Ty P. A. Ferré

Measuring soil moisture content non‐invasively at intermediate spatial scale using cosmic‐ray neutrons

[3] We present a novel non-invasive technique that utilizes the dependence of the low-energy cosmic-ray neutron intensity above the ground surface on the hydrogen content of soil. The cosmic-ray method is based on slowing down and thermalization of cosmic-ray neutrons by hydrogen atoms present in soil. Soil moisture greatly affects the rate at which fast neutrons are moderated, controlling neutron concentration in soils and prescribing their emission into the air. Dry soils have low moderating power and are therefore highly emissive; wet soils are more moderating and therefore less emissive as highly moderated neutrons are more efficiently removed from the system. The change in soil neutron emission is sufficient to produce a clear signal in the neutron intensity above the surface. For soil moisture content varying from zero to 40% volumetrically, the corresponding decrease in cosmic-ray neutron intensity above the surface is 60%, a hundredth of which can easily be measured using a neutron detector.

Improved extraction of hydrologic information from geophysical data through coupled hydrogeophysical inversion

Improved extraction of hydrologic information from geophysical data through coupled hydrogeophysical inversion A.C. Hinnell 1 , T.P.A. Ferre 1 , J.A. Vrugt 2 , J.A. Huisman 3 , S. Moysey 4 , J Rings 3 , and M.B. Kowalsky 5 Hydrology and Water Resources, University of Arizona, Tucson, AZ, 85721-0011 Center for Nonlinear Studies (CNLS), Mail Stop B258, Los Alamos, NM 87545 ICG 4 Agrosphere, Forschungszentrum Julich, 52425 Julich, Germany Environmental Engineering and Earth Sciences, Clemson University, Clemson, S.C. 29634 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 Abstract There is increasing interest in the use of multiple measurement types, including indirect (geophysical) methods, to constrain hydrologic interpretations. To date, most examples integrating geophysical measurements in hydrology have followed a three-step, uncoupled inverse approach. This approach begins with independent geophysical inversion to infer the spatial and/or temporal distribution of a geophysical property (e.g. electrical conductivity). The geophysical property is then converted to a hydrologic property (e.g. water content) through a petrophysical relation. The inferred hydrologic property is then used either independently or together with direct hydrologic observations to constrain a hydrologic inversion. We present an alternative approach, coupled inversion, which relies on direct coupling of hydrologic models and geophysical models during inversion. We compare the abilities of coupled and uncoupled

Measurement depth of the cosmic ray soil moisture probe affected by hydrogen from various sources

[1] We present here a simple and robust framework for quantifying the effective sensor depth of cosmic ray soil moisture neutron probes such that reliable water fluxes may be computed from a time series of cosmic ray soil moisture. In particular, we describe how the neutron signal depends on three near-surface hydrogen sources: surface water, soil moisture, and lattice water (water in minerals present in soil solids) and also their vertical variations. Through a combined modeling study of one-dimensional water flow in soil and neutron transport in the atmosphere and subsurface, we compare average water content between the simulated soil moisture profiles and the universal calibration equation which is used to estimate water content from neutron counts. By using a linear sensitivity weighting function, we find that during evaporation and drainage periods the RMSE of the two average water contents is 0.0070 m 3 m � 3 with a maximum deviation of 0.010 m 3 m � 3 for a range of soil types. During infiltration, the RMSE is 0.011 m 3 m � 3 with a maximum deviation of 0.020 m 3 m � 3 , where piston like flow conditions exists for the homogeneous isotropic media. Because piston flow is unlikely during natural conditions at the horizontal scale of hundreds of meters that is measured by the cosmic ray probe, this modeled deviation of 0.020 m 3 m � 3 represents the worst case scenario for cosmic ray sensing of soil moisture. Comparison of cosmic ray soil moisture data and a distributed sensor soil moisture network in Southern Arizona indicates an RMSE of 0.011 m 3 m � 3 over a

Critical steps for the continuing advancement of hydrogeophysics

Special hydrogeophysics issues published by hydrology and geophysics journals, special sessions and workshops at conferences, and an increasing number of short courses demonstrate the growing interest in the use of geophysics for hydrologic investigations. The formation of the hydrogeophysics technical subcommittee of AGUs Hydrology section adds further evidence of the recognized significance of this growing interdisciplinary field. Given the clear value of nondestructive and nonintrusive imaging for subsurface investigations, we believe the advances in the adoption of existing geophysical methods, the development of novel methods, and the merging of geophysical and other data made in hydrogeophysics could be applied to a wide range of geological, environmental, and engineering applications.

Optimization of ERT Surveys for Monitoring Transient Hydrological Events Using Perturbation Sensitivity and Genetic Algorithms

available for completion of a survey is based on the physical process to be monitored. That is, all measurements A simple yet powerful algorithm is presented for the optimal allocacomprising a survey must be made rapidly compared with tion of electrical resistivity tomography (ERT) electrodes to maximize measurement quality. The algorithm makes use of a definition of the the rate at which the electrical conductivity changes in sensitivity of an ERT array to a series of subsurface perturbations. the subsurface. The time window divided by the time An objective function that maximizes the average sensitivity of a required per measurement defines the maximum number survey comprised of a large number of arrays is defined. A simple of arrays in a survey, A. Furthermore, the rate of change genetic algorithm is used to find the optimal ERT survey if there is of the electrical conductivity, EC, may change with time, a limited time allowed for survey. We further show that this approach leading to tightening or relaxation of the time frame, and allows for user definition of the sensitivity distribution within the a change in the number of arrays comprising a survey. targeted area. Results show clear improvement in the sensitivity distriElectrical resistivity tomography (note that for the purbution. The total sensitivity of the optimized survey compared with poses of this paper only surface electrodes are considtypically used surveys composed of one array type. This improved ered) has long been seen as a promising noninvasive, sensitivity will allow for more accurate monitoring of static and transient vadose zone processes. Furthermore, the algorithm presented nondestructive method (Edlefsen and Anderson, 1941). may be fast enough to allow for real-time optimization during timeRecent advances in ERT instrumentation and inversion lapse surveys. methods have increased the use of ERT for hydrologic investigations (Barker and Moore, 1998). These improvements include the use of multicore cable, addressed elecM subsurface hydrologic processes, and trodes, improved control and recording, and increased particularly those that occur within the vadose measurement accuracy, allowing for the use of very small zone, is difficult and expensive. Many monitoring methcurrents (i.e., tens to hundreds of milliamps). ods involve drilling for sampling or for access, which can Despite the increased use of electrical geophysical disturb the process under investigation and increase the methods, including ERT, applications are largely limited cost of the monitoring. Alternatives typically include burto monitoring static or very slowly changing conditions. ied instrumentation (e.g., time domain reflectometry, Electrical resistivity tomography has been applied to minthermocouples, or tensiometers). However, these point eral exploration (e.g., Griffiths and Barker, 1993), geomeasurements provide limited spatial resolution because logic mapping (e.g., Griffiths and Barker, 1993; Storz et they require a separate probe for each measurement al., 2000), groundwater table location (e.g., Yadav et al., point. In contrast, nondestructive, noninvasive geophysi1997), and groundwater contamination mapping (e.g., cal methods may offer high spatial and temporal resoluBuselli and Lu, 2001). Barker and Moore (1998) showed tion monitoring of shallow subsurface hydrological prothat ERT could be used to monitor transient processes cesses. in the shallow subsurface. However, their examples are Monitoring of hydrological problems differs in many limited to processes that occur slowly, on the order of ways from surveying for geological purposes. The most hours. An alternative to time-lapse monitoring involves important factor is the need to consider the measurement coupling the inversion of geophysical data with the physitime in hydrologic monitoring. Other differences include cal description of the monitored process (i.e., Richards’ the spatial scales of investigation (primarily that shallower equation in the case of infiltration), using the approach targets are of greater interest in most hydrologic investiof Seppanen et al. (2001). gations) and the required spatial resolution of the image As discussed above, the successful application of ERT (higher resolution is needed for hydrologic characterfor hydrological monitoring needs to address the requireization). ment for rapid measurement, accuracy, and high resoluAlthough the time required for a single ERT measuretion. Additional problems include the separation of the ment depends on many parameters, including the properresistivity to its components (primarily water content, ties of the subsurface at the time of measurement, it can chemical properties of the water, and electrochemical be treated as a constant (e.g., ≈15 s). The time window properties of the porous or fractured media). This can be particularly difficult under unsaturated conditions, or A. Furman and T.P.A. Ferré, Hydrology and Water Resources, Univ. when high solute concentrations are involved. of Arizona, Tucson, AZ 85721; A.W. Warrick, Soil, Water, and EnviIncreased accuracy can be achieved through noise reronmental Sciences, Univ. of Arizona, Tucson, AZ 85721; A. Furman, duction (Ritz et al., 1999), improved inversion algorithms, currently Inst. Soil, Water, and Environ. Studies, ARO, Volcani Ctr., assimilation of data obtained through other means (Yeh P.O. Box 6, Bet Dagan 50250, Israel. Received 12 May 2003. Original Research Paper. *Corresponding author (alexf@agri.gov.il). et al., 2002), and optimized selection of arrays used to Published in Vadose Zone Journal 3:1230–1239 (2004).

A Sensitivity Analysis of Electrical Resistivity Tomography Array Types Using Analytical Element Modeling

The analytic element method is used to investigate the spatial sensitivity of different electrical resistivity tomography (ERT) arrays. By defining the sensitivity of an array to a subsurface location we were able to generate maps showing the distribution of the sensitivity throughout the subsurface. This allows us to define regions of the subsurface where different ERT arrays are most and least sensitive. We compared the different arrays using the absolute value of the sensitivity and using its spatial distribution. Comparison is presented for three commonly used arrays (Wenner, Schlumberger, and double dipole) and for one atypical array (partially overlapping). Most common monitoring techniques use a single measurement to measure a property at a single location. The spatial distribution of the property is determined by interpolation of these measurements. In contrast, ERT is unique in that multiple measurements are interpreted simultaneously to create maps of spatially distributed soil properties. We define the spatial sensitivity of an ERT survey to each location on the basis of the sum of the sensitivities of the single arrays composing the survey to that location. With the goal of applying ERT for time-lapse measurements, we compared the spatial sensitivities of different surveys on a per measurement basis. Compared are three surveys based on the typical Wenner, Schlumberger, and double dipole arrays, one atypical survey based on the partially overlapping array, and one mixed survey built of arrays that have been shown to be optimal for a series of single perturbations. Results show the inferiority of the double dipole survey compared with other surveys. On a per measurement basis, there was almost no difference between the Wenner and the Schlumberger surveys. The atypical partially overlapping survey is superior to the typical arrays. Finally, we show that a survey composed of a mixture of array types is superior to all of the single array type surveys. By analyzing the spatial sensitivity of the single array, and most significantly the sensitivity of the ERT survey, we set the basis for quantitative measurement of subsurface properties using ERT, with applications to both static and transient hydrologic processes.

Spatial focusing of electrical resistivity surveys considering geologic and hydrologic layering

Electrical resistivity tomography ERT has shown great promise for monitoring transient hydrologic processes. One advantage of ERT under those conditions is the ability of a user to tailor the spatial sensitivity of an ERT survey through selection of electrode locations and electrode combinations. Recent research has shown that quadripoles can be selected in a manner that improves the independent inversion of ERT data. Our ulti

Near-Surface Water Content Estimation with Borehole Ground Penetrating Radar Using Critically Refracted Waves

0.1181 c v 0.1848 [2] Zero-offset profiling (ZOP) with borehole ground penetrating radar (BGPR) is a promising tool for profiling water contents in the The preceding analysis can be used to determine the subsurface to great depths with high spatial and temporal resolution. water content at any depth if the direct wave is associThe ZOP method relies on determining the velocity of an electromagated with the first energy to arrive at the receiver at netic (EM) wave that follows a direct path from the transmitter to the that depth. However, the direct route does not always receiver. However, near the ground surface, critically refracted energy have the shortest travel time. Rather, under some condithat travels along the ground surface at the velocity of an EM wave in air may arrive before direct waves that travel through the subsurface. If tions, a critically refracted wave may arrive before the the critically refracted waves are mistakenly interpreted to be direct direct arrival (Fig. 1). Critical refraction occurs at any waves, the water content will be underestimated. As a result, the water interface across which the velocity increases (Sheriff content near the ground surface cannot be determined using standard and Geldart, 1995). The contrast between the EM wave BGPR analysis. We refer to the depth below which direct waves are velocity in air (vair 0.3 m ns 1) and in soil (0.17 vsoil the first to arrive as the refraction termination depth. An alternative 0.05 m ns 1) gives rise to critical refraction (Bohidar analysis is presented to determine the water content above the refracand Hermance, 2002) when the antennae are located tion termination depth using the slope of the travel time vs. depth below the ground surface. Critically refracted energy profile. Additionally, guidelines are presented to predict the refraction arrives at the receiver through secondary waves genertermination depth for known near-surface water contents. ated at the surface (Parkin et al., 2000), where a head wave is created. The receiver can intersect the head wave at any depth. C ground penetrating radar is a highIn picking the first arrival, it is not possible to distinfrequency EM method that can be used to profile guish a critically refracted arrival from a direct arrival the water content in the subsurface (Binley et al., 2001, from initial inspection. This complicates the analysis of 2002a,b). In ZOP mode, a transmitting antenna and a water content near the ground surface. If the travel time receiving antenna are lowered to the same depth within of a first-arriving critically refracted wave is assumed nonmetallic access tubes and an EM wave is propagated to correspond to a direct wave, the water content at the between them. Assuming that losses are low enough to measurement depth will be underestimated. For exampermit identification of transmitted energy, the travel ple, near-surface travel times, such as those shown by time of the first-arriving energy, on the order of a few Kuroda et al. (2002) can suggest low water contents in tens of nanoseconds, is measured. The velocity of the the shallow subsurface, even under ponded constant EM wave is calculated for a known antennae separation, infiltration conditions. As a result, the water content proassuming that the wave traveled along a direct path from file can only be determined quantitatively below the the transmitter to the receiver. The unitless apparent depth at which direct waves arrive before critically rerelative dielectric permittivity, Ka, of the medium can fracted waves. We refer to this depth as the refraction be calculated directly from the EM velocity, assuming termination depth, zrtd. that the frequency-dependent dielectric loss is relatively In this study, equations are derived to relate the resmall (Davis and Annan, 1989): fraction termination depth to the velocity of EM energy through the near-surface soil and to the antennae sepaK 1/2 a c v [1] ration. This is similar to the development presented by Hammon et al. (2002). The analysis is extended here to where c is the speed of light in free space (0.3 m ns 1), determine the near-surface water content and to develop which is assumed to be equal to the velocity in air, and guidelines to predict the refraction termination depth. v (m ns 1) is the velocity of propagation of the EM wave It is important to note that these developments explicthrough the medium. The volumetric water content, itly assume that the dielectric permittivity of the medium (cm3 cm 3), can be determined from the apparent dielecis both homogeneous and isotropic in the shallow subtric permittivity using the linearized form of the empirisurface. Using existing equipment to collect ZOP data, cal relationship presented by Topp et al. (1980) as given it is not possible to determine dielectric anisotropy. Simby Ferré et al. (1996): ilarly, ZOP measurements cannot be used to determine horizontal variations in dielectric permittivity between BGPR antennae. The precision with which vertical hetD.F. Rucker and T.P.A. Ferré, Department of Hydrology and Water erogeneity due to soil layering or water content distribuResources, University of Arizona. P.O. Box 210110, Tucson, AZ 85721. Received 3 Dec. 2002. Original Research Paper. *CorrespondAbbreviations: BGPR, borehole ground penetrating radar; EM, elecing author (druck@hwr.arizona.edu). tromagnetic; TDR, time domain reflectometry; ZOP, zero-offset profiling. Published in Vadose Zone Journal 2:247–252 (2003).

Tier-Scalable Reconnaissance Missions For The Autonomous Exploration Of Planetary Bodies

A fundamentally new (scientific) reconnaissance mission concept, termed tier-scalable reconnaissance, for remote planetary (including Earth) atmospheric, surface and subsurface exploration recently has been devised that soon will replace the engineering and safety constrained mission designs of the past, allowing for optimal acquisition of geologic, paleohydrologic, paleoclimatic, and possible astrobiologic information of Venus, Mars, Europa, Ganymede, Titan, Enceladus, Triton, and other extraterrestrial targets. This paradigm is equally applicable to potentially hazardous or inaccessible operational areas on Earth such as those related to military or terrorist activities, or areas that have been exposed to biochemical agents, radiation, or natural disasters. Traditional missions have performed local, ground-level reconnaissance through rovers and immobile landers, or global mapping performed by an orbiter. The former is safety and engineering constrained, affording limited detailed reconnaissance of a single site at the expense of a regional understanding, while the latter returns immense datasets, often overlooking detailed information of local and regional significance.

Correcting Water Content Measurement Errors Associated with Critically Refracted First Arrivals on Zero Offset Profiling Borehole Ground Penetrating Radar Profiles

Borehole ground penetrating radar (BGPR) operated in zero offset profiling (ZOP) mode has promise for monitoring rapidly changing water contents within the subsurface. However, the coexistence of multiple travel paths through the subsurface can give rise to measurement errors. Specifically, in layered systems with sharp changes in water content with depth, critically refracted waves may arrive before direct waves at some depths. Velocity profiles are determined based on analyses of the travel time of the first-arriving energy at each depth. Therefore, correct velocity analysis requires that these travel times be classified according to the path followed by the first-arriving energy. We establish criteria that can be used to identify first-arriving critically refracted waves from travel time profiles. Through hypothetical examples and a field experiment, we demonstrate that these criteria allow for more accurate determination of the water content profile. However, these corrections are limited if thin, high water content layers are present in the subsurface.