- Home
- Conferences
- Conference Proceedings
- Conferences
Near Surface 2007 - 13th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 03 Sep 2007 - 05 Sep 2007
- Location: Istanbul, Turkey
- ISBN: 978-90-73781-81-8
- Published: 03 September 2007
21 - 40 of 144 results
-
-
Model Analysis of Cross-hole Resistivity Tomography for Void Detection
Authors K. Leontarakis, G. Apostolopoulos and C. OrfanosModel experiments in the laboratory were used to find the best measuring parameters which affect the quality of results for the improved application of resistivity tomography between boreholes. Plastic tubes of various diameters were used to simulate the boreholes and the tunnel’s void in a big plastic tank with non-ionized water. The experiments have shown that a void can be detected using various diameter sizes and depths, and that the resolution is dependent on the distance of the target from the drillings, the distance between the drillings, the electrode array, the use of various and certain dipole lengths and distances, the measuring protocol, and the extended resistivity model. In general, sensitivity analysis is the main guide.
-
-
-
Resistivity Monitoring for Assessing Cement Grouting Effect
More LessIt would be important to investigate the actual grouting performance and to examine whether the original purpose of the grouting work would be achieved. Geophysical imaging technique has a high potential in the evaluation of the grouting effect as a non-destructive and cost-effective way. In order to study the applicability of resistivity monitoring technique to the assessment of cement grouting effect, we carried out DC resistivity monitoring before, during and after grouting work conducted for the reinforcement of weak ground. In accordance with the field monitoring, we also monitored the resistivity change of cement mortar specimens in a laboratory. The laboratory monitoring results clearly showed that electrical prospecting methods are good to locate the cement grouted zone. The field monitoring data were inverted by the four-dimensional (4-D) inversion algorithm and difference images were reconstructed. Owing to the 4-D inversion algorithm, we could more easily and clearly identify the decrease of resistivity in the region where the grout material was injected and infiltrated. Through the monitoring experiments at a field site as well as in a laboratory, we were able to know that DC resistivity monitoring technique is an effective way for evaluating the cement grouting performance.
-
-
-
Detecting Linear Features Using the Directionality of the HLEM Response
By D.C. NobesThe horizontal-loop electromagnetic (HLEM) response is sensitive to the orientation of the instrument relative to buried objects. Linear features have maximum responses when they are are oriented perpendicular to the long axis of the HLEM instrument, especially for features with relatively sharp boundaries, such as pipes and archæological targets. Thus, HLEM responses are often acquired using two orientations, parallel and perpendicular to the survey line direction, and the responses are then combined, usually by summing or averaging the bi-directional responses. If the ratio or the difference between the two responses are used instead, the contrast between the directionality of any linear anomalies is emphasised. Two examples are presented to illustrate the utility of this approach: (1) an early to mid-19th century archæological site; and (2) a soccer pitch where shallow drainage pipes and an old stream channel are located. In both examples, the quadrature response for the two orientations, separately and averaged, shows indications of linear features, but the response from other features, such as buried stream channels, obscures the linear anomalies. When the ratio or the difference are used, the object locations become clear and correlate well with results from other methods.
-
-
-
Imaging the ROOT Cause of Subsidence Using Electrical Resistivity Tomography
Authors G.M. Jones and N.J. CassidyThroughout the world, ground movement caused by moisture content variation in expansive clay soils is a major problem. In southern areas of the UK particularly, where major urban areas coincide with outcropping clay, subsidence and the resulting structural damage caused by clay shrinkage is at the forefront of both the public and insurers concerns. The principal cause of such damage however, has not only been attributed to expansive clays but more increasingly to the influence urban trees have in promoting clay subsidence through water uptake, especially during times of drought. At present the diagnosis of tree-induced subsidence (TIS) involves verifying clay desiccation via testing soil index properties (i.e. Atterberg Limits) and comparing soil water and strength profiles at points proximal and distal from the tree. These methods however, are costly, difficult, and sometimes unreliable and ultimately provide a very limited and often incomprehensive understanding of tree induced subsidence, especially in the long term. As part of an interdisciplinary study of this system, a geophysical approach is taken to bridge the knowledge gap by using Electrical Resistivity Tomography (ERT) to provide a more comprehensive understanding and visualisation of the interactions between trees, moisture movement and volume change in clays.
-
-
-
Delaunay Triangulation-based Spatially Constrained Inversion for Quasi 3D Modeling of Airborne TEM Data
Authors A. Viezzoli, A.V. Christiansen and E. AukenThe spatially constrained inversion (SCI) is a robust methodology for quasi-3D modeling of geoelectrical and EM data of varying spatial density, using a 1D forward solution. Information migrate horizontally through spatial constraints applied between nearest neighboring soundings, and allow resolution of layers that would be locally poorly resolved. The constraints are built using the Delaunay triangulation, which ensures automatic adaptation to data density variations. In this study the SCI was applied to airborne Time domain EM soundings, but it can be implemented also with other data types. The field case study proves that the SCI produces laterally smooth results that respect the 3D geological variations of sedimentary settings, while suppressing the elongated artifacts commonly seen in profile oriented data sets.
-
-
-
Deriving Biophysical Products from Airborne Electromagnetic Data
Authors K. Cahill and A.D. FitzpatrickConcerns by the Angas Bremer Irrigators over the future expansion and sustainable use of water resources in the Angas Bremer Plains of South Australia has driven a number of recent initiatives aimed at trying to understand the complex interaction between surface and ground water and, in particular, the effect this has on salinity of these resources and the potential for irrigation-induced salinisation of the region. Among these initiatives is the development of the Angas Bremer Integrated Systems Model which aims to better define strategies for a truly conjunctive approach to water management from the paddock to catchment scale. Key to the successful development of that model is the availability of good biophysical data which describe the spatial variability of salinity and water quality across the region, most particularly in the near surface (<30m). Airborne electromagnetic data have the potential to assist in delivering this information, but they require careful, constrained interpretation and analysis, if they are to deliver products with this information.
-
-
-
Spatial Patterns and Processes of Floodplain and River Salinisation – Sunraysia, Victoria, Australia
Authors A.D. Fitzpatrick and T.J. MundayThe Sunraysia irrigation region of Australia supports a nationally significant citrus, vegetable production and horticulture industry. This industry is dependent upon the Murray River supplying water for irrigation, household use, livestock consumption, and industrial water use. Salinisation of the Murray River is a significant threat to the sustainability of the irrigation resource for this region. Natural inflows of highly saline groundwater to the Murray River have been exacerbated by irrigation development and adjacent clearing of native vegetation and the development of dryland agriculture within the vicinity of the river. Airborne electromagnetic methods have the capacity to provide detailed spatial information to assist in the prediction of the impact of current and future irrigation developments, the design of salt interception schemes and protection of floodplain values. These issues are central to the regional and state strategies for the River Murray. The RESOLVE survey of the Sunraysia region has demonstrated that floodplains can exhibit varying conductivity, both laterally and with depth, over fine scales. The data from this airborne survey is being used to produce key land use management tools.
-
-
-
Influence of the Parameterization and a Priori Information on the Inversion of Surface Waves – Parametrical Study and Application
Authors M. Wathelet and D. JongmansInversion of Rayleigh dispersion curve does not provide one unique solution and the choice of the parameterization (number of layers, range of velocity and thickness values for the layers) is of prime importance for obtaining reliable results. The number of solutions can be reduced by a priori information (geologic log), as well as by the use of other sets of data, which can be easily introduced in the inversion process using direct search inversion techniques. The method is illustrated on both synthetic and real cases.
-
-
-
Determination of Dispersion Curves from 3D Seismic Surveys
Authors B. Neducza, E. Törös, P. Tildy, Z. Pronay, A.I. Kanli and L. HermannThe presented method is a modification of the well-known SASW method. The algorith is tested on a real dataset measured above a refilled mine pit. Phase velocity maps are generated directly from shot records. Local dispersion curves are determined by the most frequent values instead of using tomographic inversion. Finally, the phase velocities are plotted on velocity maps at each frequency.
-
-
-
Modeling of R-wave Dispersion Curve by Using Fast Delta Matrix Method
Authors E.E. Ekincioglu and S. KadiogluThis paper is based on computation of fundamental mode Rayleigh wave (R-wave) dispersion curve depending on layered earth model by using fast delta matrix method. In addition, this paper indicates the changes in the dispersion curve and phase velocity according to the model parameters, thickness, density, P and S-wave velocity. In this study, we introduced the mathematical solution of the fast delta matrix method which computes free mode dispersion of a layered earth model. We modelled dispersion curves for two earth model and figured out only the fundamental mode. First, we calculated dispersion curves for by increasing each parameter by 25% for a six layered earth model. Secondly, we examined changing in the thicknesses, densities, P and S-wave velocities for a two layered model. Result showed that variation in S-wave velocities has an important effect on R-wave phase velocities when it is compared with other effects of model parameters. However, this doesn’t mean that other parameters are not affective. When individual changing of the parameters examined, it is seen that all parameters have a role in changes of phase velocity and dispersion curve according to the frequency range.
-
-
-
An Integrated Wave Equation Datuming Scheme for the Overthrust Data Based on the One-way Extrapolator
Authors K. Yang, F. Jiang, J.B. Cheng and Z.T. MaThe wave equation datuming (WED) is a necessary technique to deal with datuming correction in areas where the surface-consistent assumption is not valid. The previous applications of WED usually downward or upward continue the data from the surface to a planar datum. But for overthrust-belt data, it is always hard to find a planar base for weathering layer (WL). So the wave-equation layer replacement has to be done under the assumption that the base of WL is non-planar. In this paper, an integrated WED scheme applicable to the overthrust data is proposed. Firstly, the wave-field is downward continued to a non-planar base of the WL to remove the unexpected effects caused by the WL. Then, an upward continuation is applied to extrapolate the wave-field to a final planar datum with the replacement velocity. Either the downward or the upward continuation are accomplished by the one-way extrapolator such as finite-difference (FD) operator in a recursive way which constitutes an integrated wave-equation layer replacement scheme.
-
-
-
Imaging of Different Moisture States Inside an Undisturbed Soil Monolith with Georadar Tomography
Authors J. Schmalholz, M. Heigel, U. Yaramanci, J. Köstel and A. KemnaFour different moisture states inside an undisturbed soil monolith, in our case a lysimeter, (cylindrical vessel of 1.2 m diameter and 1.5 m height) are imaged with georadar tomography. Two of the moisture states correspond to different steady state flow conditions which were adjusted by means of an irrigation device. Due to the all-around accessibility of the investigated lysimeter high resolution horizontal tomographic planes can be acquired. In this way, the lateral heterogeneity inside the lysimeter is investigated non-invasively, which is new for soil physical investigations. Based on the tomographic planes distinct lateral and vertical heterogeneous volumetric water content distributions are identified. According to the tomographic results lateral volumetric water content differences of up to 0.1 m3/m3 are encountered. In the vertical direction, volumetric water content changes of more than 0.1 m3/m3 are detected. This reflects the presence of different soil layers. Due to the different response of specific regions inside the lysimeter subject to the moisture state, supplementary information on the soil hydraulic properties is gathered.
-
-
-
Attenuation and Velocity Characteristics of Nano-to-micro Scale Magnetite Mixtures – Implications for Near-surface GPR
By N.J. CassidyFor many higher-frequency (500-2000MHz) near–surface ground-penetrating radar applications the magnetic properties of the subsurface are considered unimportant or negligible. However, if significant amounts of magnetically lossy minerals are present, such as magnetite, hematite, maghemite and/or iron in its free state, the influence of these individual components on the GPR signal attenuation or propagation velocity cannot be ignored. In an attempt to obtain an element of quantitative control on the attenuation/propagation characteristics of such materials, the results of a dielectric spectroscopy investigation into mixtures of quartz and micro-to-nano scale magnetite is presented where the relative attenuation enhancement (in decibels per metre – dB/m) and velocity retardation (in percent velocity difference) is given for a range of volume percent, ‘generic’ magnetite/quartz mixtures. The results indicate that even small amounts of magnetite can produce significant degrees of loss and velocity reduction and at the higher frequencies associated with near-surface GPR, any observed dispersion behaviour will be predominantly controlled by the magnetic loss characteristics rather than frequency-dependent variations in propagation velocity.
-
-
-
GPR Survey in Coastal Sambaqui Archaeological Site in Santa Catarina State, Brazil
Authors S.I. Rodrigues, J.L. Porsani, P.A.D. DeBlasis and P.C.F. GianniniIn this work some GPR results obtained in coastal sambaqui Jabuticabeira II are presented. Jabuticabeira II is located in Santa Catarina State, South of Brazil. GPR profiles show interesting anomalous reflectors, which can be related to targets of archaeological interest. 2D FDTD modelling studies presented good accordance with real data obtained in the field. Synthetic model has been constructed in order to evaluate the performance of GPR method in relation to materials found in that environment, besides helping interpretations of real results. So, GPR results function as reference guide to direct excavation activities in buried archaeological, and geological structures, a task that will be the next step of this research.
-
-
-
Numerical Modelling of Commercial GPR Antennas
Authors C. Warren and A. GiannopoulosThe use of Ground Penetrating Radar (GPR) has become widespread in a range of fields - civil engineering, geophysics, environmental engineering, archaeology, and remote sensing. This has led to the development of increasingly sophisticated GPR models to help progress understanding of how to detect, interpret and identify buried objects. The transmitter and receiver antenna(s) are a key component of any GPR system, but are often not accurately described and therefore not well modelled in many GPR simulators. This research aims to improve understanding and interpretation of GPR signal attributes, by incorporating accurate descriptions of commercially used antennas into GPR models. A methodology is demonstrated for creating 3D FDTD models of two real GPR antennas. It is shown that when the, often complex, geometry is accurately represented in the FDTD model, a good agreement can be obtained between the simulated results and those measured directly using real GPR transducers.
-
-
-
Geophysical Study of Salinity Distribution in Northeastern Thailand Paddy Soils
Authors J.-P. Montoroi, O. Grünberger, N. Bouzid, Y. Enet, S. Sukchan, K. Srisuk and S. NadeeSalinization process is a worldwide constraint for soil management especially in Northeast Thailand, region characterized by widespread, deep and thick continental salt deposits. The near-surface distribution of salinity is performed using ground geophysics methods including time domain electromagnetic, frequential domain electromagnetic, vertical electrical soundings and self potential. The vertical and horizontal mapping of the electrical resistivity indicate a very conductive anomaly, situated at nearly 10 m depth owing to a 3-4 m thick clayey layer. Electrical conductivity mapping made in rainy season gives better details than the one made in dry season. Self-potential contrasts are clearly shown and are mostly due to electrodiffusion potentials. A conceptual model of the hydrosystem functioning was proposed at a local scale including two groundwater systems namely a deep saline groundwater flowing up through fractures and a shallow saline groundwater overlying a low-permeable clayey layer.
-
-
-
Characterization of Contaminated Sites by Resistivity and IP Methods
Authors Z. Nyari and B. NeduczaA consortium with three partners from different fields of environmental sciences (Geophysics, Hydrogeology, Geotechnics) was formed for a three-year-long project to improve hydrogeophysical methods in order to detect and characterize special subsurface contaminants. Contaminated sites were chosen in Hungary as study-areas to improve and calibrate special geophysical methods to provide remediation experts and hydrogeologists with necessary information for reliable transport modeling. A strong collaboration between the geophysicists and hydrogeologists evolved protocols and techniques to carry out successful site assessment and remediation schemes of contaminated lands. In this multi-disciplinal project the results provided by the geophysicists were immediately applied and controlled by the end users. This study presents two case histories on detailed geophysical survey of two contaminated areas. At both sites high resolution techniques providing reliable data were applied. The results were validated and accepted by hydrogeologists and applied during the remediation processes.
-
-
-
Geophysical Investigation of the Bosherston Lily Ponds, South Wales, UK
By C. HusbandThe Bosherston Lily pond is a man made water body situated in Pembroke, South Wales, UK. Created in stages between the late 18th and mid 19th century, the lily ponds were formed by damming a river valley. The ponds are now maintained by the National Trust and have been classified under the EC Habitat Directive as a Special Area of Conservation (SAC). The lily ponds are of great importance not only due to the diverse ecology but because of the significant financial income generated in the area by tourism. The lily ponds exist due to a delicate balance of influxes and outflows; any change could lead to the lily ponds demise. It has been noted that the summer level of the lake has decreased over the last 80 years. It is believed that the water is seeping away through cracks and crevices in the underlying carboniferous limestone. The aim of the geophysical survey was to locate possible conduits which would explain the decrease in water level. A combination of geophysical techniques were employed in order to comprehensively survey the subsurface; these included electrical resistivity arrays, ground penetrating radar and self potential surveys.
-
-
-
Using GPR to Monitor Fluid Channelization in a Subhorizontal Bedrock Fracture by Tracking a High-salinity Tracer
Authors G.S. Baker, K. Burns and J.T. PardyTo observe flow channeling in situ, surface ground-penetrating radar (GPR) was used to detect a saline tracer moving through a sub-horizontal bedrock fracture. The tracer is mapped using amplitude variations resulting from the significant difference in electromagnetic properties between the tracer and natural ground water within the fracture. Results show that hydrogeophysical investigations using GPR can successfully image tracer movement in a variety of flow configurations in three spatial dimensions through time (4D). Observed tracer travel paths demonstrated meter-scale spatial variability that may be attributed to heterogeneity in fracture aperture. Subsequent controlled experiments yield empirical relationships between GPR reflection amplitudes and fracture aperture and tracer concentration.
-
-
-
Realistic GPR Simulation of a Modelled DNAPL Migration – A Novel Link between Two Numerical Models
Authors C. Power, V. Wilson, J.I. Gerhard and A. GiannopoulosIn this paper we present some initial results of our investigations in creating a meaningful and realistic link between two numerical models that can be used to enhance our understanding of the process of migration of DNAPL and of the GPR’s potential for its detection. The aim is to simulate realistic GPR scans using an electromagnetic simulator from realistic subsurface DNAPL spills as obtained using a hydrogeological fluid flow model.
-