Michael D. Knoll

Time-lapse ERT monitoring of an injection/withdrawal experiment in a shallow unconfined aquifer

Toquantifyperformanceof3Dtime-lapseelectricalresistivity tomography ERT, a sequential injection/withdrawal experiment was designed for monitoring the pump-and-capture remediation of a conductive solute in an unconfined alluvial aquifer. Prior information is incorporated into the inversion procedure via regularization with respect to a reference model according to threeprotocols:1independentregularizationinvolvingasingle reference model, 2 background regularization involving a referencemodelobtainedviainversionofpreinjectiondata,and3 time-lapseregularizationinvolvinganevolvingreferencemodel obtained via inversion of data from previous experimental stages.Emplacementandsequentialwithdrawalofthesoluteisclearly imaged for all protocols. Time-lapse regularization results in greater amounts of model structure, while providing significantcomputationalsavings.ERT-estimatedelectricalconductivity is used to predict solute concentration and solute mass in the aquifer. At any experimental stage, we are able to estimate total solute mass in the aquifer with a maximum accuracy of 60%‐85% depending on regularization protocol and survey geometry.Wealsoestimatethewithdrawnsolutemassforeveryexperimentalstagethechangeinmassbetweenexperimentalstages. Withdrawn mass estimates are more reliable than total mass estimates and do not exhibit systematic underprediction or dependence on regularization protocol. Withdrawn mass estimates areaccurateforchangesinmassbelow2‐4 kgofpotassiumbromide KBr for horizontal and vertical dipole-dipole surveys, respectively. Estimating the withdrawn solute mass does not require background subtraction and, thus, does not require backgrounddata.

Improving crosshole radar velocity tomograms: A new approach to incorporating high-angle traveltime data

To obtain the highest-resolution ray-based tomographic images from crosshole ground-penetrating radar (GPR) data, wide angular ray coverage of the region between the two boreholes is required. Unfortunately, at borehole spacings on the order of a few meters, high-angle traveltime data (i.e., traveltime data corresponding to transmitter-receiver angles greater than approximately 50° from the horizontal) are notoriously difficult to incorporate into crosshole GPR inversions. This is because (1) low signal-to-noise ratios make the accurate picking of first-arrival times at high angles extremely difficult, and (2) significant tomographic artifacts commonly appear when high- and low-angle ray data are inverted together. We address and overcome thesetwo issues for a crosshole GPR data example collected at the Boise Hydrogeophysical Research Site (BHRS). To estimate first-arrival times on noisy, high-angle gathers, we develop a robust and automatic picking strategy based on crosscorrelations, where reference wav...

Model reliability for 3D electrical resistivity tomography: Application of the volume of investigation index to a time-lapse monitoring experiment

Solution appraisal is difficult for large 3D, nonlinear inverse problems such as electrical resistivity tomography (ERT). We construct the volume of investigation index (VOI) as the sensitivity of the inversion result to a variable-reference model. This limited exploration of the model space provides an efficient and pragmatic method of appraisal for a particular data set and a 3D model domain. We present a synthetic example to demonstrate the applicability of the VOI as a tool for characterizing model reliability for 3D ERT and as a method of survey design. We show how the VOI provides a measure of model resolution and how insight gained from VOI analysis cannot be gained through similar examination of the average sensitivity distributions. In the context of ERT monitoring of an injection/withdrawal experiment, we utilize the VOI for judging the degree of reliability of hydrogeological interpretations that stem from features observed in the estimated electrical-conductivity models. We employ the VOI for the experimental data as a comparative measure of survey performance. For this experiment, the VOI shows that a larger, more artifact-free region of reliability is achieved using a circulating vertical dipole-dipole survey geometry, as opposed to a horizontal dipole-dipole survey geometry. The experimental VOI distributions exhibit dependence on the borehole infrastructure and the actual earth model.

A field comparison of Fresnel zone and ray-based GPR attenuation-difference tomography for time-lapse imaging of electrically anomalous tracer or contaminant plumes

Ground-penetrating radar GPR attenuation-difference tomographyisausefultoolforimagingthemigrationofelectrically anomalous tracer or contaminant plumes. Attenuation-difference tomography uses the difference in the trace amplitudes of tomographic data sets collected at different times to image the distribution of bulk-conductivity changes within the medium. Themostcommonapproachforcomputingthetomographicsensitivities uses ray theory, which is well understood and leads to efficient computations. However, ray theory requires the assumption that waves propagate at infinite frequency, and thus sensitivities are distributed along a line between the source and receiver. The infinite-frequency assumption in ray theory leads to a significant loss of resolution both spatially and in terms of amplitude of the recovered image. We use scattering theory to approximate the sensitivity of electromagnetic EM wave amplitude to changes in bulk conductivity within the medium. These sensitivities occupy the first Fresnel zone, account for the finite frequency nature of propagating EM waves, and are valid whenvelocityvariationswithinthemediumdonotcausesignificant ray bending. We evaluate the scattering theory sensitivities by imaging a bromide tracer plume as it migrates through a coarse alluvial aquifer over two successive days. The scattering theory tomograms display a significant improvement in resolution over the ray-based counterparts, as shown by a direct comparison of the tomograms and also by a comparison of the verticalfluidconductivitydistributionmeasuredinamonitoringwell, located within the tomographic plane. By improving resolution, the scattering theory sensitivities increase the utility of GPR attenuation-difference tomography for monitoring the movement of electrically anomalous plumes. In addition, the improved accuracy of information gathered through attenuation-difference tomography using scattering theory is a positive step toward futuredevelopmentsinusingGPRdatatohelpcharacterizethedistributionofhydrogeologicproperties.

Boise Hydrogeophysical Research Site (BHRS): Objectives, Design, Initial Geostatistical Results

The Boise Hydrogeophysical Research Site (BHRS) is a wellfield developed in a shallow, coarse (cobble-and-sand), alluvial aquifer with the goal of developing cost-effective methods for quantitatively characterizing the distribution of permeability in heterogeneous aquifers using hydrologic and geophysical techniques. Responses to surface geophysical techniques (e.g., seismic, radar, transient electromagnetics) will be calibrated against a highly characterized control volume (the wellfield) with 3-D distributions of geologic, hydrologic, and geophysical properties determined from extensive field measurements. Also, these data sets will be used to investigate relationships between properties and to test petrophysical models. Well coring and construction methods, and the well arrangement in the field, are designed to provide detailed control on lithology and to support a variety of single-well, crosshole, and multiwell geophysical and hydrologic tests. Wells are screened through the cobble-and-sand aquifer to a clay that underlies the BHRS at about 20 m depth. In addition, the wellfield design optimizes well-pair distances and azimuths for determination of short-range geostatistical structure. Initial geostatistical analysis of porosity data derived from borehole geophysical logs indicates that the omnidirectional horizontal experimental variogram for porosity (possible proxy for log permeability) is best fit with a nested periodic model structure. INTRODUCTION Permeability is the most significant aquifer parameter for quantitatively describing or modeling groundwater flow and contaminant transport, and for designing remediation systems. A number of workers have shown the potential of supplementing sparse, expensive, direct permeability measurements with geophysical surveys that can be conducted rapidly and inexpensively (e.g., McKenna and Poeter, 1995; Hyndman and Gorelick, 1996). Recent work at a groundwater remediation site in downtown Boise, Idaho has investigated a variety of geophysical and hydrologic techniques and parameter relationships in a coarse, unconsolidated alluvial aquifer (e.g., Barrash et al., 1997a; Barrash and Morin, 1997). Currently, Boise State University is developing the Boise Hydrogeophysical Research Site (BHRS) as a 3-D control volume in a natural heterogeneous aquifer to support research on use of non-invasive geophysical techniques with hydrologic measurements to map variations in permeability in shallow alluvial aquifers (Barrash and Knoll, in press). This paper briefly introduces the BHRS and discusses site design and initial geostatistical analysis of porosity data based on geophysical logs. Companion papers at this symposium (Clement and Liberty, 1999; Clement et al., 1999a,b; Knoll et al., 1999; Liberty et al., 1999; Peretti et al., 1999; Peterson et al., 1999) present geophysical testing strategies and initial results from the BHRS. BOISE HYDROGEOPHYSICAL RESEARCH SITE The BHRS is located on a gravel bar adjacent to the Boise River (Fig. 1) about 15 km east of downtown Boise where the river leaves a canyon and enters the broad western Snake River Plain. Eighteen wells were emplaced in 1997 and 1998 using a coring and construction method (Fig. 2)

Capacitive conductivity logging and electrical stratigraphy in a high-resistivity aquifer, Boise Hydrogeophysical Research Site

We tested a prototype capacitive-conductivity borehole tool in a shallow, unconfined aquifer with coarse, unconsolidated sediments and very-low-conductivity water at the Boise Hydrogeophysical Research Site BHRS. Examining suchahigh-resistivitysystemprovidesagoodtestforthecapacitive-conductivity tool because the conventional induction-conductivity tool known to have limited effectiveness in high-resistivity systems did not generate expressive well logs at the BHRS. The capacitive-conductivity tool demonstrated highly repeatable, low-noise behavior but poor correlation with the induction tool in the lower-conductivity portions of the stratigraphy where the induction tool was relatively unresponsive. Singular spectrum analysis of capacitive-conductivity logs reveals similar vertical-length scales ofstructurestoporositylogsattheBHRS.Also,majorstratigraphic units identified with porosity logs are evident in the capacitive-conductivity logs. However, a previously unrecognized subdivision in the upper portion of one of the major stratigraphicunitscanbeidentifiedconsistentlyasarelatively low-conductivity body i.e., an electrostratigraphic unit between the overlying stratigraphic unit and the relatively high-conductivity lower portion — despite similar porosity and lithology in adjacent units. The high repeatability and resolutionandthewidedynamicrangeofthecapacitive-conductivity tool are demonstrated here to extend to high-resistivity,unconsolidatedsedimentaryaquiferenvironments.

Traveltime inversion of vertical radar profiles

Traveltimes of direct arrivals in vertical radar profiles VRPs are tomographically inverted to estimate the earth’s electromagnetic EM velocity between a surface transmitter and a downhole receiver. We determine the 1D interval velocity model that best fits the first-arrival traveltimes by using a weighted, damped, least-squares inversion scheme. We assess the accuracy of the velocity model using synthetic traveltimes from a known velocity-distribution model simulating an unconfined aquifer. The inverted velocity profile closely matched the velocity profile of the input model in the synthetic examples. Using vertical radar profile data from an unconfined aquifer near Boise, Idaho, we inverted traveltimes to obtain velocity estimates at the well location. The velocity change at a depth of 2.0 m corresponds well with the measured depth to the water table of 1.95 m, and at depths between 2 and 18 m, the velocities ranged between 0.06 and 0.1 m/ns. Our estimates approximately match the velocity distribution determined from neutron-derived porosity logs at depths greater than about 2 m. An important function of inverse methods is to assess quantitatively and qualitatively the uncertainty of inverted velocity estimates. We note that the velocity values in the upper and lower parts of the inverted model are not as well constrained compared to those between the depths of 4 and 13 m. From the model resolution and model covariance matrices of the real-data inversion,we determine theuncertainty in ourvelocity model, leading to more reliable interpretations of the subsurface.

Investigating the Stratigraphy of an Alluvial Aquifer Using Crosswell Seismic Traveltime Tomography

In this study, we investigate the use of crosswell P-wave seismic tomography to obtain spatially extensive information about subsurface sedimentary architecture and heterogeneity in alluvial aquifers. Our field site was a research wellfield in an unconfined aquifer near Boise, Idaho. The aquifer consists of a 20-m-thick sequence of alluvial cobble-and-sand deposits, which have been subdivided into five stratigraphic units based on neutron porosity logs, grainsize analysis, and radar reflection data. We collected crosswell and borehole-to-surface seismic data in wells 17.1 m apart. We carefully considered the impact of well deviation, data quality control, and the choice of inversion parameters. Our linearized inverse routine had a curved-ray forward model and used different grids for forward modeling and inversion.An analysis of the model covariance and resolution matrices showed that the velocity models had an uncertainty of 10 m/s, a vertical resolution of 1 m, and a horizontal resolution of 5 m. The velocity in the saturated zone varied between 2100 m/s and 2700 m/s. Inclusion of the borehole-to-surface data eliminated the Xshaped pattern that is a common artifact in crosswell tomography, and the increased angular coverage also improved the accuracy of the model near the top of the tomogram. The final velocity model is consistent with previous stratigraphic analyses of the site, although the locations of some of the unit boundaries differ by as much as 2 m in places. The results of this study demonstrate that seismic tomography can be used to image the sedimentary architecture of unconsolidated alluvial aquifers, even when the lithologic contrasts between units are subtle.

GEOPHYSICAL SURVEYS ACROSS THE BOISE HYDROGEOPHYSICAL RESEARCH SITE TO DETERMINE GEOPHYSICAL PARAMETERS OF A SHALLOW, ALLUVIAL AQUIFER

At the Boise Hydrogeophysical Research Site (BHRS), we are characterizing the hydrogeophysical parameters of a cobble-and-sand, unconfined aquifer using a wide variety of geophysical methods. Our goal is to develop methods for mapping variations in permeability by combining non-invasive geophysical data with hydrologic measurements. We are using seismic, ground penetrating radar, and electrical methods in a variety of configurations to provide images of and parameter distributions at the BHRS. Issues such as resolution, depth of penetration, and the ability to image the desired parameters will help determine the most effective methods. Supporting data sets from the BHRS include core analyses and geophysical logs from 18 wells at the site. We will use these data to verify our geophysical interpretations. The various geophysical methods and acquisition geometries, combined with the well control, will provide an outstanding data set to characterize the heterogeneity of the subsurface beneath this alluvial aquifer, and find ways to map permeability with geophysical information. INTRODUCTION The Boise Hydrogeophysical Research Site, or BHRS (Fig. l), is being developed to support the goal of using non-invasive geophysical methods to supplement hydrologic data to map the distribution of permeability in 3-D heterogeneous shallow alluvial aquifers. In particular, the BHRS is a research wellfield in a shallow, unconfined, coarse (cobble-and-sand), braided-stream, alluvial aquifer (Fig. 2a) that is broadly similar to many other alluvial aquifers comprised of correlated random fields of facies and hydraulic parameters. Eighteen wells were cored and installed at the BHRS in 1997 and 1998, including 13 wells in the central portion of the wellfield (Fig. 1). These wells provide: (a) a dense network with direct “hard” information such as core (Fig. 2b) and permeability measurements at the individual well scale; (b) access for a wide variety of geophysical and hydrologic testing in individual wells (1 -D), between wells (2-D), and in multi-well subsurface and surface-subsurface configurations (3-D); and (c) a configuration designed to support geostatistical analysis of geophysical or hydrologic parameters (Barrash and Knoll, in press; Barrash et al., 1999). The intention is to thoroughly characterize the wellfield as a control volume with “known” 3-D distributions of sedimentary facies, geophysical parameters, and hydrologic parameters. Then responses from geophysical methods, alone and together, can be correlated against known parameter distributions to develop techniques for mapping the 3-D distribution of permeability with non-invasive geophysical methods. Initial efforts will concentrate on three generally accessible geophysical methods: seismic, ground penetrating radar (GPR), and time-domain or transient electromagnetics (TEM). Also, a variety of borehole geophysical logs are being run in all wells. The general approach at the BHRS is to establish “control” with dense, high-resolution measurements at wells (l-D), use these as calibration or reference points for level runs and tomography measurements between wells (2-D), and use these as calibration or reference 399 D ow nl oa de d 08 /2 7/ 19 to 1 32 .1 78 .1 61 .1 54 . R ed is tr ib ut io n su bj ec t t o SE G li ce ns e or c op yr ig ht ; s ee T er m s of U se a t h ttp :// lib ra ry .s eg .o rg / sections for surface profiles (2-D) and multiwell experiments (3-D). We acknowledge that differences will exist: (a) because of measurement scales and data acquisition methods, (b) because of differences in actual parameters measured with similar geophysical methods, and (c) because of differences between information contained in structural images and parameter fields. The task of analyzing any given data set will be assisted by the availability of data sets from other methods, including many coincident data sets. Indeed, part of the overall research effort includes investigation of scaling, petrophysical relationships, and multivariate relationships.

Reflectivity modeling of a ground-penetrating-radar profile of a saturated fluvial formation

Major horizons in radar reflection profiles may correlate with contacts between stratigraphic units or with structural breaks such as fault surfaces. Minor reflections may be caused by clutter or, in some cases, may indicate material properties or structure within stratigraphic units. In this study, we examine the physical basis for major and minor reflections observed in a shallow, unconfined, fluvial aquifer near Boise, Idaho, U. S. A. We compare a 2D profile from a surface ground-penetrating-radar reflection transect with the 1D modeled reflection profiles at three wells adjacent to the surface-reflection profile. The 1D models are based on dielectric constant and electrical conductivity values from borehole logs and vertical radar profile data. Reflections at the water table/capillary fringe, at the base of a sand-filled channel, and at the base of two sand-rich lenses in a cobble-dominated unit are recognizable in the surfacereflection profile and in all 1D reflectivity models. Less prominent reflections in stratigraphic units occur in both the surface-profile model and the reflectivity model. Although such minor reflections are not correlated easily, general similarities in their presence and location indicate that sometimes the reflections may be useful for recognizing internal facies structure or character.