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Measuring and analyzing ground penetrating radar data on different sand-clay soils as a function of water content

Numerical models were used to recursively compute the density profile and the seismic velocity profile of three different artificial models of the underground from primary reflection images obtained from multi-angle incident plane wave reflection data. The aim is to investigate the effect of errors in the obtained primary reflection amplitudes on the recursive construction of the vertical density-velocity profiles. The reflection coefficients needed for the computations were obtained by solving the Marchenko equation for different angles of incidence. The recursive computation shows errors occur in every layer, but the error does not necessarily grow with each step. This implies the error does not propagate into the recursive scheme. Adding a random error to the reflection coefficients yielded results in a greater error in each individual layer with respect to the values obtained without an added error. Using a too large angle of incidence can result in too few primary events in the autofocused data, distorting the computed values.

Forward modelling demonstrates that resistivity anisotropy has a huge effect on Multi-Transient ElectroMagnetic step and impulse responses. The earth is never isotropic – even a stack of isotropic layers behaves anisotropically – and there is a great need to ccount for resistivity anisotropy in order to delineate the true target depth and target transverse resistance in ElectroMagnetic surveying. I account for resistivity anisotropy by (a) deriving apparent anisotropy formulae and using them together with apparent resistivities for a fast iterative inversion scheme, and (b) by including anisotropy into a 1D full waveform inversion scheme. Full anisotropic inversions result in much smoother models than isotropic inversions. Sharp resistivity boundaries result in anisotropy anomalies, as horizontal and vertical resistivities are not affected in the same way. Anisotropic inversion results yield a good indication of the present background anisotropy. Carrying out inversions with fixed anisotropies, e.g. determined in a free anisotropic inversion, can improve the result significantly compared with an isotropic inversion.

Water-flooding in heavy oils is generally not an efficient way of production due to high viscosity of heavy oil compared to water. Therefore, thermal recovery methods are commonly used in heavy oil production. Most thermal methods involve fluid injection to transfer heat further into the reservoir. Hot water-flooding is among these methods. In hot water-flooding, thermal energy will increase oil mobility, and possibly provide a more efficient sweep. This research investigates the effect of heat on water-flood recovery. An approximate analytical model has been constructed to describe fluid flow and heat transfer, simultaneously. Furthermore, several core flooding experiments have been conducted. These experiments involve regular (isothermal water-flooding at room temperature), and non-isothermal (hot) water-flooding. X-ray Computed Tomography (CT) scans have been also taken during the experiments to detect the movement of the water phase and the stability of the displacement front. It has been observed from the experiments, that increasing the injection temperature delays water breakthrough, and increases recovery factor of the water-flood. Moreover, due to the decrease in oil viscosity, the pressure drop along the core also decreases with increasing temperature. On the other hand, the movement of the water phase cannot be detected accurately from CT images.

Seismic and electromagnetic imaging methods both provide the geophysicist with different types of medium parameters. Seismic methods are sensitive to the elastic properties of the medium, while electromagnetic methods are sensitive to the electric properties. In porous-saturated media, these two wave fields occur as a coupled system, which is known as 'seismoelectrics'. This coupling is caused by physical interactions at the grain surface boundary and is a function of several medium parameters, such as dynamic permeability. This medium parameter is valuable to the oil and gas industry, as well to the field of hydrology. By conducting a seismoelectric survey it would theoretically be possible to provide an extra control on this medium parameter. However, both practice and theory have shown that this coupling mechanism also results in a low signal-to-noise ratio. A possible solution to this problem would be to apply interferometric Green's retrieval, which is a technique based on stacking of cross-correlated data. This approach has been proved successful for the SH-TE mode in 1D. The SH-TE mode forms together with the P-SV-TM mode, the total seismoelectric system. In this thesis the first steps are taken towards the proof that this technique could also work for the P-SV-TM mode of the system. This is supported by a modelling experiment of 2D homogeneous space solutions of the seismoelectric P-SV-TM mode for different configurations. This analysis turned out that the unwanted artefacts observed in the interferometric retrieval are generated by cross-correlations between P-waves and SV-waves.

The objective of thesis is to see at which rate and depth Arsenic (As) contaminated water flows into deep wells in the subsurface of a small region in Bihar, India. The method of investigation is interpretation of geological data and implementing this in a flow model built in Comsol Multiphysics 4.2a. Interpretation of the data is done by grain size analysis on two samples, using microscopy and sieve tests. The results show extensive differences in characteristic properties: The top layer (0-28m) has a permeability of 30.7 mD and a porosity of 20 %; the bottom layer(28-50m) has a permeability of 9.34*10^5 mD and a porosity of 36%. The flow model is based on the assumption of Darcy flow and transport of diluted species. The measured value of Arsenic concentration in the well in the model is 232 µg/L. The model assumes As release to happen only in the top layer by microbiological activity at low rates of dissolved oxygen. Contamination in the well is caused only by flow from the top layer, for there is assumed no in situ release of Arsenic in the bottom layer. The research question is: is it possible to subtract water from the second layer found in Bihar to produce clean drinking water? The findings are that if the well is at a depth of 42 m, the Arsenic concentration comes to an equilibrium for longer times and stays below the value of 10 µg/L measured at the well.

Elastodynamic and electromagnetic processes are coupled together in saturated, porous media, by a phenomenon known as the electrokinetic effect. In horizontally layered media, the seismoelectric system, which contains the coupled elastodynamic and electromagnetic systems, can be separated into two independent modes of propagation: SH-TE and P-SV-TM. The SH-TE mode contains horizontally polarized shear waves coupled with transverse electric polarized electromagnetic waves. In the P-SV-TM mode, both fast and slow compressional waves are coupled with vertically polarized shear waves and transverse magnetic polarized electromagnetic waves. In this thesis, the P-SV-TM mode of the two-dimensional seismoelectric system was expressed in the form of both the two-way and one-way wave equations. The principle of normalizing energy flux across boundaries was applied, improving the matrix amplitude balance of the system and allowing for the implementation of one-way reciprocity theorems. We carried out full-waveform modelling of the flux-normalized P-SV-TM seismoelectric system in a 2-D fluid-saturated, horizontally-stratified, porous media. Both one-way and two-way wavefields were modelled, allowing the composition of one-way wavefields into two-way wavefields to be clearly observed. We investigated both the generation of electromagnetic fields due to the propagation of a seismic pertubation and the generation of seismic waves due to the propagation of a diffusive electromagnetic wave. Reciprocity of the wavefields was verified by applying reciprocity theorems to both one-way and two-way wave vectors. The electromagnetic field that is created when a seismic wave traverses a contrast in medium parameters is rapidly attenuated during propagation. To mitigate the decay in the amplitude of the signal with distance, we modelled a Vertical ElectroSeismic Profiling (VESP) survey, in which receivers could be placed in near proximity to the target layer. In another model, the sensitivity of the seismoelectric method to pore fluid contrasts was tested by simulating the influx of contaminants into an aquifer. It was observed that a small change in the conductivity of the aquifer led to a significant change in the strength of the electromagnetic signal that was generated at the top of the aquifer.

The numerical modeling with the diffusive controlled source electromagnetic (CSEM) method has been used to validate the published results of Ellis et al. (2010a) by first investigating the validity of effective medium theory for both horizontally layered isotropic and vertically transverse isotropic (VTI) layered earth. Ellis et al. (2010a) used ’moving window average method’ to calculate bulk vertical and horizontal resistivities of resistivity logs, from which they calculated effective anisotropic ratio. The question is if the application of their local averaging scheme is valid also for a layered earth. The investigation of effective medium theory is done in horizontal wavenumber-frequency domain (k−ω) which allows us to evaluate the influence of propagation and attenuation parts of the complex vertical wavenumber on the existence and stability of the effective solution for the whole range of wavenumber, k. For the layered models considered, the total thickness is held constant while the number of layers is varied with consequent changes in thickness of the individual layer. Also, reflection and transmission interactions are both ignored and included. For the horizontally layered isotropic earth, with only propagation-diffusion term, a stable effective transverse electric (TE-mode) isotropic conductivity exists from 0.5% of maximum wavenumber irrespective of the unit thickness of a layer. But with the inclusion of reflection and transmission interactions, the effective skin depth or wavelength has to be sufficiently large compared with unit thickness of the layered earth for a stable solution to exist at all values of wavenumber, k. It is noted that effective transverse magnetic (TM-mode) isotropic conductivity is not the same as that of TE-mode for large wavenumber, k, because of the different limits of their reflection coefficients at large k. However, there are no effective vertically transverse isotropic (VTI) parameters which satisfy the horizontally layered isotropic or VTI layered earth because the effective VTI parameters are wavenumber, k-dependent, though with approximately constant effective anisotropic ratio. We conclude that the effective medium theory adopted by Ellis et al. (2010a) is not a valid approach for modeling a layered earth, though it may be valid for a local measurement.

In Bihar, India, arsenic rich aquifers are used as a drinking water source by millions of people, even though the arsenic concentration in some places is so high it causes serious health issues. The arsenic concentrations in the aquifers show a large spatial variability, one that is presently unpredictable. The most important factors which play a role in arsenic distribution are the source (natural occurring minerals) and the redox conditions, which enable the arsenic to be released from the minerals. There is a relation between fluvial deposits and redox conditions. In addition, fluvial deposits are known to be heterogeneous, which has large influence on flow regimes. The objective of this study is therefore to investigate the fluvial deposits in the region, and to predict the control they have on the distribution of arsenic. To do so, the architecture of a point bar attached to a clay plug is mapped in the region of Bakhorapur, Bhojpur District, Bihar, India. This was done by interpreting Google Earth satellite images, with executing a transient electromagnetic survey and by drilling two boreholes of 50 m (including well logs); the cores were ultimately studied in detail. The three methods indicate that up to 28 m, two stacked, heterogeneous and laterally continuous 10-15 m thick point bars are present. Below these extensive conglomerate and coarse sand bodies of braided river origin are recognized. The end result is a geological model concept, in which fluvial reservoir architecture elements of importance for flow regimes are highlighted. The geological model concept suggests that both the initial place of release and the spreading of arsenic are to a large extent controlled by the 3D architecture of the fluvial deposits. The concept can be turned into a static model and used for flow calculations such that the user is able to predict arsenic spreading in the case of arsenic release from specific fluvial sediment bodies. Considering the region being full of similar point bar deposits, the concept is generally applicable throughout the region. With this, arsenic safe drinking spots can be located, helping a great number of people.

Seismic interferometry is a technique by which the Green’s function (or impulse response) between two receivers can be acquired from the crosscorrelations of wavefield responses at these receivers. Recent developments of this method has led researchers to exploit active as well as passive seismic wavefields to retrieve surface wave Green’s functions by crosscorrelation. The primary objective of these applications has been to gain near surface resolution from the high frequency content of the active data while gaining greater depth resolution from the low frequency content of the passive data. In these applications however, a Green’s function is retrieved for each data type and therefore a matching filter or a form of joint inversion is required to benefit from the additional bandwidth of both data types. Interferometry by multidimensional deconvolution (MDD) is a relatively new method of Green’s function retrieval that provides several advantages over interferometry by crosscorrelation. This thesis proposes a new method of merging active and passive data during the process of MDD. A primary advantage of this method over the alternatives is that the source signatures are disregarded and only a single Green’s function with the combined characteristics of both the active and passive data is retrieved. Using numerical modelling it is demonstrated that a broadband Green’s function response can be retrieved from combined active and passive data without the need to compensate for the differences in source signatures or variations in amplitude. Merging active and passive data prior to deconvolution may in fact improve the retrieved response due to the additional illumination provided by the supplementary data. In addition to expanding the bandwidth of the retrieved response, this method is shown to be capable of using data from one source type to spatially infill gaps in illumination in another source type when the bandwidth of the two are comparable.

We distinguish between trivial and nontrivial differences in retrieving the real or imaginary parts of the Green's function. Trivial differences come from different Green's function definitions. The energy and Lagrangian forms constitute nontrivial differences. Magnetic noise sources suffice to extract the quasistatic electromagnetic-field Earth impulse response in the Lagrangian form. This is of interest for Earth subsurface imaging. A numerical example demonstrates that all source vector components are necessary to extract a single-field vector component.

Abstract not available