Near Surface Geophysics, 2014, 12, 271-284
doi: 10.3997/1873-0604.2013066
Surface NMR instrumentation and methods for detecting and characterizing water in the vadose zone David O. Walsh1*, Elliot D. Grunewald1, Peter Turner1, Andrew Hinnell2 and Ty P.A. Ferre2 1 2
Vista Clara Inc., Mukilteo, WA, 98275, USA University of Arizona, Department of Hydrology and Water Resources, Tucson, AZ 85721-0011, USA
Received January 2013, revision accepted November 2013 ABSTRACT A commercially available surface NMR instrument was modified to address the challenges of using earth’s field surface NMR to detect and characterize water in the unsaturated (or vadose) zone. The modified instrument incorporates faster switching electronics to achieve an instrument dead time of 2.8 ms, and higher output power electronics to enable a maximum coil voltage of 8000 volts and coil current of 800 amps. The instrument was used to collect and interpret surface NMR data at several active vadose zone investigation sites in the western US. A 6-week surface NMR experiment was conducted at a managed aquifer storage and recovery facility in Arizona, to explore the measurement capabilities and limitations of the instrument, during a managed infiltration event. The resulting time lapse surface NMR data were used to map zones of held water prior to the flood event, image the influx of water through the top 15 metres of the subsurface during and after the event, quantify the spatial and temporal distribution of infiltrating water throughout the event, and characterize the distribution of water in different relative pore sizes throughout the event. Data obtained at pseudo-static vadose zone investigation sites indicate that the surface NMR instrument can detect and image some forms of water held in unconsolidated vadose zone formations, at depths up to 30 metres. Complementary NMR logging data indicate that the surface NMR instrument does not detect all of the water held in these pseudo-static formations, but that the non-invasive surface NMR data may yield valuable information nonetheless. INTRODUCTION Importance of vadose zone hydrology Understanding hydrogeologic controls of the unsaturated, or vadose zone, is crucial to understanding transport and fate of underground contaminants. Except in cases where contamination is introduced directly into an aquifer below the water table, contamination must travel through the unsaturated zone to reach the saturated zone. Unlike the saturated zone, the vadose zone exhibits time varying and spatially varying saturation effects, and critical flow parameters (e.g., hydraulic conductivity) may be highly dependent on the degree of saturation present at any given time. Understanding vadose zone transport is also critical to groundwater management and production. Natural groundwater recharge, and many forms of engineered groundwater recharge, occurs through the vadose zone; hence groundwater recharge is governed by flow through it. Vadose zone properties are especially important to assessing the feasibility and efficacy of aquifer storage and recovery (ASR) projects. An improved ability to *
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accurately parameterize the vadose zone and model recharge can benefit groundwater management and production at both the site and basin scale. The vadose zone is critical in many other respects, for example as the zone of exchange between soil moisture and atmospheric moisture affecting climate and as the root zone controlling natural ecosystem and agricultural production. Measurement of infiltration, percolation, and recharge is a major focus of research in vadose zone hydrology. Much effort has been devoted to developing chemical and isotopic methods (see Scanlon et al. 2006). In the realm of geophysics, methods range from time domain reflectometry profiling (Dahan et al. 2003), to temperature-based monitoring of the rate (e.g., Constantz et al. 1996) and timing (Blasch et al. 2002) of streambed infiltration, to gravity monitoring of mass change due to infiltration (Christiansen 2011), to electrical resistivity monitoring of water content dynamics (Jayawickreme et al. 2010). The wide range of geophysical methods that can be applied to recharge monitoring is reviewed by Ferré et al. (2007) and the broader application of geophysics to hydrologic studies is presented by Robinson et al. (2008). Traditional surface geophysical methods that have been 271
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used to characterize vadose zone water content include electrical conductivity (DC electrical resistivity sounding and tomography, and frequency and time domain electromagnetics), induced polarization, seismic, and ground penetrating radar. These traditional geophysical methods measure physical properties of the vadose zone that may be influenced by multiple factors including water content, grain size, sediment composition, and fluid properties. For example, electrical conductivity is influenced by water content as well as grain size, distribution and composition, and fluid electrical conductivity (Archie 1942). Seismic and ground penetrating radar similarly measure changes in acoustic or dielectric properties of the subsurface, which may be influenced by a combination of parameters in addition to water content, such as physical variables of the soil/ rock matrix. Traditional geophysical borehole and direct-push methods operate on similar principles. These methods can be sensitive to vadose zone properties but interpretation of electrical or acoustic logging data can be ambiguous. While invasive logging methods provide high vertical resolution, they may also disrupt the soil/ moisture matrix (and hydrology) in the immediate vicinity of the measurement and can be economically inefficient for determining lateral heterogeneities in the subsurface. NMR-based geophysics for characterizing the saturated zone NMR offers something unique among geophysical methodologies: direct detection of water, and information on the surrounding media of water via relaxation and diffusion properties. Water content ϕ is the most easily derived parameter as it is directly proportional to the NMR signal amplitude. In the saturated zone, the NMR relaxation constants T1, T2, and T2* are correlated with pore diameter, as has been demonstrated across a large body of applied research (Kenyon et al. 1988, Kleinberg 2001, Dunn et al. 2002). The science of borehole NMR in saturated media has benefited from 20 years of continuous and widespread use in the petroleum sector, and is considered a mature science (Coates et al. 1999). Surface NMR, also referred to as magnetic resonance sounding or magnetic resonance tomography, is among the newest geophysical methods. Surface NMR uses large loops of wire on the surface of the earth to activate and detect groundwater protons in the Earth’s magnetic field, at resonant frequencies between 1 kHz and 3 kHz. A significant advantage of surface NMR, compared to other geophysical methods, is that surface NMR enables direct detection and measurement of water content. Through the NMR processes of absorption and relaxation, the NMR measurement causes the water itself to produce a weak but detectable alternating magnetic field. The initial amplitude of this magnetic field is directly proportional to the total water content, and the rate of decay of this alternating magnetic field is roughly correlated to the pore diameter, or distribution of pore diameters, in the subsurface.
In surface NMR, the distribution of NMR signals (and hence water content and other NMR-derived hydraulic properties) as a function of depth is estimated by applying a sequence of different transmitted pulse moments which cause a predictable modulation of the NMR signal at each depth. Subsequent linear inversion, based on the known spatial properties of the transmitting coil field and the known range of pulse energies applied, is used to isolate the time domain NMR signals emanating from different 1D depth locations, or even 2D or 3D voxels (Walsh 2008, Müller-Petke and Yaramanci 2010). Mono- or multi-exponential estimation algorithms are applied to estimate the total water content, bound and mobile water content, relative pore size distribution, and even hydraulic conductivity at each depth layer. Until recently, the practical use of surface NMR was limited to the investigation of saturated near surface aquifers, in nonmagnetic geology, and in low-noise (usually rural) environments. These practical limitations were directly related to limitations of existing surface NMR instrumentation available at the time. The most critical instrumentation limitations were the inability of existing surface NMR instruments to cancel natural and cultural noise, and the relatively long measurement “dead time” on the order of 30 to 40 ms, which prevented detection of fast decaying NMR signals in small pores and/or magnetic geology. The dead time is the effective time it takes for the instrument to switch between the transmitting and receiving modes. Recent developments in surface NMR instrumentation have largely overcome the limitations of older generations of surface NMR equipment. The development of sensitive multi-channel surface NMR instrumentation, with highly effective multichannel noise cancellation capabilities (Walsh 2008), has enabled the use of surface NMR in urban areas, and even within 50 to 100 metres of high density urban infrastructure as demonstrated at the 2012 MRS workshop in Hannover Germany. The recent development of surface NMR instrumentation with a short measurement “dead time” of less than 10 ms has enabled surface NMR to be applied in a much wider range of magnetic geologies, and to detect and characterize groundwater in smaller pore spaces (Walsh et al. 2011). Finally, the recent development of surface NMR instrumentation with ultra-low noise levels of 0.5 nV/rt(Hz) or less (Walsh 2008), has enabled the detection of very small NMR signals associated with low water content and the use of very small detection loops. Previous use of surface NMR to characterize vadose zone hydrology Although the majority of prior research on the application of surface NMR has focused on the saturated zone, there has been significant laboratory and field-based research in recent years on the application of surface NMR to characterizing water in the unsaturated zone. Several extensive laboratory investigations explored the possibility of using low field, and specifically earth’s field, NMR measurements to detect and characterize water content and saturation dependent hydraulic properties in
© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, 271-284
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the vadose zone (Ioannidis et al. 2006, Hertzog et al. 2007, Costabel and Yaramanci 2011b). We have previously demonstrated the ability to use surface NMR to detect and image some forms of water content in unsaturated, unconsolidated sediments, and to monitor changes in water content and its distribution in the aftermath of a controlled infiltration of surface water (Walsh et al. 2011). Other research has demonstrated the ability of surface NMR equipment to detect and characterize water trapped in small zones or capillary water in the vadose zone, in different geological settings (Boucher et al. 2006, Costabel and Yaramanci 2011, Mazzilli et al. 2012). Unique instrument requirements for surface NMR in the vadose zone Surface NMR detection of water in the vadose zone generally requires both a shorter instrument dead time, and increased instrumentation power. This is due to the tendency of water in the vadose zone to inhabit the smallest available pore spaces, and the tendency of water in smaller pore spaces with larger surface to volume ratios to exhibit faster relaxation times. This is particularly true of unconsolidated sediments, where larger pore spaces can be expected to quickly drain via gravity, and where smaller pore spaces can “hold” isolated droplets of water for longer periods through capillary or bound water forces. The benefit of a shorter dead time is obvious when considering the detection of fast decaying NMR signals. If the NMR signal decays faster than the instrument dead time then the NMR signal cannot be detected. An instrument with a shorter dead time can detect water with faster T2* relaxation, which means it catches more of the water that is held in smaller pores. The requirement for higher instrumentation output power is not as well appreciated. It is commonly assumed that, since the vadose zone is closer to the earth’s surface than the saturated zone, a surface NMR instrument should require less transmit power to investigate the vadose zone than to investigate the saturated zone. This assumption would be useful only in situations where the expected relaxation times of water in the unsaturated and saturated zones are comparable. In most unconsolidated environments, however, water in the vadose zone produces much faster relaxing NMR signals than when comparable sediment types are saturated with water. The faster relaxing NMR signals from water in the vadose zone require a correspondingly shorter pulse length to enable efficient broadband excitation of the broadband NMR signal, while also avoiding the significant effects of relaxation during the pulse (Walbrecker et al. 2009). Hence to achieve shorter pulse lengths, while maintaining useful depths of investigation, the instantaneous transmit power of the surface NMR instrument must be increased. Research and development objectives The goals of this research were twofold. The first objective was to develop surface NMR instrumentation with increased power and sensitivity, and decreased dead time to provide enhanced capabil-
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ity to measure the fast relaxing and low amplitude NMR signals of water in unsaturated fine sediments. Specific research objectives were to achieve a maximum coil voltage of 6kV to 8 kV, maximum coil current of 600 A to 800 A, an input receive level of 0.5 nV/rt(Hz) or less, and a measurement dead time of 3 ms or less. The second objective was to use the newly enhanced surface NMR instrumentation to demonstrate detection, imaging and characterization of water content in a variety of practical hydrogeological investigations. To drive the research, we developed the following hypotheses to test in the course of the field studies: 1) Can surface NMR be used to image and characterize water infiltrating the vadose zone during an engineered aquifer recharge event at a working aquifer storage and recovery (ASR) facility? More specifically, to what extent can surface NMR be used to: i. detect changes in water content and its distribution in the subsurface over time, ii. determine the rate of advance of the wetting front, iii. quantify the total amount of water recharged during a flood event, and iv. image and characterize the principal subsurface processes and structures influencing the infiltration event. 2) Can surface NMR be used to characterize the residual water in the pore space, in the vadose zone of typical unconsolidated formations and sediments, in non-ASR (i.e., pseudostatic) field investigations? METHODS Instrument Development The enhanced surface NMR instrumentation was developed in three stages, starting with the commercial 4-channel GMR instrument that was available in 2008, which had a maximum output of 4 kV and 400 A, a measurement dead time of 8 ms, and a receiver input noise level of 0.5 nV/rt(Hz). The measurement dead time was reduced initially to 4.8 ms, and ultimately to 2.8 ms, through improvements in switching electronics, and incorporation of phase cycling in the pulse sequences and processing software. The maximum current output was increased to 800 A through the design and assembly of a new H-bridge power conversion module. The maximum coil voltage was increased from 4 kV to 8 kV, by routing the transmit current to the coil through two 4 kV tuning units in series, and by using #10 AWG wire (6 mm^2 conductor cross-section) with 15 kV rated silicone insulation for the surface coils. Figure 1 is a photo of the final configuration of the enhanced surface NMR instrumentation, shown at a field site in Whatcom County, Washington USA. Processing and Analysis All surface NMR data were processed using reference coils and adaptive noise cancellation to reduce cultural noise (Walsh 2008). The surface NMR instrument uses a single clock to synchronize all receive channels, and identical wideband frequency
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FIGURE 1 Enhanced surface NMR instrumentation developed specifically for vadose zone investigations. The use of two 4 kV tuning units in series (shown at centre right) enables generation of coil voltages up to 8 kV.
response on all input channels. Both of these instrumentation details are important for maximizing the effectiveness of the adaptive noise cancellation procedure. Raw data were filtered, stacked and decimated to a sampling rate of 10 kHz using the “GMR QC” software which is provided with the GMR device. We used relatively large processing bandwidths of 300 Hz to 400 Hz to minimize the post-filtered deadtime (Walsh et al. 2011). Surface NMR data were inverted using a regularized linear spatial inversion algorithm to estimate watercontent normalized NMR signals from different depth layers (Walsh 2008), followed by mono- and multi-exponential estimation algorithms to estimate the total, bound and mobile water content, and distribution of water content as a function of T2* relaxation time, in each depth layer. The effects of relaxation during the pulse were incorporated by fitting the initial amplitudes to the centre point of the transmit pulse (Walbrecker et al. 2009). Available one-dimensional electrical resistivity profiles were incorporated in the 1D inversion using forward modelling of the transmitted fields calculated by the “Lemma” software (Irons et al. 2012). FIELD EXPERIMENTS AND RESULTS Tucson Arizona: March 17–April 26, 2011 One of the primary field study sites for our surface-NMR vadose zone measurements was an infiltration pond within the Southern Avra Valley Storage and Recovery Project (SAVSARP) near Tucson, Arizona, USA. Infiltration basins within the SAVSARP facility are used to actively infiltrate surface water diverted from the Colorado River into the local groundwater aquifer. The water table of the underlying aquifer varies seasonally, but is normally at a depth greater than 60 m from the surface. The scale of the recharge facility and the availability of meteorological and inflow data provided a unique opportunity to test the use of surface NMR for infiltration monitoring.
FIGURE 2 Location of 35 m diameter surface NMR detection loops as laid out in the SAVSARP infiltration basin. The Figure also shows the location of 5 ERT survey lines and 5 core sample locations.
Layout of surface NMR loops and other geophysical measurements Surface NMR data and a suite of other geophysical data were collected at SAVSARP Basin RB-208 on a continuing basis over a 6 week period from March 17–April 26, 2011. Working in concert with the project collaborator, Tucson Water, data were collected at the site before, during, and after an infiltration event. Before the experiment, Tucson Water allowed the basin to drain, with no applied water, for several months. Surface NMR data were collected with the enhanced GMR instrumentation, on two detection loops arranged as single-turn 35 m figure 8 loops, as shown in Fig. 2 (each figure 8 loop consisting of two 35 m circles). The two detection loops were deployed on the bottom of the dry basin prior to the flooding. Two surface NMR reference loops were also deployed in the direction of nearby power lines to assist in noise cancellation. The North figure 8 loop exhibited higher overall noise after noise cancellation processing, so in this paper we focus on data from the South figure 8 loop. In addition to surface NMR data, ancillary geophysical and core sample data were collected throughout the experiment. Five electrical resistance tomography (ERT) lines were deployed in the vicinity of the surface NMR coils to provide corroborating information about the change in electrical conductivity during and after the infiltration event. All ERT data were collected using two metre electrode spacing. Each survey line had 72 electrodes for a total length of 142 m. Soil sampling pre- and post-infiltration was completed at locations offset to the south from ERT line RB208-EW01. At each sample location, six-inch long cores were collected starting at 0.0, 0.6, 1.2 and 1.8 m below ground surface.
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The maximum core sampling depth was limited by the hand auger tools that were used, and also by the existence of a laterally continuous hard calcified layer at a depth of approximately 2 metres beneath the basin surface. The cores were sealed and returned to the laboratory for analysis. The locations of the surface NMR detection loops, ERT lines, and core samples are shown in Fig. 2. Basin operation and data collection schedule The operation plan for the basin infiltration event was designed to be relatively simple to aid in later interpretation of the imposed hydrologic signal. The basin was filled as rapidly as possible to a depth of approximately 1.8 m (at the gauge) and then maintained for 7 days. The pond was then allowed to drain by infiltration. All water added to the basin either infiltrated into the soil or evaporated. (Tucson Water routinely estimates the rate of water loss to evaporation using on-site meteorological data: the average water loss to evaporation during our experiment was estimated to be only 1–2% of the total volume added to the basin and was ignored for later analyses). The actual basin operation schedule is listed in Table 1 and the measured inflow and water depths are presented in Figs 3 and 4. The inflow to the basin was measured with a mag flow meter, which is permanently installed at the inlet to the basin. The temperature and water stage in the basin was monitored using a vibrating wire transducer and thermistor embedded at the midpoint of the basin. Surface NMR and ERT measurements were collected several times per week before, during, and immediately after the infiltration. Data collection was stopped on April 30, after monitoring redistribution of water in the subsurface for approximately one month. It is noted that the surface NMR measurements were performed during periods when the surface coils were entirely submerged in water. Special precautions were taken to minimize electrical hazards. The surface NMR loops for this particular experiment used newly manufactured, extra-long, continuously polyurethane-insulated #10 AWG wire rated to 2000 Volts AC, and independently tested to greater than 5000 VDC. This precluded the possibility of having any electrical connectors or conductors exposed or in direct contact with the water. In addition, the differential coil voltage for this
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experiment was limited to 4000V peak. The GMR equipment does not require or incorporate an Earth ground; hence the risk of electrical accident by ground return or leakage was mitigated by the instrument’s ungrounded electrical design. Nonetheless, readers are cautioned not to attempt a similar underwater surface NMR experiment without experimentally-specific safety engineering, planning, safety review and formal approval from all applicable governing authorities. Results Figure 5 shows plots of the mean free induction decay (FID) signal on the South figure 8 loop before the start of infiltration on March 21 (a) and immediately after the surface water in the recharge pond had infiltrated below ground surface on April 5 (b). These data were post-processed using a filter bandwidth of 300 Hz, resulting in an effective dead time (after all processing) of 7.1 ms from the end of the transmit pulse. The “mean FID” shown here is the average NMR signal computed over a single
FIGURE 3 Inflow rate and cumulative length of water added to basin RB-208.
TABLE 1 SAVSARP RB-208 basin operation schedule.
Date
Event
Jan 18 – Mar 24 16:00
Drying of RB-208 pre experiment
Mar 24 16:00 – Mar 25 10:00
Rapidly fill RB-208 (~25,00 gpm flow rate)
Mar 25 10:00 – Mar 31 16:00
Maintain depth at ~1,8 m (~5,000 gpmflow rate)
Mar 31 16:00 – May 1 0:00
No inflow
May 1 0:00
End of experiment
FIGURE 4 Water depth (24 hr moving average filter) in the centre of RB-208 during the infiltration experiment (blue line). The temperature at the observation station (also with a 24hr moving average filter applied) is shown in red.
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FIGURE 5 Mean FID NMR signals collected on the south GMR loop immediately before the infiltration (top) and immediately after the infiltrating water has drained below ground surface (bottom). Blue signals are before noise cancellation, red signals are after noise cancellation.
stack of 48 different pulse moments, using an FID (single pulse) sequence with a 6 ms pulse. Signals before noise cancellation are shown in blue; signals after noise cancellation are shown in red. These two data sets clearly show that the enhanced surface NMR instrumentation can detect water content in the vadose zone even before the infiltration when the subsurface is in its “dry” condition, and can also detect a significant change in the bulk subsurface water content as a result of the infiltration. (The data acquisition sequence incorporates phase cycling on alternate pulses to cancel any transient response from the instrumentation itself. Thus any observed short duration signals are ensured to be due to an NMR response of the subsurface water.) It is notable that the peak amplitude of the detected mean NMR signal has approximately doubled as a result of the infiltration. The NMR signal following infiltration also exhibits longer T2* relaxation components, which is consistent with infiltrating water filling larger available pore spaces. The data from the SAVSARP infiltration experiment also demonstrate the benefit of using very short transmit pulses, and hence higher instantaneous power output, for vadose zone surface NMR measurements. Figure 6 shows plots of the FID signal on the south figure 8 loop, for the same pulse moment of 0.5 A*s, using a 12 ms transmit pulse (top) and a 6 ms transmit pulse (bottom). Both data sets were collected on April 5, which was the first day of collection after the surface water in the recharge pond had infiltrated below ground surface. The processing parameters and effective dead times of these two data sets are identical. The FID signal produced by the 6 ms pulse exhibits almost twice the initial amplitude, compared to the FID signal produced by the 12 ms transmit pulse. It is notable that the largest difference
between the two signals is the presence of very fast relaxing signals in the early portion of the NMR signal produced by the 6 ms pulse, which are absent in the NMR signal produced by the 12 ms pulse. This comparison clearly demonstrates the importance of broad-band excitation and the influence of relaxation during the pulse (Walbrecker et al. 2009), highlighting the need to use high-power, short pulses for surface NMR studies of the vadose zone. The analysis of the data on March 29, when the pond had 1.2 m of standing water, was complicated by the large bulk water signal present in the raw data. This large bulk water signal is evident as the large concentration of water with long T2* between depths of 0 m and 5 m, in Fig. 7 (left). The authors used two different post-processing approaches in an attempt to remove the bulk water signal from the March 29 data set. First, we estimated and subtracted a mono-exponential signal from the data record for each pulse moment, using non-linear least squares fitting with a constraint that the mono-exponential component have a T2* between 700 ms and 2 s. This result is shown in Fig. 7 (centre). Second, we used data concurrently collected on the North figure 8 coil as a reference for the bulk water signal, and used the standard noise cancellation software to cancel the long NMR signal. The North coil was submerged and its response was dominated by long T2* signals from the bulk water in its vicinity. The water in this vicinity experienced small tip angles as a result of the B1 fields generated by the south figure 8 coil during the transmit phase. The North figure 8 data were recorded on one of the available input channels, and were used as a “noise reference” channel during noise cancellation processing. The result of this correlation cancellation processing is shown in Fig. 7 (right).
FIGURE 6 Comparison of free induction decay (FID) signals, using a constant pulse moment of 0.5 A*s, but differing pulse lengths of 12 ms (top) and 6 ms (bottom). These data sets were collected a few hours apart on the same day, April 5, 2011, on the SAVSARP south detection loop.
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Comparing the three results in Fig. 7, it appears the two approaches to removing the bulk water signal yield somewhat different results, at least after processing them via multi-exponential inversion. The estimation and removal of a mono-exponential component with T2* constrained to be between 700 ms and 2 s (Fig. 7 centre) effectively eliminates all of the bulk water signal, but also changes the apparent distribution of short T2*water. This is evident between 10 m and 13 m where some short T2* components are present where they were not apparent in the T2* distribution prior to bulk water signal removal. The second bulk water cancellation method, using the north figure 8 coil as a reference with adaptive correlation cancellation (Fig. 7 right), eliminates the bulk water signal, and does so without changing the apparent T2* distribution below the depth of 13 m. It is not clear which result is better at estimating the distribution of water in the sediments below the pond. In retrospect, in-situ NMR logging measurements would have been useful for this purpose. Additional development and testing of signal processing methods to remove the large bulk water signal would be useful. The data shown in Fig. 8 depicts the evolution of surfaceNMR-estimated water content as a function of depth and T2* relaxation rate, over the course of the infiltration experiment. In Fig. 8, multi-exponential analysis is used to estimate the distribution of water content as a function of both T2* relaxation rate and depth, on each day. The resolution matrix (Müller-Petke and Yaramanci 2008) for these spatial inversions indicated a maximum resolution depth of approximately 15 metres. For March 29, when the pond was flooded, we use the data processed with adaptive correlation cancellation using the North figure 8 coil as a “noise” reference. Figure 9 depicts the evolution of total water content, and estimated bound and mobile fractions, as a function of depth and time. For the March 29 data, we use the data set processing with adaptive correlation cancellation to remove some of the bulk water signal. For the data in Fig. 9, we segregate the detected water into short and long T2* components, using a cutoff value of 30 ms. The selected cutoff value of 30 ms is based upon an often-cited T2 cutoff of 33 ms, which was derived empirically to estimate the cutoff between bound and mobile fluid fractions in NMR logging of consolidated oil-bearing sandstone formations (Straley et al. 1997). From the results of Fig. 9, it is clear that a portion of the water with T2* less than 30 ms exhibits drainage over the course of the 6 week experiment. This result demonstrates that the T2* cutoff value of 30 ms is not universally applicable for estimating the transition from bound to mobile water in surface NMR, as would be anticipated given the variable influence of magnetic inhomogeneity on the T2* response (Grunewald and Knight 2011). More research is needed to illuminate the relationship(s) between surface NMR derived relaxation rates (T1, T2 and T2*) and true water mobility, in both the saturated and unsaturated zones.
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Figure 10 depicts the evolution of the surface NMR-estimated total water content and FID integral as a function of depth and time. Total water content was estimated by integrating the water content distribution shown in Fig. 8, at individual depth samples, over all T2* components. For a mono-exponential distribution, the FID integral is directly proportional to both the total water content and T2*, and is less susceptible to noise than water content and T2* estimates obtained through least squares fitting (Walsh 2008). It was computed as the sum of the sampled FID signal after demodulation to baseband. For this study, the FID integral provides another measure of the distribution of water with long T2*, in space and time. In Fig. 10, the FID integral indicates a transient increase in the saturation of larger pores, between the depths of 5 and 12 metres, peaking on April 5, 2011. A summary observation and interpretation of the surface NMR data in this infiltration event follows. On March 21, prior to the filling of the pond, the surface NMR data show apparently held water content between depths of 0 m and 5 m ranging from 2% to 4%, and also apparently held water content below 10 m. On March 29, after a week of flooding and with 1.2 m of standing water in the recharge pond, the top 5 metres of the subsurface appears to be saturated, with NMR-detected water contents ranging from 12% to 25% in the top 5 metres. At this stage,
FIGURE 7 Multiexponential T2* distributions of data from March 29, when 1.2 metres of standing water was in the pond and over the coil. Left: no additional preprocessing, centre: least squares fit and removal of single long T2* exponential term, right: use of 2nd underwater detection coil as reference with adaptive correlation cancellation software.
FIGURE 8 Evolution of surface-NMR-derived water content as a function of T2* relaxation rate and depth, at five points in time over a 6-week measurement period (March 21–April 26, 2011). Water content density is shown on a fixed linear color scale.
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of water content vs. depth and T2* on April 26 appears similar to the starting stage on March 21, though there is still elevated water content held in apparently small pores throughout the top 12 m.
FIGURE 9 Evolution of surface-NMR-estimated bound, mobile and total water content as a function of depth, at five points in time over a 6-week measurement period (March 21–April 26, 2011).
FIGURE 10 Surface NMR estimated total water content (left) and calculation of (FID integral) ^2 (right) as functions of depth and time.
according to Figs 8 and 9, the wetting front appears to have reached a depth of approximately 8 metres. Interpretation on this day is complicated by the presence of the large bulk water NMR signal form the standing water in the pond. On April 5, the first day of measurements after the water had completely infiltrated below surface, the wetting front appears to have reached the maximum depth of resolution of 15 m. The water content peaks at approximately 16% at the depth of 7.5 metres. The water content is progressively smaller closer to the surface, indicating that gravity drainage has caused significant reduction of saturation in the top 5 metres. The water content in the zone between 6 m and 13 m shows a notably longer T2* relaxation, indicating that the water in this zone is inhabiting larger pore spaces than the zones immediately above and below it. On April 12, the peak water content continues to migrate to lower depths, and saturation is reduced in the zones closer to the surface. This observed spreading of water with depth through time is entirely consistent with vadose zone flow processes (Hinnell et al. 2010). The water content in the intermediate zone from 6 m to 10 m has noticeably shorter T2* relaxation than on April 5, indicating that some water has drained from the larger pores, and remaining water is tending to reside in the smaller available pores. On April 26, three weeks after the water in the pond infiltrated the surface, the centre of mass of the infiltrating water has continued to migrate deeper, and the water content and T2* relaxation in all zones has trended lower. The final distribution
Electrical Resistance Tomography and Core Sample Data 2D Electrical resistivity profiles from the ASR infiltration experiment in Tucson Arizona indicated relatively high resistivity (32 to 50 ohm-m ) in the top 5 m of the subsurface, relatively lower resistivity (4 to 10 ohm-m) below 5 metres, and ambiguity below 10 metres. The inverted ERT resistivity profile along line RB208EW05, which intersects the south Fig. 8 NMR loop, is shown in Fig. 11 before the infiltration (top), after the infiltration (middle), and with the difference shown in Fig. 11 (bottom). The ERT data were not conclusive. There is evidence of reduced electrical resistivity associated with increased water content to 8 m depth. However, we later concluded that water infiltrated through a hydraulically restrictive layer, at depth, leading to relatively small water content changes over a deeper region. Given the nonlinear dependence of electrical conductivity on water content, it is reasonable to conclude that ERT was simply not sufficiently sensitive to this unexpected hydrologic response. The soil core data, collected before and after infiltration, were intended to ground truth the range of water content change measured with surface NMR. The available hand auger sampling methods limited the depth of cores to less than 2 metres. Soil cores were collected at sampling locations spaced 10 m apart offset from ERT line RB208-EW04 (Fig. 2). At each sample location 15 cm cores were collected at 0.00, 0.60, 1.20 and 1.85 m depth on March 15, 2011 (pre infiltration), and April 6, 2011 (post infiltration). Prior to April 6 the floor of the basin was too soft to access for sampling. The soil profile at this location had approximately 2 days of drying and subsurface water redistribution before the post flood samples were collected. The cores were collected with an AMS hand auger and 6” soil core sampler. The post infiltration boreholes were located 30 cm from the pre infiltration boreholes. Once the core was retrieved, the ends were sealed and returned to the laboratory for analysis using a low field (279 kHz) NMR spectrometer. The NMR laboratory measurements were performed with a CPMG measurement with echo spacing of 2.0 ms. NMR-measured water content was calibrated by conducting measurements of same sized core containers filled with water. The measured volumetric water contents are presented in Table 2. At each sample location the upper number is the pre infiltration water content and the lower number is the post infiltration water content. The pre-infiltration water contents are very low, as expected for this environment. Surprisingly, the postinfiltration data also show relatively low water contents as well. On average, the laboratory NMR data show that the post infiltration water contents were increased by 0.03 to 0.11 cm3/cm3 volumetric water content, compared to the pre-infiltration water contents. NMR results were obtained on cores within four weeks of the end of the experiment and the cores were sealed before
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measurement. To check these results, half of the cores were sacrificed and their water contents were determined by oven drying in September 2011. The laboratory NMR-measured water contents are consistently lower than the gravimetrically-determined values. However, the water content change associated with flooding is highly consistent, with most results well within the accuracy of any water content measurement method. This suggests that even though water was ponded to several feet depth in the basin, locally only a small amount of additional water was held in the top 2 metres immediately after the infiltration. Table 3 compares the total water contents over the top 2 meters of the subsurface, estimated using a) oven drying of all core samples, b) low field laboratory NMR measurement of all core samples, and c) the surface NMR inversions of the south figure 8 loop. These are compared before and after the flood. Comparison of the average NMR-derived and gravimetric water contents show that there is some variation in estimated water content between the various measurements, and all three measurements indicate increased water content in the top 2 m on April 5, compared to before the flood. The laboratory NMR and surface NMR measurements generally estimate lower content than the gravimetric measurement. This indicates that there is a fraction of the water that exhibits T2 relaxation that is too short to measure using the laboratory NMR measurement with a 2.0 ms echo spacing, as well as with T2* too short to measure with the surface NMR with a post-processed dead time of 7 ms. While the ancillary data (ERT and cores) were not as useful as hoped for ground-truthing the surface NMR interpretations, the difficulties in collecting and interpreting these data provide a useful comparison for the performance and utility of surface NMR in this challenging setting.
FIGURE 11 ERT measurements on line RB208EW05 (see Fig. 2 for location). Top: electrical resistivity (log10 ohm-m) on March 18, 2011, before flooding. Middle: electrical resistivity (log10 ohm-m) on April 6, 2011, after flooding. Bottom: Pre-flood minus post-flood electrical resistivity map (ohm-m).
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Even discounting the possible effects of bulk water in the March 29 data, the results of the infiltration experiment demonstrate the ability of surface NMR to quantify changes in subsurface water content in the vadose zone, in time and space. The results clearly show a significant increase in water content due to the infiltration, and also a clear shift towards longer T2* relaxation, which is expected and consistent with transient water filling more pore spaces, and larger pore spaces. The individual snapshots of water content and distribution on March 21, March 29, April 5, April 12, and April 26, indicate complex, spatially distinct processes alternately holding water or impeding drainage at some depths, and facilitating rapid drainage at other depths. The snapshot on March 21, before the infiltration had started, shows small amounts of water apparently held in small pores in the upper 2 metres, and also depths greater than 5 metres, with less held water in the intervening layers. The analysis of the data on March 29, when the pond had 1.2 m of standing water, is complicated by the very large bulk water signal present in the raw data. Nonetheless the data show a clear increase in short T2* water in the upper 5 metres, apparently due to increased saturation of fine sediments in these depth ranges. The first data set after the surface water had completely drained below surface, on April 5, indicates significant water held in small pores (short T2*) in the upper 6 metres, and significant amounts of water in apparently larger pores (longer T2*) between 7 m and 12 m. The observation that the total water content shows a bulge at 5 m depth, is unexpected based on simple infiltration and drainage in a homogeneous medium. But, this could be consistent with the presence of a low permeability layer at this depth hypothesized based on observations of infiltration versus time. Subsequent measurements on April 12 and April 26 depict the water content trending lower, deeper, and generally inhabiting smaller pore spaces (indicated by shorter T2* relaxation) as time moves forward. Dix, Nebraska USA A vadose zone investigation using the enhanced surface NMR instrument was conducted in Dix Nebraska, USA, in November 2011, with the assistance the USGS Water Sciences Center in Nebraska. This site was on irrigated farmland. A 2.0 inch PVCcased monitoring well was located approximately 200 metres south of the test site. This provided an opportunity to ground truth the surface NMR data with NMR borehole logging measurements. We went into this experiment with little information on the subsurface, other than a description that the subsurface consisted mainly of the Ogallala formation (semi-consolidated sand, gravel, sandstone, silt and clay), and that the water table was at a depth of approximately 80 metres. Our hypothesis was that we could use the enhanced surface NMR instrument to detect water in the unsaturated zone, and that the detected water would yield some insight into the structures influencing water content and groundwater transport in the top 15 metres of the vadose zone. NMR
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logging data (Walsh et al. 2013) were acquired in a 2” PVC-cased borehole located approximately 200 m south of the surface NMR measurement location. The land surface was also slightly elevated compared to the surface NMR measurement location. NMR logging was performed in an unscreened section of the well, between depths of 5 m and 20 m, using an echo spacing of 2.0 ms. Surface NMR data were collected using a single turn 44 m figure 8 detection coil, and a second surface coil was deployed towards a power line to aid in noise cancellation. The mean FID signal, averaged over a single stack of 58 pulse moments, using a 6 ms transmit pulse length , is shown in Fig. 12. This figure depicts a very small amplitude, short T2* FID signal. Although this observed NMR signal is small, in the average of 12 stacks it exhibits peak amplitude and phase coherence in the middle range of the acquired pulse moments (peaking at a pulse moment of around 1 A*s). The multi-exponential inversion from this experiment is shown in Fig. 13. The inversion yields low estimated water content in general, with concentrations of water held in apparently fine sediments between 2.5 and 4.5 m, and between 6 and 9 m. The multi-exponential inversion also indicates possible small amounts of water with longer T2* relaxation rates, in apparently larger pore spaces at 2 m, and between 8 m and 12 m. The resolution matrix for this inversion indicated the lower limit for spatial resolution was around 14 metres. The NMR logging data from the nearby well is shown in Fig. 14. Comparing the surface NMR to the logging NMR data, both NMR measurements indicate two zones of elevated water
content with short relaxation. Both measurements also indicate a very small amount of water with longer T2 or T2* relaxation between the two zones of short T2/T2* relaxation. A major difference between the two measurements is the total water detected content, which peaks at around 8% for the surface NMR data but reaches a peak of over 30% in the NMR logging data. Lower water content estimates from the surface NMR measurements indicate that a portion of the signals detected by the logging tool are too short to be detected by surface NMR. One reason some signals may not be detected are that the NMR logging measurement has a shorter processed data dead time (2.0 ms vs. 5.1 ms for the surface NMR data). Also the NMR logging instrument measures T2 relaxation, which is always longer than T2* relaxation. Some discrepancy may further be attributed to lateral heterogeneity and the significant 200 m lateral offset between measurement locations. Colby, Kansas USA The test site, selected by colleagues at the Kansas Geological Survey (KGS), was part of an active KGS research program that was attempting to characterize and monitor groundwater recharge processes affecting the High Plains Aquifer in Western Kansas. This site was on irrigated farmland near Colby, Kansas. The subsurface consisted of approximately 18 m of loess, underlain by a mostly unsaturated Ogallala formation. The water table at the time of the test was approximately 66 m below the surface. The saturated portion of the Ogallala was approximately 17 m thick, running to a depth of about 83 m.
TABLE 2 Pre- and post-infiltration results from soil coring. The NMR volumetric water content was measured using a low field NMR spectrometer operating at a frequency of 279 kHz.
Oven VWC Preflood
Flood
30–0
0.153
40–0
0.131
50–0 40–2
NMR VWC Change
Preflood
0.234
0.08
0.247
0.12
0.107
0.205
0.068
0.136
60–2
0.179
70–2 30–4
Oven vs. NMR
Flood
Change
WC difference
0.03
0.1
0.07
0.01
0.07
0.180
0.11
0.01
0.10
0.06
0.16
0.1
0.00
0.07
0.04
0.15
0.11
–0.04
0.273
0.09
0.19
0.26
0.07
0.02
0.099
0.238
0.14
0.03
0.14
0.11
0.03
0.069
0.099
0.03
0.08
0.17
0.09
–0.06
50–4
0.214
0.239
0.03
0.15
0.18
0.03
0.00
60–6
0.192
0.267
0.08
0.07
0.14
0.07
0.01
TABLE 3 Comparison of total volumetric water content estimated over the top 2 metres of the subsurface using gravimetric, laboratory NMR and surface NMR measurements.
Oven VWC
Lab NMR VWC
Surface NMR VWC
Preflood
.135
.094
.027
Flood
.215
.154
.061
Change
.080
.060
.034
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The objective of this experiment was to determine whether the enhanced surface NMR instrumentation could detect thin lenses of perched water within the vadose zone. The KGS researchers and their cooperators in the Colby KS area hypothesized that such thin lenses of water existed at depth in the vadose zone, where there were deposits of sand overlain deposits of silt and/or clay. A borehole electrical resistivity log indicated numerous transitions from low to high resistivity throughout the unsaturated zone (Young et al. 2007). The surface NMR equipment was deployed using a 70 m square single turn detection loop, with an additional surface loop deployed 200 m to the west to cancel noise from a nearby power line. We used an FID pulse sequence with several different pulse lengths ranging from 40 ms to 15 ms. Based on previous experience, it was anticipated that a shorter pulse would be preferable for detection of very short vadose zone water signals. In this instance, however, the objective was to detect signals at fairly deep levels in the vadose zone (i.e., deeper than 15 m). It was, therefore, necessary to use a fairly large loop and a somewhat longer pulse length to achieve the required depths of investigation. We hypothesized that any perched water in larger pores would exhibit longer T2* and hence would still be detectable using longer transmitted pulses. Ancillary data included an EM induction log of a nearby 2 inch diameter PVC-cased monitoring well, located 400 m west of the surface NMR site. Complementing the surface NMR data, a section of this same monitoring well was logged with a 1.75 inch diameter NMR logging tool (Walsh et al. 2013). A 1D multi-exponential surface NMR inversion of a 15 ms FID pulse sequence data set is shown in Fig. 15. This inversion incorporated a 1D electrical resistivity profile derived from the EM induction log in the nearby 2 inch diameter PVC-cased monitoring well. The resolution matrix for this result indicates a maximum resolution depth of around 33 metres. The inversion result indicates concentrations of water content in small pores (short T2* relaxation), between depths of 8 m and 12 m, and between 16 m and 22 m. A partial NMR log of the nearby 2 inch diameter PVC-cased monitoring well is shown in Fig. 16. This NMR log depicts significant water content in small pores (short T2 relaxation), in the depth range from 16 m to 26 m. The water content detected by the NMR logging tool (4% to 18%) is much higher than that detected by the surface NMR measurement (less than 3%) over the same depth interval. This could be explained by: 1) the relatively long pulse length of the surface NMR measurement (15 ms, compared to 120 us for the NMR logging data) which would ensure a significant and probably large loss of signal amplitude due to relaxation during the pulse; and 2) the short echo spacing of 2 ms for the logging NMR measurement, in contrast to the longer 5.1 ms post-processed dead time of the surface NMR measurement. DISCUSSION In this section, we discuss the results of the various field experiments in the context of the hypotheses that we had set out to test. Returning to the specific hypotheses tested in the ASR infiltration experiment:
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FIGURE 12 Dix Nebraska. Mean FID (top) and frequency spectrum (bottom) averaged over a single stack 600 of 58 pulse moments, using FID sequence with 6 ms pulse. Short free induction decay (FID) signals are clearly evident. Blue graph is before noise cancellation, red graph is after noise cancellation.
FIGURE 13 Dix, NE, Inversion of surface NMR data. This multi-exponential inversion result indicates significant water content in very small pores in the range from 2 m to 10 m, and possibly detection of water in somewhat larger pores between 8 and 12 m.
(i) The infiltration experiment demonstrated that it is possible to utilize surface NMR to detect changes in water content and its distribution in the subsurface over time. This hypothesis was clearly validated by the data in Figs 5–10. In addition to changes in total water content, these time lapse NMR images clearly show changes in the way water is filling pore spaces of different sizes. This valuable information on the time and space-dependent distribution of water in different pore spaces, is uniquely provided by NMR.
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FIGURE 15 Colby Kansas, USA. Multi-exponential inversion of surface NMR data, using a 70 m square loop and 15 ms FID pulse sequence.
FIGURE 14 Borehole NMR log Dix, NE, November 9, 2011. This borehole NMR log was measured using a 1.75” diameter NMR logging tool, in a 2” PCVcased well located approximately 200 m south of the surface NMR measurement location. This borehole NMR log was processed by averaging 2 adjacent 1m levels, to produce a spatial resolution of 2m, similar to the spatial resolution of the surface NMR inversion at a depth of 10 m. The increase in longer T2 water content in the range from 11–14 m is similar to the trend in the surface NMR inversion.
(ii) The infiltration experiment indicated it may be possible to use surface NMR to determine the infiltration rate of the wetting front, as shown by the evolution of water content vs. depth and time depicted in Figs 7–9. In this experiment, we were limited to resolving spatially in the top 15 m. The data set from March 29 indicates that a wetting front extends to a depth of approximately 10 m, after of 3–4 days of infiltration. The data set from April 5 indicates that the wetting front had reached a depth of at least 13 m, after 10–11 days of infiltration. (iii) Comparison of the total surface NMR-estimated stored water, before and immediately after infiltration, suggests that surface NMR might be capable of quantifying the amount of water recharged during this type of aquifer recharge event. However, the unknown lateral variation of infiltration rates across the larger basin leaves inherent ambiguity in this example. The total water infiltrated across the basin is known and equates to an average infiltrated water flux or equivalent column length of 3.4 m. The resolution matrices for these surface NMR inversions indicate that the maximum depth at which individual layers can be resolved is around 15 m. The maximum depth of
resolution is a function of the coil size and geometry, the electrical conductivity of the subsurface and the maximum applied pulse moment (which in turn is a function of the maximum power output and the pulse length). The total stored water content estimates, from the inversion of the surface NMR data on March 21 (before infiltration) was integrated over the top 15 metres establishing a background water column length of 0.46 m. The total water content estimated from the inversion of the data on April 5 (immediately after the pond had drained) was also integrated over the top 15 metres, establishing an equivalent water column length of 1.25 m. The surface NMRestimated infiltrated flux of 0.79 metres is approximately 23% of the known average value of 3.4 metres. Some discrepancy may be attributed to water that may have infiltrated to depths greater than 15 metres by the time the pond had drained below the surface on April 5. It is also possible that some of the infiltrated water spread laterally during the infiltration, and it should be expected that drainage occurred at different rates across the laterally extensive recharge basin. (iv) The infiltration experiment demonstrated that it is possible to use surface NMR to characterize the processes and structures influencing the infiltration event. The data set obtained before the infiltration shows water held in apparently small pores between depths of 0 to 2 m, and also at depths greater than 5 m. These two zones appear to contain fine sediments capable of holding water for long periods (months). The intermediate zone between 2 and 5 m, in contrast, appears to consist of coarser sediments with larger pore spaces that exhibit longer T2* relaxation (when saturated) and that drain relatively quickly. The depth of 2 m also corresponds to the known depth of a calcified layer under the basin surface The evolution of water content in all depths, and the observed shift in T2* relaxation rates, indicates water moving over time into larger pore spaces at higher saturation and into smaller pore spaces at lower saturation.
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The hypothesis for the pseudo-static vadose zone investigations was: can surface NMR be used to image water content and other hydrologically-relevant structures or properties in the vadose zone, in typical unconsolidated formations and sediments, in non-ASR (i.e., pseudo-static) field investigations? The experimental results from Dix Nebraska and Colby Kansas demonstrated that surface NMR can detect, measure and localize at least a portion of the held water in presumably fine unconsolidated sediments in portions of the vadose zone tens of metres above the water table. In the case of Colby Kansas, there is some indication of detection of a small amount of perched water, or increased saturation of large pores residing above a finer grained sediment layer, at a depth of around 20 metres. This was an important question posed by local hydrologists, and one which had not been previously answered using a variety of borehole geophysical measurements. CONCLUSIONS The results of this research, and prior related research, indicate that surface NMR has a significant role to play in characterization of vadose zone hydrology. The most basic challenge is simply detection of the typically low-amplitude and fast-relaxing NMR signals in the vadose zone. This basic detection challenge has been overcome to a significant degree through purposedesigned surface NMR instrumentation that incorporates:
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1) a very short measurement dead time; 2) higher instantaneous transmit power for broadband excitation and minimization of relaxation during the pulse; 3) very low noise electronics; and 4) effective multichannel noise cancellation hardware and software to enable reduction of system and environmental noise levels to the level of a few nanovolts over a 300 Hz bandwidth. The infiltration experiment demonstrated the capability of surface NMR to monitor the progress of a flood-induced wetting front over time, and to estimate the total amount of water recharged in a flood induced recharge event. The infiltration experiment also demonstrated the ability to use surface NMR measurements to characterize the pore structure of subsurface sediments and measure the saturation of different pore spaces at different depths over time. It is possible that these results could be successfully extended in the future to characterize recharge during naturally occurring flood events. This research has demonstrated the ability of surface NMR to detect and image at least a portion of the held water content in a pseudo-static vadose zone environment. In practical vadose zone investigations, this information on the location of held water content could presumably shed light on aquifer recharge and contaminant migration processes, and could reduce ambiguity when combined with other geophysical data in such investigations. Based on comparison with available NMR logging data, and laboratory NMR and gravimetric measurements of core samples as part of this study, presently available earth’s field surface NMR technology cannot be expected to detect all forms of water in the vadose zone. ACKNOWLEDGEMENTS This work was supported by the United States Department of Energy under Grant # DE-FG02-08ER84979. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the United States Department of Energy. The authors thank Tuscon Water, Jim Cannia of the US Geological Survey, and Jim Butler and Ed Reboulet of the Kansas Geological Survey for their generous assistance in facilitating the field studies presented in this work. The authors also thank two anonymous reviewers whose careful review and thoughtful suggestions greatly increased the quality of this submission. REFERENCES
FIGURE 16 NMR log of PVC-cased monitoring well, located ¼ mile west of the surface NMR collection location in Fig. 15.
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