Investigating peat hydrological properties using field and laboratory methods: application to the Lanoraie peatland complex (southern Quebec,Canada)

Flow dynamics within a peatland are governed by hydraulic parameters such as hydraulic conductivity, dispersivity and specific yield, as well as by anisotropy and heterogeneity. The aim of this study is to investigate hydraulic parameters variability in peat through the use of different field and laboratory methods. An experimental site located in the Lanoraie peatland complex (southern Quebec, Canada) was used to test the different approaches. Slug and bail tests were performed in piezometer standpipes to investigate catotelm hydraulic conductivity. Combined Darcy tests and tracer experiments were conducted on cubic samples using the modified cube method (MCM) to assess catotelm hydraulic conductivity, anisotropy and dispersivity. A new laboratory method is proposed for assessing acrotelm hydraulic conductivity and gravity drainage using a laboratory experimental tank. Most of slug tests' recovery curves were characteristic of compressible media, and important variability was observed depending on the initial head difference. The Darcy experiments on cubic samples provided reproducible results, and anisotropy (K_h > K_v) was observed for most of samples. All tracer experiments displayed asymmetrical breakthrough curves, suggesting the presence of retardation and/or dual porosity. Hydraulic conductivity estimates performed using the experimental tank showed K variations over a factor of 44 within the upper 40 cm of the acrotelm. The results demonstrate that the intrinsic variability associated with the different field and laboratory methods is small compared with the spatial variability of hydraulic parameters. It is suggested that a comprehensive assessment of peat hydrological properties can be obtained through the combined use of complementary field and laboratory investigations. Copyright © 2007 John Wiley & Sons, Ltd.     

Comparative Evaluation of Single‐Well LNAPL Tracer Testing at Five Sites

Light nonaqueous phase liquid (LNAPL) tracer testing is a technique used to directly measure LNAPL flow in situ and evaluate LNAPL mobility and recoverability. The test method consists of adding a fluorescent oil‐soluble tracer to LNAPL within a well, isolating small volumes of LNAPL with known tracer concentrations for use as in‐well calibration standards, and measuring the rate of tracer concentration decline in the well over time. The test measures LNAPL flux through the well, which is directly related to LNAPL mobility and recoverability in the surrounding formation. Test results for a total of 29 wells at five sites are presented. Results from LNAPL tracer testing were comparable to results obtained through other methods, and the method offers a time‐averaged result measured over a relatively long period, in ambient conditions, and reflects the influences of heterogeneity and hydraulic changes. In some cases, tracer concentration decline followed unexpected patterns, and these data have led to a better understanding of test assumptions, mechanisms influencing tracer distribution, and options to improve test execution and data interpretation. Method improvements developed over the course of the field studies included refinement of pre‐test screening of LNAPL fluorescence and improvements to measurement equipment. Overall, the field studies confirmed the technical validity and usefulness of the LNAPL tracing technique to support LNAPL mobility and recoverability assessments.     

A Method for Conducting Simultaneous Convergent Tracer Tests in Multilayered Aquifers

Forced gradient tracer tests between two boreholes can be used to study contaminant transport processes at the small field scale or investigate the transport properties of an aquifer. Full depth tests, in which tracer samples are collected just from the discharge of the abstraction borehole, often give rise to breakthrough curves with multiple peaks that are usually attributed to different flow paths through the aquifer that can rarely be identified from the test results alone. Tests in selected levels of the aquifer, such as those between packer‐isolated sections of the boreholes, are time consuming, expensive; and the identification of major transport pathways is not guaranteed. We present a method for simultaneously conducting multiple tracer tests covering the full depth of the boreholes, in which tracer sampling and monitoring is carried out by a novel multilevel sampling system allowing high frequency and cumulative sampling options. The method is applied to a tracer test using fluorescein conducted in the multilayered sandstone aquifer beneath the city of Birmingham, UK, producing six well‐defined tracer breakthrough curves.     

Interpreting Variations in Groundwater Flows from Repeated Distributed Thermal Perturbation Tests

To better understand the groundwater resources of southern Nye County, Nevada, a multipart distributed thermal perturbation sensing (DTPS) test was performed on a complex of three wells. These wells penetrate an alluvial aquifer that drains the Nevada National Security Site, and characterizing the hydraulic properties and flow paths of the regional groundwater flow system has proven very difficult. The well complex comprised one pumping well and two observation wells, both located 18 m from the pumping well. Using fiber‐optic cables and line heaters, DTPS tests were performed under both stressed and unstressed conditions. Each test injects heat into the water column over a period of one to two days, and observes the rising temperature during heat injection and falling temperatures after heating ceases. Aquifer thermal properties are inferred from temperature patterns in the cased section of the wells, and fluxes through the 30‐m screened section are estimated based on a model that incorporates conductive and advective heat fluxes. Vertical variations in flux are examined on a scale of tens of cm. The actively flowing zones of the aquifer change between the stressed and unstressed test, and anisotropy in the aquifer permeability is apparent from the changing fluxes between tests. The fluxes inferred from the DTPS tests are compared to solute tracer tests previously performed on the same site. The DTPS‐based fluxes are consistent with the fastest solute transport observed in the tracer test, but appear to overestimate the mean flux through the system.     

Single Well Tracer Tests in Aquifer Characterization

When the purpose of aquifer testing is to yield data for modeling aqueous mass transport, pumping tests and gradient measurement can only partially satisfy characterization requirements. Effective porosity, ground water flow velocity, and the vertical distribution of hydraulic conductivity within the aquifer are left as unknowns. Single well tracer methods, when added to the testing program, can be used to estimate these parameters. A drift, and pumpback test yields porosity and velocity, and point-dilution testing yields depth-discrete hydraulic information, A single emplacement of tracer into a test well is sufficient to conduct both tests. The tracer tests are facilitated by a simple method for injecting and evenly distributing the tracer solution into a wellbore, and by new ion-selective electrode instrumentation, specifically designed for submersible service, for monitoring the concentration of tracers such as bromide.     

Combining 3D Hydraulic Tomography with Tracer Tests for Improved Transport Characterization

Hydraulic tomography (HT) is a method for resolving the spatial distribution of hydraulic parameters to some extent, but many details important for solute transport usually remain unresolved. We present a methodology to improve solute transport predictions by combining data from HT with the breakthrough curve (BTC) of a single forced‐gradient tracer test. We estimated the three dimensional (3D) hydraulic‐conductivity field in an alluvial aquifer by inverting tomographic pumping tests performed at the Hydrogeological Research Site Lauswiesen close to Tübingen, Germany, using a regularized pilot‐point method. We compared the estimated parameter field to available profiles of hydraulic‐conductivity variations from direct‐push injection logging (DPIL), and validated the hydraulic‐conductivity field with hydraulic‐head measurements of tests not used in the inversion. After validation, spatially uniform parameters for dual‐domain transport were estimated by fitting tracer data collected during a forced‐gradient tracer test. The dual‐domain assumption was used to parameterize effects of the unresolved heterogeneity of the aquifer and deemed necessary to fit the shape of the BTC using reasonable parameter values. The estimated hydraulic‐conductivity field and transport parameters were subsequently used to successfully predict a second independent tracer test. Our work provides an efficient and practical approach to predict solute transport in heterogeneous aquifers without performing elaborate field tracer tests with a tomographic layout.     

Dilution tests in a low-permeability fractured aquifer: matrix diffusion effect

A point dilution test is commonly used in single-borehole tracer experiments designed to determine the Darcy velocity of a formation. This method is based on the concept that, in a borehole, a tracer's concentration declines as a consequence of the water flux. Based on theoretical simulations and field observations, this study indicates that for low-permeability, yet highly porous fractured formations, the common practice of excluding the effect of diffusive mass flux between the dissolved tracer within the borehole and the surrounding matrix may lead to significant errors in the assessment of the Darcy velocity. This conclusion was confirmed by a model adapted to simulate experimental data collected from a tracer test performed in a vertical, large-diameter (25-cm) borehole drilled along a subvertical fracture intersecting a chalk formation.     

改进的各向异性标准化方差探测斜磁化磁异常源边界

针对存在强剩磁作用磁化方向不明的磁异常,本项研究探索直接处理斜磁化磁异常的识别,提出了基于磁力梯度张量模的各向异性边界探测方法.首先利用各向异性尺度改进了各向异性标准差的核函数,突出各向异性高斯函数的作用;结合磁力梯度张量模来消弱斜磁化的影响.数值实验模拟了一组复杂磁异常模型,在斜磁化条件下分析该研究方法的边界探测效果.实验表明：改进方法,即磁力梯度张量模的各向异性标准化方差,它可以探测非垂直磁化磁异常的磁源边界;同时指出,改进方法比基于三维解析信号振幅的各向异性标准化方差对磁化方向的依赖性更小.将该方法应用于中国西部某磁铁矿集区的精细探测,在非垂直磁化条件下对实测磁异常直接进行边界探测,获得了较为理想的处理结果.     

Evaluating the Use of In‐Well Heat Tracer Tests to Measure Borehole Flow Rates

Recent research has demonstrated the use of in‐well heat tracer tests monitored by a fiber optic distributed temperature sensing (DTS) system to characterize borehole flow conditions in open bedrock boreholes. However, the accuracy of borehole flow rates determined from in‐well heat tracer tests has not been evaluated. The purpose of the research presented here is to determine whether borehole flow rates obtained using DTS‐monitored in‐well heat tracer tests are reasonable, and to evaluate the range of flow rates measureable with this method. To accomplish this, borehole flow rates measured using in‐well heat tracer tests are compared to borehole flow rates measured in the same boreholes using an impeller or heat pulse flowmeter. A comparison of flow rates measured using in‐well heat tracer tests to flow rates measured with an impeller flowmeter under the same conditions showed good agreement. A comparison of in‐well heat tracer test flow rate measurements to previously‐collected heat pulse flowmeter measurements indicates that the heat tracer test results produced borehole flow rates and flow profiles similar to those measured with the heat pulse flowmeter. The results of this study indicate that borehole flow rates determined from DTS‐monitored in‐well heat tracer tests are reasonable estimates of actual borehole flow rates. In addition, the range of borehole flow rates measurable by in‐well heat tracer tests spans from less than 10^?1 m/min to approximately 10¹ m/min, overlapping the ranges typically measurable with an impeller flowmeter or a heat pulse flowmeter, making in‐well heat tracer testing a versatile borehole flow logging tool.     

An Unsteady State Tracer Method for Characterizing Fractures in Bedrock Wells

Evaluating contaminants impacting wells in fractured crystalline rock requires knowledge of the individual fractures contributing water. This typically involves using a sequence of tools including downhole geophysics, flow meters, and straddle packers. In conjunction with each other these methods are expensive, time consuming, and can be logistically difficult to implement. This study demonstrates an unsteady state tracer method as a cost‐effective alternative for gathering fracture information in wells. The method entails introducing tracer dye throughout the well, inducing fracture flow into the well by conducting a slug test and then profiling the tracer concentration in the well to locate water contributing fractures where the dye has been diluted. By monitoring the development of the dilution zones within the wellbore with time, the transmissivity and the hydraulic head of the water contributing fractures can be determined. Ambient flow conditions and the contaminant concentration within the fractures can also be determined from the tracer dilution. This method was tested on a large physical model well and a bedrock well. The model well was used to test the theory underlying the method and to refine method logistics. The approach located the fracture and generated transmissivity values that were in excellent agreement with those calculated by slug testing. For the bedrock well tested, two major active fractures were located. Fracture location and ambient well conditions matched results from conventional methods. Estimates of transmissivity values by the tracer method were within an order of magnitude of those calculated using heat‐pulse flow meter data.     

Comparisons of Three Methods to Determine the Vertical Stratification of Pore Fluids

Three methods were used to investigate the possibility of vertical stratification of the pore fluid in an aquifer. The wells available for the study were fully screened and had a 5cm disturbed annulus around them. The first method used a pump with a straddle packer to isolate a short section of the secreened interval. A tracer test demonstrated that most of the pumped sample came from the well bore, presumably by piping through the disturbed. annulus. The second method used induction logs to measure the formation electrical conductivity as a function of depth. Due to the presence of clays and an inability to obtain porosity information, it was not possible to determine the pore fluid electrical conductivity using induction logs. A third method, dilution sampling, was developed that used a straddle packer to isolate a segment of the well screen. A tracer was injected into the packed-off segment, and the tracer concentration in the well was monitored. The tracer was removed from the packed-off segment by dilution and advection by ground water. When the tracer was completely removed from the packed-off segment, the fluid in the segment was considered to be representative of the adjacent pore fluid. Only the dilution sampling method determined unambiguously that the pore fluid was not stratified.     

Loading Rate Dependence of Tensile Strength Anisotropy of Barre Granite

Granitic rocks usually exhibit strongly anisotropy due to pre-existing microcracks induced by long-term geological loadings. The understanding of the rock anisotropy in mechanical properties is critical to a variety of rock engineering applications. In this paper, Brazilian tests are conducted statically with a material testing machine and dynamically with a split Hopkinson pressure bar system to measure both static and dynamic tensile strength of Barre granite. To understand the anisotropy in tensile strength, samples are cored and labelled using the three principle directions of Barre granite to form six sample groups. For dynamic tests, a pulse shaping technique is used to achieve dynamic equilibrium in the samples during the dynamic test. The finite element method is then implemented to formulate equations that relate the failure load to the material tensile strength by employing an orthotropic elastic material model. For samples in the same orientation group, the tensile strength shows clear loading rate dependence. The tensile strengths also exhibit clear anisotropy under static loading while the anisotropy diminishes as the loading rate increases, which may be due to the interaction of pre-existing microcracks.     

Process-based interpretation of tracer tests in carbonate aquifers

A tracer test in a carbonate aquifer is analyzed using the method of moments and two analytical advection-dispersion models (ADMs) as well as a numerical model. The numerical model is a coupled continuum-pipe flow and transport model that accounts for two different flow components in karstified carbonate aquifers, i.e., rapid and often turbulent conduit flow and Darcian flow in the fissured porous rock. All techniques employed provide reasonable fits to the tracer breakthrough curve (TBC) measured at a spring. The resulting parameter estimates are compared to investigate how each conceptual model of flow and transport processes that forms the basis of the analyses affects the interpretation of the tracer test. Numerical modeling results suggest that the method of moments and the analytical ADMs tend to overestimate the conduit volume because part of the water discharged at the spring is wrongly attributed to the conduit system if flow in the fissured porous rock is ignored. In addition, numerical modeling suggests that mixing of the two flow components accounts for part of the dispersion apparent in the measured TBC, while the remaining part can be attributed to Taylor dispersion. These processes, however, cannot reasonably explain the tail of the TBC. Instead, retention in immobile-fluid regions as included in a nonequilibrium ADM provides a possible explanation.     

Determination of longitudinal dispersivity in an unconfined sandy aquifer

Study of the mobility of contaminants in an aquifer is an important issue for the proper remediation of contaminated groundwater. Determination of associated solute transport parameters therefore is essential for investigation of the extent to which groundwater can be contaminated. This study aimed at determining solute transport parameters for an unconfined sandy aquifer at a laboratory scale through various tracer tests using a conservative solute as a tracer. Tracer tests consisted of both well‐tracer tests (single and double wells) and an aquifer tracer test using a plume‐capturing device such as time domain reflectometry (TDR). The results showed that longitudinal dispersivities estimated from the single and double well‐tracer tests were 2·2 cm and 13·5 cm for a travel distance of 9·3 cm and 13·5 cm from the injection point respectively. These results agreed reasonably well with the results of the aquifer tracer test. The solute transport parameters obtained at multiple points in the aquifer through the aquifer tracer test revealed that the dispersivity length was proportional to the travel distance by a factor of 0·3, which was moderately higher than the value of 0·1 given in the literature. Copyright © 2002 John Wiley & Sons, Ltd.     

Point dilution tests to calculate groundwater velocity: an example in a porous aquifer in northeast Italy

ABSTRACT

The point dilution test is a single-well technique for estimating horizontal flow velocity in the aquifer surrounding a well. The test is conducted by introducing a tracer into a well section and monitoring its decreasing concentration over time. When using a salt tracer, the method is easy and inexpensive. Traditionally, the horizontal Darcy velocity is calculated as a function of the rate of dilution and is based on the simple assumption that the decreasing tracer concentration is proportional both to the apparent velocity into the test section and to the Darcy velocity in the aquifer. In this article, an alternative approach to analyse the results of point dilution tests is proposed and verified using data acquired at a test site in the middle Venetian plain, northeast Italy. In this approach, the one-dimensional equilibrium advection–dispersion equation is inverted using the CXTFIT model to estimate the apparent velocity inside the test section. Analysis of the field data obtained by the two approaches shows good agreement between the methods and suggests that it is possible to use the equilibrium advection–dispersion equation to estimate apparent velocity over a wide range of velocities.

Editor D. Koutsoyiannis; Associate editor K. Heal     

地壳分层各向异性介质接收函数及其粒子群反演

地壳不同深度介质的地震各向异性是研究地壳不同深度范围变形方式的重要依据.鉴于地壳介质的复杂性,如何从远震体波接收函数中提取不同深度的各向异性参数仍是一个有待深入研究的课题.在已有研究的基础上,本文利用广义反射-透射系数矩阵方法计算的合成地震图,研究了复杂地壳分层各向异性介质的接收函数随反方位角(back azimuth)变化及不同层位各向异性参数对接收函数波场的影响,为各向异性介质接收函数的解释提供了新的理论依据.通过引入粒子群优化理论,发展了分层各向异性介质接收函数全局反演算法.数值及观测数据的验证结果表明,在各向同性速度模型确定的前提下,我们的方法能够可靠地提取地壳分层各向异性参数;在反演中引入曲波变换去噪技术,对于正确解析不同层位的各向异性参数具有重要价值.     

Multitracer test approach to characterize reactive transport in karst aquifers

A method to estimate reactive transport parameters as well as geometric conduit parameters from a multitracer test in a karst aquifer is provided. For this purpose, a calibration strategy was developed applying the two-region nonequilibrium model CXTFIT. The ambiguity of the model calibration was reduced by first calibrating the model with respect to conservative tracer breakthrough and later transferring conservative transport parameters to the reactive model calibration. The reactive transport parameters were only allowed to be within a defined sensible range to get reasonable calibration values. This calibration strategy was applied to breakthrough curves obtained from a large-scale multitracer test, which was performed in a karst aquifer of the Swabian Alb, Germany. The multitracer test was conducted by the simultaneous injection of uranine, sulforhodamine G, and tinopal CBS-X. The model succeeds to represent the tracer breakthrough curves (TBCs) of uranine and sulforhodamine G and verifies that tracer-rock interactions preferably occur in the immobile fluid region, although the fraction of this region amounts to only 3.5% of the total water. However, the model failed to account for the long tailing observed in the TBC of tinopal CBS-X. Sensitivity analyses reveal that model results for the conservative tracer transport are most sensitive to average velocity and volume fraction of the mobile fluid region, while dispersion and mass transfer coefficients are least influential. Consequently, reactive tracer calibration allows the determination of sorption sites in the mobile and immobile fluid region at small retardation coefficients.     

最小二乘逆时偏移中黏弹性和各向异性的校正:以渤海湾地区地震数据为例

地下介质中存在黏弹性和各向异性,在应用最小二乘逆时偏移时,如果没有对黏弹性和各向异性的影响进行校正,在观测数据和模拟数据的匹配过程中就会发生错误,最终使得最小二乘逆时偏移无法得到准确的成像结果.因此,本论文首先对最小二乘逆时偏移的黏弹性和各向异性进行分别校正,然后对两种影响进行同时校正.在实际资料试算部分,通过对渤海湾数据进行测试,分别采用常规声波最小二乘逆时偏移、补偿黏弹性的最小二乘逆时偏移、校正各向异性的最小二乘逆时偏移及同时校正两种性质的最小二乘逆时偏移进行比较测试,证明了在同时校正了黏弹性和各向异性影响之后,最小二乘逆时偏移得到了更高质量的成像结果,具体表现在低频噪音压制、震源效应压制、深部能量改善、分辨率提升、目的层刻画等方面都明显优于其他三种最小二乘逆时偏移的成像结果,同时也证明了同时校正黏弹性和各向异性最小二乘逆时偏移方法的正确性及在实际资料处理中的适用性.     

Performance of the steady-state dipole flow test in layered aquifers

Steady-state numerical simulations of the dipole flow test in layered aquifers demonstrate that the test produces a good estimate of the equivalent hydraulic conductivity anisotropy ratio for the part of the aquifer spanned by the well chambers. The effects of chamber size, different conductivity of layers and layer location on the estimated anisotropy ratios are presented. The steady-state dipole flow test, when performed at different levels in the well, can yield estimates of the down-hole anisotropy ratio distribution. Numerical simulations also illustrate that the skin effect can significantly distort the anisotropy estimates produced by the dipole flow test. © 1997 John Wiley & Sons, Ltd.