermore, the conversion parameter from T2 relaxation time to the pore size for hydrate free muddy sediments r2 = 0.009 (mm/s) was confirmed based on the result of mercury injection capillary pressure analysis, which is lower than r2 = 0.0325 (mm/s) of hydrate-bearing sediments.

The effects of temperature on shallow borehole NMR measurements in permafrost

High fidelity observations of the amount and state of water within permafrost help constrain the seasonal behavior of soil moisture and the effects of soil moisture on the surface energy balance. This work emphasizes the necessity for temperature-specific calibrations of low-frequency borehole NMR measurements. Constraining the effects of temperature on NMR signatures will allow for more reliable NMR inspection of hydrogeochemical parameters in permafrost ecosystems. We find that calibration at typical laboratory temperatures (20 °C) and subsequent measurement at typical permafrost active layer temperatures (∼0 °C) can result in an 18% bias in reported NMR water content values, and therefore temperature compensation is required under most scenarios. This is particularly important for active layer conditions that may include steep vertical temperature gradients. Similarly, seasonal time-lapse measurements of permafrost active layer may encounter substantial soil temperature variations which would also require temperature compensation on the observed NMR water content estimate.

Fast-forward modeling of borehole nuclear magnetic resonance measurements acquired in deviated wells and spatially heterogeneous formations

High-angle and horizontal (HAHz) wells are routinely drilled to enhance reservoir exposure and increase fluid productivity. The interpretation of nuclear magnetic resonance (NMR) measurements entails major technical challenges in horizontal layers penetrated by HAHz wells or through dipping formation layers penetrated by a vertical well. Three-dimensional geometric effects, coupled with spatially and petrophysically heterogeneous rocks, may bias petrophysical estimates obtained from borehole NMR measurements using methods designed for vertical wells and horizontal layers. Reliable interpretation requires an accurate and rapid forward-modeling method that integrates NMR physics and tool/instrument specifications, borehole, and 3D formation geometric properties. The latter is possible with the recent development of a forward-modeling algorithm that accurately and efficiently simulates NMR measurements based on 3D spatial sensitivity functions (SSFs). We implement and quantify this forward-modeling approximation across dipping formations penetrated by deviated wells in the presence of mud-filtrate invasion. We validate and verify the fast approximation against a 3D multiphysics model using challenging synthetic cases, in highly apparent dip wells with the varying radial front of the mud-filtrate invasion. Our work compares the effect of radial length of investigation from the three distinct NMR acquisition shells to better understand borehole NMR measurements acquired in 3D complex geometries. On average, the fast approximation via SSFs reproduces NMR measurements in 3 s of central processing unit time with maximum root-mean-square errors below 1% and can therefore be used for real-time calculations and interpretations. Modeling results indicate that thinly bedded formations and their petrophysical properties can be resolved with relatively low measurement resolution in HAHz wells and dipping formations.

Prospects of borehole NMR instruments and applications

Borehole nuclear magnetic resonance (NMR) is a powerful technology to characterize the petrophysical properties of underground reservoirs in the petroleum industry. The rising complexity of oil and gas exploration and development objectives, as well as the novel application contexts of underground reservoirs, have led to increasingly demanding requirements on borehole NMR technology including instrument design and related processing methods. This mini review summarizes the advances and applications of borehole NMR instruments along with some future possibilities. It may be helpful for researchers and engineers in the petroleum industry to understand the development status and future trends of borehole NMR technology.

Surface relaxivities and bulk relaxation times from nuclear magnetic resonance T1-T2 logging in unconventional reservoirs

In borehole nuclear magnetic resonance (NMR) logging of unconventional reservoirs, important fundamental parameters such as surface relaxation, bulk fluid relaxation, pore size, and wettability often are ignored because their controls are nontrivial. We developed a data-driven workflow to estimate surface relaxivities and bulk relaxation times from borehole wireline NMR [Formula: see text]-[Formula: see text] measurements for practical modeling of petrophysical properties in unconventional reservoirs. The workflow, in a series of analytical steps, initially implements petrophysical multimodal analysis of [Formula: see text]-[Formula: see text] maps to identify naturally clustered responses of key relaxation effects. Subsequently, it then jointly derives apparent bulk relaxation times ([Formula: see text]) and surface relaxivities, i.e., transverse ([Formula: see text]) and longitudinal ([Formula: see text]), per distinct petrophysically relevant pore and fluid types. The results, and associated uncertainties, in an unconventional field are within ranges of published experiments in the literature for siliciclastic clay- and organic-rich formations. This approach for in situ, in contrast to laboratory, derivations of surface relaxivities in unconventional reservoirs creates novel opportunities to estimate NMR-based pore-size distributions and permeability in the subsurface.

Evaluation of slim-hole NMR logging for hydrogeologic insights into dolostone and sandstone aquifers

This study assesses the performance and limitations of slim-hole borehole nuclear magnetic resonance (NMR) technology from a hydrogeologic perspective in fractured, porous rock. NMR logging was carried out in dolomitic and sandstone bedrock boreholes at two research test sites in Ontario, Canada, where aquifer and aquitard units provide a range of clay contents as well as a variety of primary and secondary porosity types (e.g. discrete fractures, reefal structures, vugs and karstic conduits). Results were compared to core measurements, geophysical logs, and hydrogeophysical testing. The vertical response curve of the instrument tested was found to produce 60% of the signal from within a 0.2m span surrounding the measuring point. The repeatability of the total porosity measurements in stationary mode is excellent where the porosity is greater than 0.15. Below that threshold, repeatability is scattered at ±0.05 porosity about the mean, with the variability primarily within the clay- and capillary-bound fractions. The NMR porosity estimates agreed with core measurements to within ±0.04 porosity in both the dolostone and sandstone, but the correlation deteriorates in finely bedded lithologies, and where fracturing is present. Much of the discrepancy is attributed to scaling in a finely layered geologic sequence, as the core samples are much smaller than the entire volume measured with NMR probes. Data collection with the probe in motion (continuous logging) added variability to the response when compared to stationary recordings. Although broadscale trends were comparable, the details and depth-specific insights of the bound fluid fractions varied with logging rates. Overall, NMR provides a robust measurement of the bulk matrix porosity and pore size distribution of lithologies intersected, both of which are critically important parameters in understanding hydrogeologic conditions and contaminant distributions in layered sedimentary rock systems.

Quality Control of Nuclear Magnetic Resonance Measurements Using Pseudo-T2 Distributions

Summary Nuclear magnetic resonance (NMR) measurements are extremely valuable in the assessment of fluid-flow properties of rocks. However, inverted NMR transverse-relaxation time (T2) distributions are often biased by positivity constraints and the implementation of regularization (stabilization) methods. In some cases, it is difficult to determine whether the estimated pore-size distributions are reliable or byproducts of the inversion. On-site quality control of NMR inversion results is thus essential to avoid erroneous analyses based on nonphysical interpretations across conventional or unconventional rocks. It is also important to adjust the measurement acquisition process in real time in response to variable signal/noise conditions in the borehole environment. We introduce and compare three quality-control approaches; two of them are based on two different signal processing practices, namely the semilogarithmic derivative and the Hilbert transform, while the third is based on a nonlinear optimization technique. Validation of the inverted T2 distribution is performed by applying either method to the echo train decay of proton magnetization to estimate a pseudo-T2 distribution that is not affected by inversion artifacts. These three methods are data-driven processes for on-site, continuous quality control of borehole NMR measurements. In the first method, a pseudo-T2 distribution is constructed by taking the derivative of the echo train decay with respect to the natural logarithm of the relaxation time. For the second method, the Hilbert transform is applied to the raw proton magnetization decay to approximate the T2 distribution. The latter method consists of an approximation of the derivative of the time decay of magnetization using least-squares minimization. NMR laboratory measurements were performed on a wide variety of rocks, the associated T2 distributions were estimated using linear inversion, and the three quality-control methods were applied for comparison. The reliability of our quality-control procedures was verified by benchmarking them against the inverted T2 distributions. Furthermore, these quality-control methods, when applied continuously to borehole NMR measurements, enable the optimization of the measurements in real time to mitigate the effects of biasing noise. By verifying the location of the dominant mode of the T2 distribution during NMR measurement acquisition, the operator can determine whether (a) more stacking of proton magnetization decays is needed, or (b) longer- or shorter-time sampling is needed to attain a high-quality estimation of the pore-size distribution before performing the T2 inversion. Our work provides the basis of effective quality-control methods for NMR measurements for the petrophysical interpretation of rocks with complex pore-size distributions, especially in unconventional rocks, where noise present in the measurements (notably at early acquisition times), can mask the useful signal originating from pore-size distributions and fluids. The fast and reliable quality control of estimated T2 distributions is not affected by inversion artifacts, relying only on unfiltered, raw data.

Rapid modeling of borehole measurements of nuclear magnetic resonance via spatial sensitivity functions

Borehole measurements of nuclear magnetic resonance (NMR) are routinely used to estimate in situ rock and fluid properties. Conventional NMR interpretation methods often neglect bed-boundary and layer-thickness effects in the calculation of fluid volumetric concentrations and NMR relaxation-diffusion correlations. Such effects introduce notable spatial averaging of intrinsic rock and fluid properties across thinly bedded formations or in the vicinity of boundaries between layers exhibiting large property contrasts. Forward modeling and inversion methods can mitigate the aforementioned effects and improve the accuracy of true layer properties in the presence of mud-filtrate invasion and borehole environmental effects across spatially complex formations. We have developed a fast and accurate algorithm to simulate borehole NMR measurements using the concept of spatial sensitivity functions (SSFs) that honor NMR physics and incorporate tool, borehole, and formation geometry. Tool sensitivity maps are derived from a 3D multiphysics forward model that couples NMR tool properties, magnetization evolution, and electromagnetic propagation. In addition, a multifluid relaxation model based on Brownstein-Tarr’s equation is introduced to estimate layer NMR porosity decays and relaxation-diffusion correlations from pore-size-dependent rock and fluid properties. The latter model is convolved with the SSFs to reproduce borehole NMR measurements. The results indicate that NMR spatial sensitivity is controlled by porosity, electrical conductivity, excitation pulse duration, and tool geometry. We benchmark and verify the SSF-derived forward approximation against 3D multiphysics simulations for a series of synthetic cases with variable bed thickness and petrophysical properties, and in the presence of mud-filtrate invasion in a vertical well. Results indicate that the approximation can be executed in a few seconds in a central processing unit, by a factor of 1000 times faster than rigorous multiphysics calculations, with maximum root-mean-square errors of 1%.

The Biophysical Role of Water and Ice Within Permafrost Nearing Collapse: Insights From Novel Geophysical Observations

AbstractThe impact of permafrost thaw on hydrologic, thermal, and biotic processes remains uncertain, in part due to limitations in subsurface measurement capabilities. To better understand subsurface processes in thermokarst environments, we collocated geophysical and biogeochemical instruments along a thaw gradient between forested permafrost and collapse‐scar bogs at the Alaska Peatland Experiment site near Fairbanks, Alaska. Ambient seismic noise monitoring provided continuous high‐temporal resolution measurements of water and ice saturation changes. Maps of seismic velocity change identified areas of large summertime velocity reductions nearest the youngest bog, indicating potential thaw and expansion at the bog margin. These results corresponded well with complementary borehole nuclear magnetic resonance measurements of unfrozen water content with depth, which showed permafrost soils nearest the bog edges contained the largest amount of unfrozen water along the study transect, up to 25% by volume. In situ measurements of methane within permafrost soils revealed high concentrations at these bog‐edge locations, up to 30% soil gas. Supra‐permafrost talik zones were observed at the bog margins, indicating talik formation and perennial liquid water may drive lateral bog expansion and enhanced permafrost carbon losses preceding thaw. Comparison of seismic monitoring with wintertime surface carbon dioxide fluxes revealed differential responses depending on time and proximity to the bogs, capturing the controlling influence of subsurface water and ice on microbial activity and surficial emissions. This study demonstrates a multidisciplinary approach for gaining new understanding of how subsurface physical properties influence greenhouse gas production, emissions, and thermokarst development.

Fractal and multifractal analysis of soil particle-size distribution and correlation with soil hydrological properties in active layer of Qinghai–Tibet Plateau, China

The soil hydrological properties (HPs) in active layers regulate soil water migration and affect the evolution of permafrost and biogeochemical cycles. There is relatively little understanding of the changes in soil HPs as a result of soil particle-size distribution (PSD) in the permafrost regions of the Qinghai–Tibet Plateau. In this study, different land-cover types, namely alpine wet meadow (AWM), extremely degraded wet meadow (EDWM), alpine meadow (AM), degraded alpine meadow (DAM), alpine steppe (AS), and alpine desert (AD), were used to determine the relationships between soil PSD and HPs. Soil HPs were determined using borehole nuclear magnetic resonance, and soil PSD (0–2 mm) was analyzed using fractal and multifractal theory. The results showed that vegetation degradation caused coarse soil particle accumulation in the root zone, reflected by the soil single-fractal dimension (Dv: 2.12–2.50). The Dv values showed significant positive correlations with soil clay and silt content as well as a negative correlation with sand content. Results from the generalized fractal and singularity spectra suggested an even distribution of soil particles in the root zone of the AWM, AM, and AD sites. Vegetation degradation increased the soil PSD capacity dimension (D0), information dimension (D1), correlation dimension, D1/D0 ratio, and Hölder exponent of order zero. This was indicative of a narrow soil PSD range and non-uniform soil texture in the root zone. Although capillary water dominated the total soil water content in the alpine soils, soil water composition was also dependent on the vegetation type and soil stratigraphic distribution. Soil particle composition and multifractal dimensions were found to be effective parameters for forecasting soil HPs in the non-root zone of alpine soils. However, the presence of vegetation weakens the relationship between soil PSD and HPs. This study contributes to the accurate evaluation of hydrological and biogeochemical cycles in permafrost regions.

Borehole Nuclear Magnetic Resonance Study at the China University of Petroleum

The research of borehole nuclear magnetic resonance (NMR) began in the 1950 s, but the maturity and large-scale applications of relevant instruments started in the mid-1990. To date, borehole NMR is an important means for borehole in-situ analysis and oil and gas evaluation, which significantly improves the success rate of exploration and the evaluation accuracy of oil and gas reservoirs. Its development has also contributed importantly to low-field and industrial NMR theories and experimental methodologies. Companies and individuals in the United States, China and other countries have developed the capabilities to engineer and deploy borehole NMR instruments and measurements independently. NMR imaging and evaluation of heterogeneous reservoirs and unconventional oil and gas are worldwide problems, involving the innovation of borehole NMR and the advanced manufacture of instruments and equipment.The commercial technology of borehole oil and gas exploration is highly competitive and proprietary. It is difficult to gain full insight into the details of the technologies and development from published literatures. Based on the research of the author's NMR laboratory at the China University of Petroleum (CUP), this paper reviews the core technologies of borehole NMR and its applications, discusses selected important issues that have not been fully solved, and looks forward to the direction and prospects of future development.

Quantifying Dynamic Water Storage in Unsaturated Bedrock with Borehole Nuclear Magnetic Resonance

AbstractQuantifying the volume of water that is stored in the subsurface is critical to studies of water availability to ecosystems, slope stability, and water‐rock interactions. In a variety of settings, water is stored in fractured and weathered bedrock as rock moisture. However, few techniques are available to measure rock moisture in unsaturated rock, making direct estimates of water storage dynamics difficult to obtain. Here, we use borehole nuclear magnetic resonance (NMR) at two sites in seasonally dry California to quantify dynamic rock moisture storage. We show strong agreement between NMR estimates of dynamic storage and estimates derived from neutron logging and mass balance techniques. The depths of dynamic storage are up to 9 m and likely reflect the depth extent of root water uptake. To our knowledge, these data are the first to quantify the volume and depths of dynamic water storage in the bedrock vadose zone via borehole NMR.

Toward a new conceptual model for groundwater flow in merokarst systems: Insights from multiple geophysical approaches

AbstractMerokarst aquifers — relatively thin (<1–2 m) karstified carbonate units interbedded between mudstone, shale, or sandstone — constitute a significant proportion of carbonate terrain and underlie a large portion of the west‐ and south‐central USA, yet few advances have been made in our understanding of porosity development and flow‐path generation in these complex systems in decades. Toward this end, we used a multi‐geophysical approach at the well‐studied Konza Prairie Biological Station (KPBS), a part of the larger Flint Hills (25,734 km2), underlain by thin limestone units (1–2 m thick) interbedded with mudstone/shale units (2–4 m thick), to elucidate hydrologic connectivity and potential controls on known groundwater flow directions. We combined electrical resistivity tomography (ERT), surface and borehole nuclear magnetic resonance (NMR), and ground penetrating radar (GPR) measurements across a low order catchment where over 25 boreholes and groundwater wells sampling perched aquifers could be used to constrain interpretation of lithology, potential flow paths, and permeability. Data revealed that groundwater export may be an unappreciated component of lateral‐flow‐dominated models used to represent merokarst in that: (a) potentiometric surfaces from two limestone units showed groundwater flows toward a hydrologic depression, opposite the direction of stream flow, in the upstream portion of the catchment, (b) long term measures of groundwater levels revealed a greater variance and overall water storage in this same upstream area compared to wells near the outlet, and (c) ERT and NMR results indicate pronounced lateral heterogeneity within a given unit, suggestive of a greater degree of vertical hydrological connectivity than usually considered for horizontally‐layered merokarst. Our data suggest vertical connectivity can shunt water to depth in these “sandwiched” merokarst aquifers, yielding atypical groundwater flow directions and unrealized deep export of weathering solutes and carbon.

Design of a new LWD NMR tool with high mechanical reliability

Nuclear magnetic resonance (NMR) provides useful information for formation evaluation on pore size and pore volume. Depending on field operations, there are two types of borehole NMR: wireline logging and logging-while-drilling (LWD). The latter type is more convenient. But due to the high mechanical failure risk caused by insufficient mechanical strength of the tool and vibrations during measurement, the quality of LWD NMR measurements are severely affected. In this paper, to enable high reliability and vibration tolerance, we propose a new design and implement a new LWD NMR tool, whose features includes a double hollow cylindric magnet (DHCM) structure and a solenoid-optimized antenna. The DHCM structure greatly reduces the proportion of the magnet in the cross-section of the tool without reducing B0 field strength. Thus, the mechanical strength and the reliability of the tool is greatly improved. The antenna is improved by optimizing the parameters of each coil of the solenoid for finding the generated B1 distribution best matching B0. The new design has a saddle sensing area in the axial and radial plane with a width greater than 50 mm, which is wide enough to ensure the validity of the T2 measurement under strong vibrations. We had demonstrated that this new tool performs well when TE = 0.6 ms, ensuring the measurement richness is suitable for LWD prospecting for unconventional oil and gas reservoirs. Besides, this design is suitable for that in slim holes.

Uniform and lateral preferential flows under flood irrigation at field scale

AbstractFlood irrigation is globally one of the most used irrigation methods. Typically, not all water that is applied during flood irrigation is consumed by plants or lost to evaporation. Return flow, the portion of applied water from flood irrigation that returns back to streams either via surface or subsurface flow, can constitute a large part of the water balance. Few studies have addressed the connection between vertical and lateral subsurface flows and its potential role in determining return flow pathways due to the difficulty in observing and quantifying these processes at plot or field scale. We employed a novel approach, combining induced polarization, time‐lapse electrical resistivity tomography, and time‐lapse borehole nuclear magnetic resonance, to identify flow paths and quantify changes in soil hydrological conditions under nonuniform application of flood irrigation water. We developed and tested a new method to track the wetting front in the subsurface using the full range of inverted resistivity values. Antecedent soil moisture conditions did not play an important role in preferential flow path activation. More importantly, boundaries between lithological zones in the soil profile were observed to control preferential flow pathways with subsurface run‐off occurring at these boundaries when saturation occurred. Using the new method to analyse time‐lapse resistivity measurements, we were able to track the wetting front and identify subsurface flow paths. Both uniform infiltration and preferential lateral flows were observed. Combining three geophysical methods, we documented the influence of lithology on subsurface flow processes. This study highlights the importance of characterizing the subsurface when the objective is to identify and quantify subsurface return flow pathways under flood irrigation.

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

Core Ideas Unsaturated saprolite, saturated saprolite, and fractured rock have unique NMR responses. Surface NMR signals will be dominated by water in saturated saprolite. The heterogeneous nature of fractured rock results in low‐amplitude surface NMR signals. Understanding critical zone (CZ) structure below the first few meters of Earth's surface is challenging and yet important to understand hydrologic and surface processes that influence life on Earth. Nuclear magnetic resonance (NMR) is an emerging geophysical tool that can quantify the volume of groundwater and pore‐scale properties. Nuclear magnetic resonance has potential to aid in CZ studies, but it can be difficult to collect high‐quality NMR data in weathered and fractured rock. We present data from seven surface NMR soundings and six borehole NMR profiles collected on a weathered and fractured granite in the Laramie Range, Wyoming. First, we show that it is possible to collect high‐quality surface NMR data in a fractured rock. Second, we use the NMR data to delineate the weathering profile into three distinct zones—unsaturated saprolite, saturated saprolite, and fractured rock—and show that the surface NMR signal is dominated by saturated saprolite. Third, we show that lateral heterogeneity significantly reduces the surface NMR signal magnitude, which suggests that the boundary dividing saprolite and fractured rock is laterally heterogeneous. The NMR measurements, when combined with previously collected seismic refraction data, provide a unique opportunity to define the lateral heterogeneity of the boundary dividing saprolite and weathered bedrock in an eroding landscape underlain by crystalline rock.

Using AEM and GMR Methods for Non-Invasive, Rapid Reconnaissance Mapping and Characterisation of Groundwater Systems in the Kimberley Region, Northern Australia

The East Kimberley Region of north-western Australia has been identified as a priority for potential agricultural development. Within this region, the Ord Bonaparte Plain is remote, with limited access in an area of great cultural and environmental sensitivity. Initially, spatio-temporal mapping using remote sensing (and potential field) data, combined with data on the deeper basin geology was used to plan an airborne electromagnetics (AEM) survey. The relatively resistive nature of the basin sediments has enabled the AEM to map the hydrostratigraphy to depths of 300-500m, except in the coastal zone affected by seawater intrusion. Two overlying aquifers, separated by a faulted, ‘leaky’ aquitard, have been identified.The AEM and remote sensing data were subsequently used to plan a ground magnetic resonance (GMR) survey. The latter has enabled a water table map to be constructed in an area with almost no drilling, while also enabling key aquifer properties to be determined. The target aquifer has a high free water content and high transmissivity. The GMR results have been validated by drilling, borehole Nuclear Magnetic Resonance (NMR), and induction logging.Integration of AEM, GMR and temporal (Landsat) remote sensing data has enabled rapid mapping and characterisation of the groundwater system in a data-poor, culturally and environmentally sensitive area. These data have also revealed complex faulting within and bounding the aquifer system, delineated the sea-water intrusion interface, and mapped groundwater dependent ecosystems. These data have been used to target drilling and pump testing that will inform groundwater modelling, water allocations and development decisions.

Application of Magnetic Resonance Data for Groundwater Prospectivity in The Fitzroy Basin, Western Australia

In the Fitzroy Basin of Western Australia, both ground and borehole Nuclear Magnetic Resonance (NMR) techniques have been applied during a groundwater prospectivity assessment of the Cenozoic sediments, and the Palaeozoic and Mesozoic sandstone aquifers. Ground magnetic resonance (GMR) data was acquired at ten sites from the within the basin and the data was inverted to obtain estimates of the one-dimensional water content for the top 100m. Where these sites were co-located with boreholes, the results were consistent with inverted borehole NMR data and other litho-stratigraphic information although with a reduced horizontal resolution. Both the GMR and borehole data suggest that the Palaeozoic Grant Group and Poole Sandstone have high water content and have the potential to be highly productive aquifers. Furthermore GMR can be used estimate the depth to water table for unconfined aquifer, using 5 volume % as the threshold for saturated zone.