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An important development providing improved resolution in surface NMR images has been the advent of the inversion of complex NMR signals (Braun et al., 2005), instead of an amplitude only inversions. The signal phase is generally attributed only to conductivity effects, and a secondary geophysical survey is used to characterize the subsurface conductivity structure allowing the phase to be well constrained in the inversion. We propose a novel strategy for data acquisition motivated by similar reasoning as studies demonstrating improved resolution through inversion of a complex signal.

Conductivity behavior of continuum percolation in restricted two-dimensional domains is simulated by considering systems of randomly distributed disks. The domain is restricted in that conducting objects are permitted to lie in only a portion of the domain. Such a restricted domain might better approximate some natural systems. Simulations of two-dimensional systems, based on three distributions of local conductances, are examined and found to demonstrate a power-law behavior with conductivity exponents smaller than those arising in regular lattice and continuum percolation

Laboratory measurements of the effects of steam injection on the electrical conductivity of sands can aid in the interpretation of electrical surveys used to monitor subsurface steam‐injection projects. The effect of variations in injected steam quality was measured in the experiments presented here. The injection of low‐quality steam, boiled from a 0.01 mol/L NaCl solution, into clean sand saturated with 0.01 mol/L NaCl, resulted in a net decrease in conductivity and a constant equilibrium conductivity in the steam zone. The injection of high‐quality steam, using the same saturating and injection salinities, caused the conductivity to first drop to a minimum and then to increase to an equilibrium value similar to that seen in the low‐quality injection. The local conductivity minimum deepened with time and traveled with the steam front. The appearance of the conductivity minimum at the steam front can be attributed to the formation of a dilution bank, which temporarily decreases the local salinity. The extent of the dilution increases with time, resulting in the decrease of the conductivity over time. The conductivity then increases as injected salt moves through the sand. The steam quality controls the appearance of this minimum because it determines the relative speeds of the steam front and the steam liquid: a minimum will not occur if the steam front moves more slowly than the steam liquid.

The dielectric constant of a partially saturated sandstone varies as a function of the level of water saturation. Experimental data indicate that rock/water interaction, at low saturations, has a large effect on the measured dielectric response. To theoretically predict the dielectric constant of the rock/water/air system, this rock/water interaction must be accounted for by including the effect of the water wetting the rock solid. Alternatively, if the rock/water interaction can be eliminated, a three‐component (dry rock, water, and air) mixing law can be used to model the dielectric behavior. In this laboratory study, a chemical treatment is used to change four water‐wet sandstones into hydrophobic sandstones. In three hydrophobic samples the rock/water interaction, seen in the data for the water‐wet samples, is eliminated and the dielectric constant of the partially saturated sandstones can be simply modeled as a dry rock/water/air system. This experimental study illustrates the importance of the chemical state of the rock surfaces in determining the dielectric behavior of sandstones.

A field experiment was conducted in the High Plains Aquifer, central United States, to explore the mechanisms governing the non-invasive Surface NMR (SNMR) technology. We acquired both SNMR data and logging NMR data at a field site, along with lithology information from drill cuttings. This allowed us to directly compare the NMR relaxation parameter measured during logging, T2, to the relaxation parameter T2 * measured using the SNMR method. The latter can be affected by inhomogeneity in the magnetic field, thus obscuring the link between the NMR relaxation parameter and the hydraulic conductivity of the geologic material. When the logging T 2 data were transformed to pseudo- T2 * data, by accounting for inhomogeneity in the magnetic field and instrument dead time, we found good agreement with T 2 * obtained from the SNMR measurement. These results, combined with the additional information about lithology at the site, allowed us to delineate the physical mechanisms governing the SNMR measurement. Such understanding is a critical step in developing SNMR as a reliable geophysical method for the assessment of groundwater resources. Citation: Knight, R., E. Grunewald, T. Irons, K. Dlubac, Y. Song, H. N. Bachman, B. Grau, D. Walsh, J. D. Abraham, and J. Cannia (2012), Field experiment provides ground truth for surface nuclear magnetic resonance measurement, Geophys. Res. Lett., 39, L03304

Borehole nuclear magnetic resonance (NMR) measurements may be used to estimate permeability at several depth intervals in the saturated zone. The empirical Schlumberger-Doll Research (SDR) equation utilizes NMR-derived porosity and an average NMR relaxation time (representative of surface-area-to-volume ratio) to estimate permeability (kSDR) using three fitting parameters. While it would be possible to optimize the fitting parameters for each depth, it is more useful to have SDR equation parameter values that are valid for the entire length of a borehole or for a study site.

There is increasing use, throughout the world, of groundwater as the primary source of freshwater. The evaluation and management of this resource requires information about the extent and connectivity of groundwater aquifers, the contained volume of producible water, the changes in stored water, and the processes that can impact the quantity and/or quality of the water. Such information is required at a density of spatial and temporal sampling best provided by various forms of geophysical data. For the past decade, we have been working in partnerships with groundwater districts and managers to advance the use of geophysical methods as a central component of groundwater evaluation and management. Examples include the use of surface and logging nuclear magnetic resonance to estimate water content and hydraulic conductivity, electrical resistivity tomography for imaging saltwater intrusion along the California coast, and satellite InSAR data for estimating changing hydraulic head levels in confined aquifers in the San Luis Valley, Colorado. Such examples illustrate the tremendous potential for – and need for – geophysical methods to ensure the long-term health of our groundwater resources.

In the San Luis Valley (SLV), Colorado legislation passed in 2004 requires that hydraulic head levels in the confined aquifer system stay within the range experienced in the years 1978–2000. While some measurements of hydraulic head exist, greater spatial and temporal sampling would be very valuable in understanding the behavior of the system. Interferometric synthetic aperture radar (InSAR) data provide fine spatial resolution measurements of Earth surface deformation, which can be related to hydraulic head change in the confined aquifer system. However, change in cm-scale crop structure with time leads to signal decorrelation, resulting in low quality data. Here we apply small baseline subset (SBAS) analysis to InSAR data collected from 1992 to 2001. We are able to show high levels of correlation, denoting high quality data, in areas between the center pivot irrigation circles, where the lack of water results in little surface vegetation. At three well locations we see a seasonal variation in the InSAR data that mimics the hydraulic head data. We use measured values of the elastic skeletal storage coefficient to estimate hydraulic head from the InSAR data. In general the magnitude of estimated and measured head agree to within the calculated error. However, the errors are unacceptably large due to both errors in the InSAR data and uncertainty in the measured value of the elastic skeletal storage coefficient. We conclude that InSAR is capturing the seasonal head variation, but that further research is required to obtain accurate hydraulic head estimates from the InSAR deformation measurements.

Randomised controlled trials (RCTs) are generally regarded as the gold standard for evaluating health care interventions. The level of uncertainty around a trial's estimate of effect is, however, frequently linked to how successful the trial has been in recruiting and retaining participants. As recruitment is often slower or more difficult than expected, with many trials failing to reach their target sample size within the timescale and funding originally envisaged, the results are often less reliable than they could have been. The high number of trials that require an extension to the recruitment period in order to reach the required sample size potentially delays the introduction of more effective therapies into routine clinical practice. Moreover, it may result in less research being undertaken as resources are redirected to extending existing trials rather than funding additional studies. Poor recruitment to publicly-funded RCTs has been much debated but there remains remarkably little clear evidence as to why many trials fail to recruit well, which recruitment methods work, in which populations and settings and for what type of intervention. One proposed solution to improving recruitment and retention is to adopt methodology from the business world to inform and structure trial management techniques. We review what is known about interventions to improve recruitment to trials. We describe a proposed business approach to trials and discuss the implementation of using a business model, using insights gained from three case studies.

Background Loss to follow-up of trial participants represents a threat to research validity. To date, interventions designed to increase participants’ awareness of benefits to society of completing follow-up, and the impact of a telephone call from a senior female clinician and researcher requesting follow-up have not been evaluated robustly. Purpose Trial 1 aimed to evaluate the effect on trial follow-up of written information regarding the benefits of participation to society. Trial 2 aimed to evaluate the effect on trial follow-up of a telephone call from a senior female clinician and researcher. Methods Two single-blind randomized controlled trials were nested within a larger trial, Txt2stop. In Trial 1, participants were allocated using minimization to receive a refrigerator magnet and a text message emphasizing the benefits to society of completing follow-up, or to a control group receiving a simple reminder regarding follow-up. In Trial 2, participants were randomly allocated to receive a telephone call from a senior female clinician and researcher, or to a control group receiving standard Txt2stop follow-up procedures. Results Trial 1: 33.5% (327 of 976) of the intervention group and 33.8% (329 of 974) of the control group returned the questionnaire within 26 weeks of randomization, risk ratio (RR) 0.99; 95% confidence interval (CI) 0.88–1.12. In all, 83.3% (813 of 976) of the intervention group and 82.2% (801 of/974) of the control group sent back the questionnaire within 30 weeks of randomization, RR 1.01; 95% CI 0.97, 1.05. Trial 2: 31% (20 of 65) of the intervention group and 32% (20 of 62) of the control group completed trial follow-up, RR 0.93; 95%CI 0.44, 1.98. Conclusions In presence of other methods to increase follow-up neither experimental method (refrigerator magnet and text message emphasizing participation's benefits to society nor a telephone call from study's principal investigator) increased participant follow-up in the Txt2stop trial.

Proton nuclear magnetic resonance (NMR) has the potential to provide a wealth of information about the physical and chemical properties of geological materials. Probed in the NMR measurement is the pore-scale environment of hydrogen nuclei (protons) in the water, or the other fluids, within the pore space of a soil, sediment or rock. Like many other forms of geophysical measurement, NMR has been developed to obtain estimates of material properties, defined at the macroscopic scale, for applications at the field-scale. But to truly understand the measurement, and thus improve the accuracy of the derived material-property estimates, we need to understand the pore-scale physics in which molecular-scale processes interact with nanoscale to macroscale heterogeneity to yield the field-scale response.

Throughout the western U.S., and in numerous other regions around the world, there is a growing need to monitor and regulate groundwater levels. Remotely sensed data offer spatially and temporally dense measurements over very large areas. of particular relevance to the characterization of groundwater systems is interferometric Synthetic Aperture Radar (inSAR), a remote sensing method that maps relative ground surface deformation. We present a methodology, using an advanced inSAR processing technique known as Small Baseline Subset Analysis (SBAS), that allows us to observe seasonal deformation and relate it to changes in the thickness of the confined aquifer due to recharge and withdrawal of groundwater. We have applied our methodology in the San Luis Valley (SLV), an 8000 km2 region in southern Colorado that is home to a thriving agricultural economy. the state has developed a hydrogeologic database and a groundwater flow model in order to quantitatively study possible management schemes for water resources in the SLV. Because well-sampled hydrographs, accurate lithologic logs and high quality aquifer tests are scarce, the model has not yet been able to characterize the spatially heterogeneous, time-varying behavior of the groundwater system. the more complete coverage offered by inSAR allows us to augment these sparse measurements. We infer hydraulic head change in the confined aquifer system by combining inSAR measurements with aquifer test data and lithologic logs. Conversely, where well-sampled hydrographs exist we use inSAR data in conjunction with the thickness of the producing zone to estimate the specific storage of the producing sediments. We propose a framework to combine all existing data together with the inSAR deformation estimates in order to improve the groundwater flow model. Ultimately all data types can be incorporated within this mathematical framework to better inform the decision-making process for groundwater management.

A mathematical formulation is presented which incorporates the effects of surface phenomena into the dielectric response of a heterogeneous medium composed of a background matrix and embedded ellipsoidal inclusions. Initially, the individual inclusions are viewed as two-component confocally layered systems in which the surface phenomena occur in the outer shell. An approximation for a homogeneous inclusion is obtained by requiring the outer shell to be infinitesimally thin while the total numbers of unit charge carriers and unit dipoles within this region are conserved. The resulting expression for the response of an inclusion explicitly depends on both inclusion shape and size. Numerical simulations demonstrate that the effects of surface conduction on the dielectric response of a heterogeneous medium change significantly as a function of inclusion shape.

Background Recruitment is a major challenge for trials but there is little evidence regarding interventions to increase trial recruitment. We report three controlled trials of interventions to increase recruitment to the Txt2stop trial. Purpose To evaluate: Trial 1. The impact on registrations of a text message regarding an online registration facility; Trial 2. The impact on randomizations of sending £5 with a covering letter to those eligible to join the trial; Trial 3. The impact on randomizations of text messages containing quotes from existing participants. Methods Single blind controlled trials with allocation concealment. Interventions: Trial 1: A text message regarding our new online registration facility; Trial 2: A letter with £5 enclosed; Trial 3: A series of four text messages containing quotes from participants. The control group in each trial received standard Txt2stop procedures. Results Trial 1: 3.6% (17/470) of the intervention group and 1.1% (5/467) of the control group registered for the trial, risk difference 2.5% (95% CI 0.6—4.5). 0% (0/ 470) of the intervention group and 0.2% (1/467) of the control group registered successfully online, risk difference —0.2 (95% CI —0.6—0.2); Trial 2: 4.5% (11/246) of the intervention group and 0.4% (1/245) of the control group were randomized into the Txt2stop trial, risk difference 4.0% (95% CI 1.4—6.7); Trial 3: 3.5% (14/405) of the intervention group and 0% (0/406) of the control group were randomized into the Txt2stop trial, risk difference 3.5 (95% CI 1.7—5.2). Limitations There were no baseline data available for trial 1. Allocation of participant IDs in trials 2 and 3 were systematic. Conclusion Sending a text message about an online registration facility increased registrations to Txt2stop, but did not increase online registrations. Sending a £5 reimbursement for participants’ time and sending text messages containing quotes from existing participants increased randomizations into the Txt2stop trial. Clinical Trials 2010; 7: 265—273. http://ctj.sagepub.com

Summary Calculating permeability from NMR data has been accomplished through the use of empirical relationships based on the Kozeny-Carman (K-C) relationship. The parameters in the K-C relationship are surface area to volume ratio (S/V), porosity and tortuosity. Due to the link between NMR measured parameters and S/V, there is potential for using NMR data to obtain estimates of permeability, where the NMR parameter T2ML replaces S/V. To examine the validity of the K-C relationship in an anisotropic media, two systems were examined, 1) anisotropic grain packs and 2) layered sand/clay systems. In the anisotropic grain pack, permeability was calculated numerically in the vertical (kv) and horizontal (kh) directions using lattice-Boltzmann and compared with K-C derived permeability (kK-C). Results show that K-C was a good predictor of both kv and kh. In the sand/clay layered system, values of permeability and porosity were assigned to the sand and clay, and the volume fraction of clay was increased from 0 to 1.0. The kv and kh were calculated analytically. Predicted permeability values were calculated with the K-C relationship using an average S/V, an amplitude-weighted average S/V, and an amplitude weighted mean log S/V (which corresponds to T2ML). Results show strong directional dependence in the agreement between the K-C-predicted permeability and true permeability. When S/V is calculated in a way so that it corresponds to T2ML, K-C is a good predictor of kh up to a critical volume fraction clay content; beyond this, K-C underestimates kh. When S/V is calculated in a way so that it corresponds to the average T2, the K-C relationship was a good predictor of kv only after a critical volume fraction clay content. Further research is needed to quantify the errors associated with the permeability predicted from NMR data using the K-C relationship when averaging over an anisotropic material occurs.

Summary Non-invasive surface nuclear magnetic resonance (SNMR) measurements can yield direct and quantitative estimates of water content in the near surface (Hertrich 2008). Fundamental to the inversion and analysis of SNMR data is a basic assumption that the measured signal exhibits an exponential decay. It can be shown, however, that in the presence of an inhomogeneous magnetic field, the signal decay may be non-exponential in form. We show that in such cases, erroneous assumptions about the decay shape could lead to significant errors in estimated water content. Here, we present numerical experiments to illustrate how environmental conditions will influence the shape of the SNMR signal and the resulting accuracy of water content

Microscopic fluid distribution can have a significant effect on the dielectric properties of partially saturated rocks. Evidence of this effect is found in the laboratory data presented by Knight and Nur in which different methods for controlling saturation produced very different results for the dependence of the dielectric response on water saturation. In this study, previously derived models for the dielectric response of a heterogeneous medium are generalized and the case of a pore space occupied by multiple pore fluids is considered. By using various geometrical distributions of water and gas, it is observed that both the pore geometry in which saturation conditions are changing and the gas–water geometry within a given pore space are critical factors in determining the effective dielectric response of a partially saturated rock. As an example, data for a tight gas sandstone undergoing a cycle of imbibition and drying are analysed. Previous research has demonstrated that significantly different microscopic fluid distributions result from the application of these two techniques to control the level of water saturation. By approximating these microscopic fluid distributions using simple geometrical models, good agreement is found between experimental data and calculated dielectric properties.

Laboratory measurements of the resistivity of three sandstone samples collected during imbibition (increasing Sw) and drainage (decreasing Sw) show pronounced hysteresis in resistivity throughout much of the saturation range. The variation in resistivity can be related to changes in pore‐scale fluid distribution caused by changes in saturation history. The form of the hysteresis is such that resistivity measured during imbibition is consistently less than that measured, at the same saturation, during drainage. This can be attributed to the presence of conduction at the air/water interface in partially saturated samples; an effect that is enhanced by fluid geometries associated with the imbibition process. The results of this study suggest that the dependence of geophysical data on saturation history should be considered when interpreting data from the unsaturated zone.

Pore‐scale fluid distribution has a significant effect on the physical properties of a partially saturated porous medium. Experimental data for the dielectric response and elastic wave velocities for a tight gas sandstone undergoing a cycle of water saturation change through imbibition and drainage are analyzed. Mathematical formulations describing the internal geometrical configuration of a porous medium in terms of a rock matrix background with embedded oblate spheroidal inclusions representing the porosity are used to theoretically predict the dielectric constant and elastic wave velocities of the partially saturated sandstone. Simple geometrical models, incorporating homogeneous and heterogeneous inclusions, are used to simulate the pore‐scale fluid distribution which should result from the two saturation methods employed. It is found that these simple scenarios accurately predict the functional form and magnitude of the observed saturation‐induced hysteresis in the experimental data for both the dielectric constant and the elastic wave velocities.

[1] In this laboratory study, we assessed the measurement of nuclear magnetic resonance (NMR) relaxation times as a means of monitoring iron mineralization processes. We conducted experiments in which columns containing ferrihydrite-coated quartz sand reacted with aqueous Fe(II) solutions to form goethite, lepidocrocite and magnetite. An observed increase in the volume of water relaxing with long relaxation times in the NMR relaxation time distribution corresponds to the formation of goethite and lepidocrocite; a decrease in the average (mean log) relaxation time, and a broadening of the relaxation time distribution, corresponds to the formation of magnetite. These results indicate that NMR relaxation times are sensitive to changes in iron mineralogy and illustrate the potential use of NMR for monitoring iron mineralization processes. Citation: Keating, K., R. Knight, and K. J. Tufano (2008), Nuclear magnetic resonance relaxation measurements as a means of monitoring iron mineralization processes, Geophys. Res. Lett., 35, L19405

[1] The modeling of fluvial systems is constrained by a lack of spatial information about the continuity and structure of streambed sediments. There are few methods for noninvasive characterization of streambeds. Invasive methods using wells and cores fail to provide detailed spatial information on the prevailing architecture and its continuity. Geophysical techniques play a pivotal role in providing spatial information on subsurface properties and processes across many other environments, and we have applied the use of one of those techniques to streambeds. We demonstrate, through two examples, how electrical resistivity imaging can be utilized for characterization of subchannel architecture. In the first example, electrodes installed in riparian boreholes and on the streambed are used for imaging, under the river bed, the thickness and continuity of a highly permeable alluvial gravel layer overlying chalk. In the second example, electrical resistivity images, determined from data collected using electrodes installed on the river bed, provide a constrained estimate of the sediment volume behind a log jam, vital to modeling biogeochemical exchange, which had eluded measurement using conventional drilling methods owing to the boulder content of the stream. The two examples show that noninvasive electrical resistivity imaging is possible in complex stream environments and provides valuable information about the subsurface architecture beneath the stream channels.

[1] Laboratory NMR relaxation time (T2) measurements at 2 MHz were used to quantify the amount of toluene and bis(2-ethylhexyl)phthalate (BEHP) in the presence of water in sand-clay mixtures. NMR measurements were made on water, toluene, or BEHP in each sand-clay mixture to understand the role of clay and iron content on the T2 of each fluid; we then made NMR measurements on mixtures of water and the contaminants in each sand-clay mixture. Our ability to resolve the NMR response of the contaminant from that of water and to quantify the amount of the contaminant varied significantly with the clay and iron content of the sand-clay mixtures. Detection limits as low as 3% contaminant (i.e., percent of the pore fluid, by weight) were observed in some sand-clay mixtures. Our results suggest that NMR measurements could quantify toluene if some clay is present but are unlikely to be useful for quantifying BEHP.

Laboratory data have been collected during a continuous imbibition/drainage experiment that show a clear dependence of elastic wave velocities on the details of the pore scale distribution of water and air in a sandstone. Compressional wave velocity (Vp) was measured at a frequency of 1 MHz; shear wave velocity (Vs) was measured at a frequency of 600 kHz. During the experiment, Vp showed little variation with the level of water saturation (Sw) during increasing Sw through imbibition until Sw = 0.80, at which point Vp increased dramatically. When Sw was decreased, pronounced saturation-induced hysteresis was observed in the region 03 0.4. As a simple model, we consider the imbibition process as producing a partially saturated state in all pores; i.e. all pores contain both air and water. The drainage process, in contrast, favors the existence of either air-filled or water-filled pores. As elastic wave velocities are very sensitive to the saturation state in the smaller, “crack-like” pores, these variations in fluid distribution cause related variations in velocities.

Experimental data for the real part of the dielectric constant (K′) of three sandstone samples are considered as a function of the level of water saturation [Formula: see text] in the frequency range 60 kHz to 4 MHz. Existing theoretical models have previously shown poor agreement with K′ versus [Formula: see text] data for rock samples, undoubtedly due to the complexity involved in adequately accounting for geometrical and electrochemical effects. In analyzing the data presented here, we find a pronounced increase in K′ in the low saturation region which in all cases can be attributed to the establishment of geometrical and surface effects associated with the rock‐water interface. When this increase in K′ is accounted for by defining wetted matrix parameters, the data show excellent agreement with existing theoretical models.

Surface-based ground-penetrating radar data were collected at the Hanford Site in Washington, U.S.A. to assess the use of radar reflection images as a means of quantifying the spatial variability of subsurface water content. Available at the selected test site were two sets of water content data derived from neutron probe measurements that had been made to a depth of ~18 m in 32 wells. The comparison of probe-derived water content data, synthetic radar data, and the acquired radar data indicated a good correspondence between the changes in probe-derived water content and the location of reflections in the radar data. Geostatistical analysis was conducted on the two sets of probe-derived water content values and the amplitudes of the reflections in the radar reflection image to determine the horizontal correlation length of water content. The experimental semivariograms for the water content data were fit with a single exponential model with a correlation length of 10 m. The semivariogram for the radar data was fit with a nested structure containing a dominant long-range structure with a correlation length of 14 m, and a smaller-scale structure with a correlation length of 0.3 m. Quantifying the scale triplet – the spacing, extent, and support –more » for the two forms of measurement provided a framework for comparing and assessing the derived correlation structures.« less

Background Securing and managing finances for multicentre randomised controlled trials is a highly complex activity which is rarely considered in the research literature. This paper describes the process of financial negotiation and the impact of financial considerations in four UK multicentre trials. These trials had met, or were on schedule to meet, recruitment targets agreed with their public-sector funders. The trials were considered within a larger study examining factors which might be associated with trial recruitment (STEPS). Methods In-depth semi-structured telephone interviews were conducted in 2003–04 with 45 individuals with various responsibilities to one of the four trials. Interviewees were recruited through purposive and then snowball sampling. Interview transcripts were analysed with the assistance of the qualitative package Atlas-ti. Results The data suggest that the UK system of dividing funds into research, treatment and NHS support costs brought the trial teams into complicated negotiations with multiple funders. The divisions were somewhat malleable and the funding system was used differently in each trial. The fact that all funders had the potential to influence and shape the trials considered here was an important issue as the perspectives of applicants and funders could diverge. The extent and range of industry involvement in non-industry-led trials was striking. Three broad periods of financial work (foundation, maintenance, and resourcing completion) were identified. From development to completion of a trial, the trialists had to be resourceful and flexible, adapting to changing internal and external circumstances. In each period, trialists and collaborators could face changing costs and challenges. Each trial extended the recruitment period; three required funding extensions from MRC or HTA. Conclusion This study highlights complex financial aspects of planning and conducting trials, especially where multiple funders are involved. Recognition of the importance of financial stability and of the need for appropriate training in this area should be paralleled by further similar research with a broader range of trials, aimed at understanding and facilitating the conduct of clinical research.

[1] Rock physics attempts to relate the geophysical response of a rock to geologic properties of interest, such as porosity, lithology, and fluid content. The geophysical properties estimated by field-scale surveys, however, are impacted by additional factors, such as complex averaging of heterogeneity at the scale of the survey and artifacts introduced through data inversion, that are not addressed by traditional approaches to rock physics. We account for these field-scale factors by creating numerical analogs to geophysical surveys via Monte Carlo simulation. The analogs are used to develop field-scale rock physics relationships that are appropriate for transforming the geophysical properties estimated from a survey into geologic properties. We demonstrate the technique using a synthetic example where radar tomography is used to estimate water content.

In order to obtain tomograms with the highest possible resolution from crosshole ground-penetrating radar (GPR) data, raypaths covering a wide range of angles between the boreholes are required. In practice, however, the inclusion of high-angle ray data in crosshole GPR inversions often results in tomograms dominated by inversion artifacts. We show that problems associated with high-angle rays in crosshole GPR tomography can arise from the standard assumption that all first-arriving energy travels directly between the centers of the antennas. As shown with a synthetic data set, an inversion scheme that accounts for radiation and reception from both the antenna feed and end points can lead to a more accurate reconstruction of the subsurface velocity model.

[1] Geostatistical analysis of a ground penetrating radar reflection image can be used to quantify the maximum correlation direction and the range of horizontal and sub-horizontal radar reflections. A review of previous work, and an analysis of a photograph of layered sediments, suggest that the vertical resolution of a radar image strongly affects its lateral correlation structure. Numerical modeling was used to generate synthetic radar sections and investigate the effect of the vertical resolution of the radar measurement on the link between the correlation structure of the radar reflections and the true correlation structure of subsurface water content. The horizontal range of the radar reflections decreased as the vertical resolution improved, closely matching that of the water content when the vertical resolution was approximately equal to the vertical range of the water content.

[1] Ground-penetrating radar (GPR) can be used to obtain information about the variation in the dielectric constant of the subsurface. The sensitivity of a soil's dielectric constant to the presence of water therefore makes water content estimation by GPR possible. The dielectric constant derived from GPR data, however, is also influenced by the geometric distribution of water in the subsurface. We show that this causes the relationship between the dielectric constant and water content to become scale dependent in complex geologic systems. We have derived dielectric constant-water content relationships that account for subsurface geometries in spatially correlated random media that can be characterized using geostatistics. From these relationships we illustrate that the importance of scale effects are strongly dependent on the variance and the anisotropy of the water content in the subsurface; in some cases, ignoring scale effects will not significantly impact the estimation of water content, while in other cases, large biases can occur. This work provides a conceptual framework for the predictive modeling of field-scale dielectric constant-water content relationships.

Accurate information is needed about the hydrogeologic properties at the Department of Energy Hanford site in southeastern Washington. This information is required in order to model the fate and transport of subsurface contaminants. Ground penetrating radar (GPR) data were collected in two areas at Hanford to assess the potential usefulness of GPR for site characterization. The sediments at Hanford are such that we found the penetration depth of the radar measurement to be limited to approximately 10 m, in most locations, when using 100 MHz antennas. This means that GPR can only be used to obtain information about the sediments in the top of the vadose zone. One survey objective involved the use of GPR to locate clastic dikes, which can have a large impact on contaminant transport. The dikes were clearly imaged as diffraction hyperbolae in the GPR data. The second objective was to test the use of GPR to describe the spatial variability in water content. Geostatistical analysis of GPR data from a test site yielded a correlation length comparable to that obtained from analysis of neutron probe data from the site. This result suggests that GPR data can be used to quantify the subsurface distribution in water content at Hanford.

Layered geological materials can potentially exhibit significant velocity anisotropy when the thickness of the layers is small compared to the propagating wavelength. For electromagnetic (EM) waves in the ground-penetrating radar (GPR) frequency range, velocity anisotropy due to fine-scale layering can often be ignored in the saturated zone because changes in velocity between layers are relatively small. In the vadose zone, however, significant velocity anisotropy can result in a layered system due to the strong dependence of dielectric properties on saturation, and the pronounced saturation heterogeneity that can exist. As the overall saturation in the vadose zone decreases, finegrained layers preferentially retain water while coarse-grained layers preferentially drain; this process enhances the dielectric contrast between the layers. If anisotropy is present in the subsurface and not accounted for in a crosswell tomographic inversion, significant artifacts can appear in the resulting tomogram and important subsurface features can thus be obscured. Analysis of crosswell GPR data collected above and below the water table at a site near Abbotsford, British Columbia shows that, in the vadose zone, anisotropy is more significant than in the saturated zone.

Moisture content is a critical parameter affecting both liquid-phase and vapor-phase contaminant transport in the vadose zone. The objective of our three-year research project is to determine the optimal way to use radar methods as a non-invasive means of determining in situ moisture content.

Wavelet dispersion caused by frequency-dependent attenuation is a common and significant problem in ground-penetrating radar (GPR) data. In the radar image, it is displayed as a characteristic 'blurriness' that increases with depth. Correcting for wavelet dispersion in GPR data is an important step that should be performed before either qualitative interpretation or quantitative determination of subsurface electrical properties are attempted. Over the bandwidth of a GPR wavelet, the attenuation of electromagnetic waves in many geologic materials is approximately linear with frequency. For this reason, the change in shape of a radar pulse as it propagates through these materials can be described using one parameter, Q*, which is related to the slope of the linear region. Assuming that all subsurface materials can be characterized by some Q* value, the problem of estimating and correcting for wavelet dispersion in GPR data becomes one of determining Q* in the subsurface and deconvolving its effects through the use of an inverse Q filter. A method for the estimation of Q* from GPR data based on a technique developed for seismic attenuation tomography is presented. Essentially, Q* is determined from the downshift in the dominant frequency of the GPR wavelet with time down a trace. Once Q* has been obtained, we propose an inverse Q filtering technique based on a causal, linear model for constant Q as a means of removing wavelet dispersion. Initial tests on field data indicate that this technique is very effective at enhancing the resolution of the GPR image.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The 1 H NMR spin–lattice relaxation time, T 1 , of saturated sands depended on the chemistry of the pore fluid, pore size distribution, and relaxivity of the surface. In the absence of paramagnetic impurities, surface relaxivities of quartz sand and silica gel samples of known porosity and surface area at any pH were lower than any previously reported values. Relaxation rate of the bulk pore fluid increased linearly with increasing Fe(III) concentration and varied with speciation of the ion. With only 0.01% of the silica surface sites occupied by sorbed Fe(III) ions, surface relaxivity increased by an order of magnitude. In addition, low concentrations of Fe(III)-bearing solid phases present as surface coatings or as separate mineral grains increased surface relaxation as much as two orders of magnitude. We believe that observations of relatively constant surface relaxivity in rocks by previous researchers were the result of consistently high surface concentrations of paramagnetic materials.

Summary Ground penetrating radar (GPR) is a near-surface geophysical technique that can provide high resolution images of modern and ancient depositional systems. The analysis of GPR images can improve the understanding of the lithologic variation and internal structure of ancient reservoirs, in much the same way that studies of analog outcrops are used. Geostatistical methods can be used to quantify the correlation structure of the GPR image, which can be taken as representative of the spatial variability of the subsurface. The spatial variability of the subsurface is known to be scaledependent; that is the specific nature of the heterogeneity, or the correlation structure, depends upon the scale at which you “sample” a region of the subsurface. Large-scale sampling, on the order of 10’s of meters for example, will capture the structure of the large-scale depositional features. Sampling at the meter or sub-meter scale will capture the finer-scale variability. In the analysis of GPR data, it is important to recognize that there is a “sampling volume” that is determined by the vertical and horizontal resolution; the resolution is related to the frequency of the GPR data. We would therefore expect GPR data collected at different frequencies, over the same sedimentary package, to provide information about spatial variability at different scales. Geostatistical analysis of three examples of such multifrequency GPR data shows a dependence of the determined correlation structure on the “sampling volume” or frequency of the GPR data. This introduces the potential for using multifrequency GPR data to characterize the scale dependence that exists in the spatial variability of the subsurface.

We present a number of new ways to use the seismic cone penetrometer (SCPTU). These applications include the prediction of stratigraphic boundaries in advance of the cone as well as the refinement of the first break shear wave velocities. These applications depend upon the acquisition of vertical seismic profile (VSP) data using the cone. The VSP acquisition is achieved using standard SCPTU accelerometers and is an extension of SCPTU shear wave velocity surveys. The data consist of both an upgoing and downgoing seismic wavefield. These wavefields are easily identifiable and can be separated with a series of simple processing techniques. VSP data are most commonly associated with common depth point (CDP) reflection seismic surveys. We have demonstrated how the data can also be used effectively to obtain geotechnical information.