1,721,020 research outputs found
Electric characterization of construction materials through radar data inversion
No full textThe non-destructive evaluation with the aim of characterizing objects before or after treatment has taken place, and the monitoring of long-term performance is analyzed in this thesis. Generally, these test methods measure material properties or changes in these properties that decision makers are interested in. There is a variety of non-destructive testing (NDT) methods to choose from, depending on the aim of the analysis. Generally speaking, different non-destructive testing methods can be used for investigating the inner structure of materials. It is worth noticing that there is not one single analytical method able to provide all the necessary information. Therefore, a combined test series capable of providing complementary information is usually adopted. Most of the methods used in NDT generate pictures of the object interior that can help locate structural flaws. Even though not common, these methods can give quantitative results. The use of electromagnetic waves within the radio frequency bandwidth, in particular, can be valuable in detecting defects, for assessing the deterioration and the success of refurbishment activities. A particular radar technology was used in this study as a tool for assessing the ability to characterize materials in specific built environments. Numerical and laboratory studies were carried out to evaluate the feasibility of the proposed methodology for the mentioned applications. A secondary objective was to provide a contribution to the road safety, preventing the risk of severe damage of pavement, induced by clay content in sub-asphalt layers, and to improve the operations of rehabilitation and maintenance through an effective inspection. In Chapter 2 the electric properties of multiphase aggregate mixtures were evaluated for a given mineralogic composition at frequencies between 300 kHz and 3 GHz. Two measurement techniques were employed: a coaxial transmission line and a monostatic stepped-frequency ground-penetrating radar. The propagation matrices analytical method was used to retrieve the electrical permittivity and conductivity of the mixtures from the measured scattering parameters in the coaxial transmission line. The effect of increasing water content was analyzed in several sand-clay mixtures. For the end-member case of maximum clay (25% by weight) and increasing water content, investigations were compared between the two measurement techniques. The electrical properties of materials are influenced by the amount of water, but clay affects the frequency dependency of soils showing distinctive features regardless of the mineralogy. The microwave attenuation, expressed by the quality factor Q, is partly dependent on frequency and on water content. The performance of one empirical and one volumetric mixing model was evaluated to assess the capability of indirectly retrieving the volumetric water content for a known mixture. The results obtained were encouraging for applications in the field of pavement engineering with the aim of clay detection. The models used show similar behaviors, but measured data were better modeled using third order polynomial equations. High-frequency, ultra-wideband penetrating radar has the potential to be used as a non-invasive inspection technique for buildings, providing high-resolution images of structures and possible fractures affecting constructions. To test this possibility, in Chapter 3 numerical and laboratory experiments were conducted using a proximal, stepped-frequency continuous-wave radar system operating in zero-offset mode, spanning the 3-8 GHz frequency range. The reconstruction of material electrical properties is achieved by resorting to full-waveform inverse modeling. Numerical experiments showed that for typical electric permittivity and electrical conductivity values of concrete and plaster, it is possible to retrieve the physical properties of the material and to detect fractures less than 1 mm thick. Laboratory experiments were conducted on non-reinforced concrete and plaster test slabs in different configurations. The results showed the good potential of this method: (1) to provide a thorough fracture response model in buildings or artworks and (2) to non-invasively characterize the samples in terms of their electromagnetic properties. The characterization of the subsurface can be performed by full-waveform inversion of electromagnetic data relating to a particular model. The modeling process relies on the ability of retrieving the scattered field Green’s function from the measured data. This is achieved using sets of antenna characteristic global reflection and transmission coefficients to describe the media in terms of their scattered field impulse response. As described in Chapter 4, crucial for a successful implementation of this technique is the understanding of uncertainties involved in the acquisition of the antenna calibration and survey measurements, and how these propagate in the parameter estimation results. It was found that averaging a number of possible Green’s functions obtained from one measurement with several antenna characteristic coefficients sets works remarkably well in reducing the uncertainties. The accuracy of the inversions improved using characteristic coefficients acquired as close as possible to the measurement conditions. Moreover, a clear relation between dynamic range and system resolution was highlighted, based on the number of effective bits contained in the data. The presence of cohesive soils as bearing courses of road pavement frequently causes damages and defects (e.g., transversal and longitudinal cracks, deformations and ruts). In Appendix A, different ground-penetrating radar (GPR) methods and techniques were used to non-destructively investigate the clay content in sub-asphalt compacted soils. The experimental layout provided the use of typical road materials, employed for road bearing courses construction. Three types of soils classified as A1, A2, A3 by the American Association of State Highway and Transportation Officials (AASHTO) were used and adequately compacted in electrically and hydraulically isolated test boxes. Analyses were carried out for each clay content using two different GPR instruments. A pulse radar with ground-coupled antennae at 500 MHz center frequency and a vector network analyzer (VNA) with a 1–3 GHz bandwidth were used. Signals were processed in both time and frequency domains, and the consistency of results was validated by the Rayleigh scattering method, the full-waveform inversion and the signal picking techniques. Promising results were obtained for the detection of clay content or cohesive soils affecting the bearing capacity of sub-asphalt layers.Geoscience & EngineeringCivil Engineering and Geoscience
Electrical Survey of Peat Deposits
No full textGeoscience & EngineeringCivil Engineering and Geoscience
Fast Frequency and Time Domain Integral Equation Modelling for Marine CSEM Applications
No full textIn this study we developed algorithms for fast frequency and time domain integral equation modelling for marine controlled-source electromagnetic (CSEM) applications. Solutions of integral equations for CSEM applications in a three-layered earth with an assumed reservoirs is examined using the conjugate gradient fast Fourier transformation (CG-FFT) method, which is used as a reference. For 3D configurations fast computational methods are relevant for both forward and inverse modelling studies. The Born approximation, extended Born approximation, and iterative extended Born approximation are implemented and compared with the full solution of the CG-FFT, even with a reservoir consisting of two separated compartments. We also showed that the approximate results are accurate at the receiver level, which is usually the sea bottom, and inside the reservoir. This iterative method is suitable as a modelling algorithm for solving the inverse scattering problem as well. These methods are based on an electric field domain integral equation formulation. With the help of this method sensitivity analysis using 3D modelling is possible in a timely manner, which is vital for CSEM applications. Our modelling studies investigate to which extent the iterative extended Born approximation method is fast and accurate for forward modelling and could be used for inverse modelling. Sensitivity analysis as a function of the source position and different reservoir sizes validates the accuracy of the iterative extended Born approximation. We also looked into the question in what model configurations two-and-a-half dimensional modelling is a good modelling choice for three-dimensional reservoir response to the diffusive EM field. It is investigated how the accuracy of 2.5D modelling compares with 3D modelling depending on the configurations. Because 2.5D modelling is much faster than 3D modelling, proper use of 2.5D CSEM modelling provides sensitivity analysis of subsurface parameters for many different scenarios in a limited amount of time. Depending on size and depth of the target, hydrocarbon thickness, and extension in cross-line dimension, the required frequency content of the emitted diffusive field and the number of offsets must be determined to assess if the target can be detected. At this stage a decision can be made on whether or not the target reservoir is detectable. From here an optimum survey can be designed based on the forward modelling results. Similarly, in this way the detectability from time-lapse monitoring data of changes in the reservoir due to production can be assessed. Finally we investigated how frequency-domain data can be efficiently transformed to the time-domain. CSEM methods are generally divided into frequency-domain electromagnetic (FDEM) and time-domain (transient) electromagnetic (TDEM) methods, depending on the waveform of the transmitted electrical current. We compared a quasi-analytical method to transform frequency-domain CSEM data back to the time-domain with a numerical transformation. The quasi-analytical method exploits the fact that the kernel of the integral equation has a known behavior as a function of frequency and that the solution to the integral equation can be written as a sum of repeated applications of the kernel to the incident field. A set of expansion functions is found, which have analytically known time domain counterparts, which need only a limited number of frequency values for the transformation back to time. We compared this quasi-analytical method, coined the Diffusive Expansion Method in CSEM applications with two other numerical methods, Gaver-Stehfest method and an optimized form of the fast Fourier transformation method where the data is required at an minimum number of discrete frequency values such that the data at intermediate frequency values can be accurately obtained by interpolation. We found that the Diffusion Expansion Method is a good candidate for CSEM frequency-to-time conversion of data for any kind of subsurface model and survey configuration.Geoscience & EngineeringCivil Engineering and Geoscience
Measurements of capillary pressure and electric permittivity of gas-water systems in porous media at elevated pressures: Application to geological storage of CO2 in aquifers and wetting behavior in coal
No full textSequestration of CO2 in aquifers and coal layers is a promising technique to reduce greenhouse gas emissions. Considering the reservoir properties, e.g. wettability, heterogeneity and the caprocks sealing capacity, the capillary pressure is an important measure to evaluate the efficiency, the success and the safety of storage applications. In this research, the capillary pressure behavior was investigated for the CO2-water system in quartz and coal. Measurements were conducted at pressures and temperatures, ranging from ambient to reservoir conditions, where CO2 is present as supercritical fluid. Furthermore, the relation between capillary pressure hysteresis and interfacial area was investigated, measuring the capillary pressure and the electric permittivity as a function of frequency, simultaneously. The results from the quartz samples, showed a dependence of the capillary pressure on the CO2 pressure. Moreover, only dissolution rate effects for gaseous CO2 in the water were observed. Significant capillary pressure fluctuations and negative values during imbibition were observed at near supercritical conditions. From the coal experiments it was observed that with increasing CO2 pressures the wettability of medium rank coal altered from water-wet to CO2-wet. High rank coal was CO2-wet during primary imbibition experiments in the entire pressure range. The relation between capillary pressure and interfacial area has been investigated by measuring the capillary pressure and the electric permittivity at 100 kHz as function of the water saturation. The permittivity data showed hysteresis between drainage and imbibition. Furthermore, non-monotonic behavior was observed which was attributed to polarization of the gas-water and water-solid interfaces. The permittivity hysteresis is provoked by the different phase distributions and geometries. From these results it was concluded that the capillary pressure is a unique function of the permittivity and the water saturation.Geoscience & EngineeringCivil Engineering and Geoscience
Electric characterization of sands with heterogeneous saturation distribution
No full textIn the soil sciences it is of great relevance to accurately determine the electromagnetic properties of the soils, specially the complex permittivity. But in the high frequency regime (relevant band for geophysical applications) accurate permittivity measurements are complicated as the relation of the measured quantities with the permittivity itself, becomes highly non-linear. Moreover, in this type of measurements (core scale), the medium is taken to be homogeneous and there is no study about the limitations of this approximation. The scope of our research is to set accurate measurements of the electromagnetic properties of the soils in the lab that will enhance the resolution of existing geophysical models and techniques. For that purpose we have developed a measurement tool and carried out a set of experiments to test the validity of measuring the permittivity as an effective property.Civil Engineering and Geoscience
Borehole radar for oil production monitoring
No full textThe area of smart well technology, or closed-loop reservoir management, aims at enhancing oil recovery through a combination of monitoring and control. Monitoring is performed with a wide range of sensors deployed downhole or at the surface. These sensors allow for capturing changes in the reservoir conditions, mainly the fluid movement, at different resolutions. Downhole sensors give information of the fluid entering the well and sample only the region immediately adjacent to the well. Reservoir-imaging techniques are based on downhole or surface sensors and image large reservoir volumes typically with a resolution at the ten meter scale. Control is performed by installation of downhole flow control devices that can regulate the fluid inflow from the reservoir into the well ranging from on/off to a large number of settings. Combining monitoring and inflow technology allows using control strategies that mitigate undesired events such as premature water or gas breakthrough. Premature breakthrough of undesired fluids can reduce drastically the oil production and may cause the production well to be shut down. Generally the near-well region in the order of ten meters is poorly imaged. However, in specific reservoir environments the monitoring of the near-well region is strongly required. For example, thin oil rim reservoirs usually have a thickness in the order of few tens of meters and are characterized by early water breakthrough in individual segments of the well. Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery technique used in heavy oil reservoirs, where oil is extremely viscous and steam injection is used to facilitate the oil flow. A pair of horizontal wells is drilled into the reservoir only a few meters apart to allow for steam injection and oil production; however, the steam chamber growth and the oil flow are largely unknown. In both these examples a better understanding of the oil displacement process in the first ten meters from the production well could help preventing early breakthrough of unwanted fluids and allow for an implementation of more effective control strategies. We have investigated radar technology as a potential tool able to cover the monitoring requirements needed in specific oilfield environments. This feasibility study was carried out through numerical modeling and laboratory experiments. Through the numerical simulations we conclude that a borehole radar system can be used as a monitoring tool to probe the near-well region of several meters. The main constraint is the formation water electrical conductivity; high conductivity makes attenuation and phase distortion too high for wave propagation. Water/steam front reflections are detectable in low conductivity reservoirs (? < 0.02 S/m). A system performance above 80 dB is necessary to detect reflections in the range of 10 m (chapters 2-3). Additional reservoir constraints are given by a high degree of time-lapse heterogeneity changes of the EM properties and the length of the transition zone from oil to water bearing rocks. The effects of changes in the reservoir can be solved by increasing the data acquisition frequency relative to the rate of the local temporal changes. A gradual transition zone reduces the water reflections, which are not detectable when the transition is in the order of the dominant wavelength of the EM signal (chapters 2-3). Numerical simulations were performed for both simple and complex geological scenarios. A sophisticated analysis was performed coupling electromagnetic and reservoir simulations. This allowed to evaluate the GPR performance in a realistic reservoir environment. Plotting the amplitude of the two-way-time reflected signal as the water advances toward the production well, where the radar system was located, appeared in clear up-dipping events (chapter 3). The metal components of the wellbore casing can destructively interfere with the signal emitted by the radar sensor; however a high dielectric medium around the sensor can increase the amplitude of the reflected signal and overcome the interference problem (chapter 2). Through the laboratory experiments we conclude general considerations on the GPR ability in monitoring oil displacement process governed by water. Water was injected in a meter-scale sand box and all the water flooding experiments presented similar characteristics. As for the modeling results, the amplitude of the two-way-time reflected signal as a function of the experiment time resulted in up-dipping events ascribable to the water front advance. According to the initial water saturation and porosity distribution continuous down-dipping events were associated to the up-dipping ones, forming wedgeshaped reflection features. The monitoring of the flow reflection features could be supported by attribute analysis, in particular, instantaneous frequency demonstrated to be a powerful tool to enhance wedge-shaped events. The analysis of the GPR data agreed with impedance measurements taken simultaneously during the water flooding experiments. The main limitation to the GPR monitoring potential is the electrical conductivity of the residual water. The experiments at a high salinity water injection showed a strong attenuation of the signal and a reduction of the resolution (chapter 4). Through an analysis of measured and modeled GPR signal it was possible to take in consideration the effect of uncertainties on subsurface characterization through full-waveform inversion. Subsurface characterization through full-waveform inversion relies heavily on the accuracy with which the forward model represents the actual GPR-subsurface system. Model errors can propagate through the inversion procedure resulting in wrong parameter estimates. The relative errors in the measured Green’s function are mainly determined by the antenna transfer functions uncertainties. Averaging over a large number of transfer function sets leads to a high-accuracy Green’s function estimate from the data, which leads to small errors in the estimated parameters obtained from full-waveform inversion. Provided the measurement conditions are respected, the inversion experiment adequately reproduces the estimated parameters. As soon as the measurement conditions are not completely respected, e.g., presence of extraneous objects, inversion experiments indicated that the accuracy of the estimates improves when calibration measurements to determine the transfer functions are acquired as close as possible to the measurement location (chapter 5).Geoscience & EngineeringCivil Engineering and Geoscience
Electrokinetic conversion
No full textIn their search for improved and new exploration tools, geophysicists have improved seismic and electromagnetic techniques. Since the 1930s it is known that there is a coupling between seismic and electromagnetic waves in the shallow subsurface of the earth. Electroseismic surveying and its reciprocal process, seismic-to-electromagnetic conversion, are methods for remotely identifying the presence of hydrocarbons in the subsurface of the earth. In this study we investigate electrokinetic coupling theoretically and experimentally. The origin of this effect lies in a very thin nano-layer which is conventionally present at all solid-fluid interfaces where an excess charge density with respect to the bulk charge density in the pore fluid exists. Any hydraulic disturbances of this nano-layer cause electric currents that are opposed by ionic counterflows generating electric fields. These effects become manifest when acoustic waves impinge upon the interface between two adjacent porous layers having different electro-mechanical properties. The theoretical basis for the coupling phenomena under investigation is conceptually imbedded in the framework of the combined Biot-Maxwell equations. An important aspect of the theory is the so-called dynamic (i.e., frequency-dependent) coupling factor. This coupling factor is studied in a dedicated experimental set-up where an oscillating flow through a porous material generates electric fields. We address the frequency-dependency of the coupling coefficient, the mathematical description and experimental detection results. We validate that the governing model is capable of modeling the coupling effect.Geoscience & EngineeringCivil Engineering and Geoscience
Controlled-Source Electromagnetics for Reservoir Monitoring on Land
Full text linkThe main goal of exploration geophysics is to obtain information about the subsurface that is not directly available from surface geological observations. The results are primarily used for finding potential reservoirs that contain commercial quantities of hydrocarbons. A number of possible geophysical methods exists these days to achieve such a goal. One of them is the controlled-source electromagnetic (CSEM) method. CSEM data can provide resistivity maps of the subsurface. Because the bulk resistivity depends on the resistivity of the pore fluid, these maps may enable us to estimate the nature of the fluid content in the reservoir. The CSEM method exploits electromagnetic fields to remotely characterize the nature of the fluid content in the pores. When a dipole current source is stuck into the ground or placed in the seawater, current flows from one pole to the other through the sediments, creating an electrical field in the subsurface. If highly resistive bodies are present in the subsurface, the electrical field measured at some distance from the source will be larger in amplitude than the field in the absence of these bodies. As hydrocarbon-bearing rock is highly resistive, one may link the larger amplitude to the presence of hydrocarbon reservoirs. A logical consequence of this phenomenon is that the CSEM method may also be suited for monitoring a hydrocarbon reservoir during production. The reason is that water flooding or steam injection for oil production creates resistivity changes in the reservoir, and if those changes are large enough, we can expect differences in the CSEM response with time-lapse surveys. This consideration led us to further investigate the EM monitoring problem. We tried to answer two questions: are the time-lapse changes in the reservoir detectable, particularly in the presence of noise, and if so, could we use timelapse signals to locate where the time-lapse changes happened in the subsurface? In this thesis, we considered land CSEM and found that the resistivity change due to displacement of oil by brine can produce a small but measurable difference in the CSEM response. Interestingly, those response differences at the surface are confined to the lateral extent of resistivity changes in the subsurface, even in the presence of various kinds of repeatability noise. We found a simple and effective method to remove the repeatability noise due to the airwave. Finally, results obtained when incorporating nonlinear EM inversion into the monitoring problem suggest that this application of the CSEM method has the potential to play a significant role in the oil and gas industry.Geoscience & EngineeringCivil Engineering and Geoscience
Marine Controlled-Source Electromagnetic Interferometry
Full text linkIn marine Controlled-Source Electromagnetics, a boat tows an electric source, whose signal is travelling on various paths to the receiver stations at the ocean bottom. Unfortunately, the signal does not only travel via the subsurface to the receivers, but also directly through the water and via the air-water interface. Signals travelling on the latter two travelpaths do not contain any information about the subsurface. On the contrary, they cover a possible response from a subsurface reservoir. Therefore, one aims to suppress the signal travelling along those paths. Interferometry by multidimensional deconvolution replaces the overburden by a homogeneous halfspace suppressing any interactions with the air-water interface. Furthermore, the direct field is removed and the source is redatumed to a receiver position. Since interferometry by multidimensional deconvolution is a data-driven method, no information about the ocean or the subsurface is needed, except the material parameters at the receiver level. This thesis investigates the benefits and limitations of interferometry by multidimensional deconvolution applied to marine Controlled-Source Electromagnetic data.Geoscience & EngineeringCivil Engineering and Geoscience
Interferometric redatuming by multidimensional deconvolution
No full textSeismic reflection imaging is a popular method to image, characterize and monitor the Earth's subsurface. In this method, seismic signals are sent into the subsurface and their reflections are collected. Strong heterogeneities in upper sections of the subsurface often pose a problem for imaging deeper sections. To overcome these problems, it has been proposed to place receivers in a horizontal, deviated or vertical borehole and to turn these receivers into virtual sources by seismic interferometry. In this thesis, the correlation-based formalism that undergirds seismic interferometry is replaced by multidimensional deconvolution, yielding several important advantages. It is shown that multidimensional deconvolution improves the radiation pattern of the generated virtual sources and that it removes undesired artifacts. A range of applications is being discussed, including the retrieval of signals from background noise, subsalt imaging and reservoir monitoring.Geoscience & EngineeringCivil Engineering and Geoscience
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