1 

Focusing one dimensional models by using the ‘Singlesided’ autofocusing procedure of Rose for layered materials

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2 

GroundPenetrating Radar
Measuring and analyzing ground penetrating radar data on different sandclay soils as a function of water content

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3 

Computing the vertical density and seismic velocity profiles from multiangle reflection data: error analysis
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 multiangle 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 densityvelocity 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.

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4 

Electromagnetic & Seismoelectric sensitivity analysis using resolution functions
In the field of exploration geophysics various methods are applied to determine the physical properties of the subsurface of the Earth. Some of the methods most widely used are seismic and electromagnetic surveys, which are each used according to the type of information that is being sought and their ability to provide that information. The method of seismoelectrics is in that regard a promising technique because theoretically it should be sensitive to a wide range of subsurface parameters, spanning both the acoustic and the electromagnetic methods.
We aim to perform a parameter sensitivity analysis for the seismoelectric problem, investigating how perturbations in different parameters affect the data and how well these perturbations can be inverted for. We will first study some elements of inverse theory with a special focus on how to construct resolution functions from the basic integral equation of scattering theory. This integral equation can be derived using both superposition and reciprocity principles. This will be shown for both the electromagnetic and the acoustic case, before moving on to the seismoelectric case; it will become clear that the latter poses additional challenges. In addition, the results for the electromagnetic case can be compared directly with the results obtained by [Slob and Mulder, 2011] who performed an electromagnetic parameter sensitivity analysis. The results of this comparison could serve as a validation of the seismoelectric forward modelling code ESSEMOD [Grobbe and Slob, 2013], which will be used to generate synthetic data for this thesis.
It is concluded that the superposition principle and the reciprocity theorem provide identical expressions for a scattered field. This conclusion was the basis for deriving the complete reciprocity theorem for the seismoelectric system, which was in turn used to derive resolution functions for a perturbation in bulk density and a perturbation in the seismoelectric coupling coefficient. For the electromagnetic case, we show that the resolution functions computed, using explicit analytical Green's function solutions are identical to resolution functions computed with data produced by ESSEMOD and that this indeed serves as a validation of ESSEMOD. Using the same approach, we have successfully computed a resolution function for inversion for the coupled seismoelectric system for a perturbation in bulk density. It is recommended that this innovative result is used as the basis for the analysis of the seismoelectric sensitivity to more complex parametercontrasts, which could serve as an assessment of the true potential of the seismoelectric method.

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5 

Inversion of multitransient EM data from anisotropic media
Forward modelling demonstrates that resistivity anisotropy has a huge effect on MultiTransient 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.

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6 

Thermal Enhancement of Waterflooding in MediumHeavy Oil Recovery
Waterflooding 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 waterflooding is among these methods. In hot waterflooding, thermal energy will increase oil mobility, and possibly provide a more efficient sweep.
This research investigates the effect of heat on waterflood 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 waterflooding at room temperature), and nonisothermal (hot) waterflooding. Xray 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 waterflood. 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.

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7 

Analysis of 2D homogeneous space solutions of the seismoelectric PSVTM mode for interferometric purposes
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 poroussaturated 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 signaltonoise ratio. A possible solution to this problem would be to apply interferometric Green's retrieval, which is a technique based on stacking of crosscorrelated data. This approach has been proved successful for the SHTE mode in 1D. The SHTE mode forms together with the PSVTM mode, the total seismoelectric system. In this thesis the first steps are taken towards the proof that this technique could also work for the PSVTM mode of the system. This is supported by a modelling experiment of 2D homogeneous space solutions of the seismoelectric PSVTM mode for different configurations. This analysis turned out that the unwanted artefacts observed in the interferometric retrieval are generated by crosscorrelations between Pwaves and SVwaves.

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8 

Arsenic Contamination: Deep wells as a solution for clean drinking water in Bihar, India
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 (028m) has a permeability of 30.7 mD and a porosity of 20 %; the bottom layer(2850m) 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.

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9 

Seismoelectric Modelling of the FluxNormalized PSVTM Propagation Mode
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: SHTE and PSVTM. The SHTE mode contains horizontally polarized shear waves coupled with transverse electric polarized electromagnetic waves. In the PSVTM mode, both fast and slow compressional waves are coupled with vertically polarized shear waves and transverse magnetic polarized electromagnetic waves. In this thesis, the PSVTM mode of the twodimensional seismoelectric system was expressed in the form of both the twoway and oneway 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 oneway reciprocity theorems.
We carried out fullwaveform modelling of the fluxnormalized PSVTM seismoelectric system in a 2D fluidsaturated, horizontallystratified, porous media. Both oneway and twoway wavefields were modelled, allowing the composition of oneway wavefields into twoway 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 oneway and twoway 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.

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10 

Control of Fluvial Architecture on the Spatial Distribution of Arsenic Rich Ground Water
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 1015 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.

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11 

CSEM modeling of a VTI layered medium with the effective anisotropic medium
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 wavenumberfrequency 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 propagationdiffusion term, a stable effective transverse electric (TEmode) 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 (TMmode) isotropic conductivity is not the same as that of TEmode 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, kdependent, 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.

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12 

Elektromagnetische signalen in de aarde

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13 

Shear Wave Seismic Interferometry for Lithospheric Imaging
Green's function retrieval by seismic interferometry (SI) exists in a variety of forms. Many of the applications of SI allow the creation of new seismic traces by crosscorrelating a wavefield recorded at two separate locations. The sum of this operation over multiple sources results in the creation of a new signal such that one of the recording locations acts as a virtual source to the other. We demonstrate that it is possible to use teleseismic shear wave transmission responses to create virtual source records for lithospheric imaging through SI. We utilize teleseismic shear wave phases for primarily two reasons: namely, (a) at large offsets the incoming wavefront approximates a plane wave due to spread of the wavefront in relation to the comparatively finite seismic array, and (b) the large distances act as a natural temporal filter to separate the incident P and S arrivals. We develop a method that allows such shear wave phases to image the subsurface.
A series of forward modelling experiments with 2D elastic propagation were conducted. SP converted energy maps primarily to the vertical component. At large ray parameters, decomposition was capable of separating the P and S fields. Sufficient illumination by many ray parameters is essential to clearly resolve subsurface features; however, decimation studies indicated that it is still possible to identify strong reflectors with suboptimal sampling. Attempts to mitigate the complex effective wavelet generated by earthquake faulting behaviour were met with difficulty; deconvolution was unsuccessful due to the nonminimum phase nature of the effective source wavelet, but spectral whitening is a necessary preprocessing step to SI was necessary. Correlation panel filtering techniques, including singular value decomposition and wavenumber filtering, were tested to remove spurious energy present in the virtual source records created through SI. Spurious terms introduced by the long duration earthquakegenerated wavelet were further suppressed in the commonoffset domain. After creating virtual source records at every model receiver location, imaging was performed. A conventional migration technique was applied to each virtual source record and stacked to produce an image of the model subsurface. All major features of the model were resolved with a high degree of accuracy using the approach developed here.
Finally, a field dataset was selected from southern Mexico with sufficient S phase sampling and prepared for imaging. By azimuthally limiting a window around the relatively linear orientation of the seismometer array, earthquakes that vary in distance correspond to sampling of unique ray parameters. A total of 43 S phase transmission responses were selected that met our criteria for magnitude and location. Geometric and topographic corrections were applied to each transmission response. The processing steps tested on our forward models were subsequently applied to this dataset and virtual source records were created via SI. The virtual source records are then migrated to form an image of the lithosphere. The resulting image of the Cocos subduction zone reproduces features present in the literature and also reveals new details at greater depths; this demonstrates the utility of the method developed within.

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14 

Simulation of Interferometric Seismoelectric Green’s Function Recovery: For the SHTE propagation mode
A recent novel technique known as seismic interferometry makes use of seismic ‘noise’ to reconstruct a Green’s function between two receivers by crosscorrelation. This technique can be applied for example in permanent subsurface monitoring using passive seismics or the creation of virtual sources on positions where only recordings were made. The aim of this thesis is to derive and understand equations for seismoelectric interferometry. The first part of this thesis focusses on a the calculation of SHTE seismoelectrical responses in a 2D horizontally stratified earth. We decompose the twoway wave equation for SHTE waves into upgoing and downgoing waves which we relate through a reflectivity formulation. The reflectivity formulation is based upon reflection matrices only, even though we can simulate both reflection and transmission experiments. We solve the SHTE seismoelectric system in a 1D homogeneous world.
In the second part of this thesis we derive interferometric Green’s fucntion representations from reciprocity theorems that relate two different states in one domain. Interferometric Green’s function representations express the Green’s function between two receivers as a function of crosscorrelations of responses of sources throughout a domain and on it’s boundary. We cast the seismoelectric system in a general diffusion, flow and wave equation and define a Green’s matrix for all different field and source types. Using this formulation we derive a sourcereceiver reciprocity relation for the Green’s matrix from the convolution type reciprocity theorem. The correlation type reciprocity theorem for the Green’s matrix is modified using sourcereceiver reciprocity to obtain the interferometric Green’s function representation.
We study the SHTE seismoelectrical interferometric representation 1D and 2D in homogeneous media. A seismoelectric interferometric representation was written to recover the causal response of the particle velocity at position B due to a electrical current source at position A, as a function of cross correlations of electric field recordings at A and particle velocity recordings at B. Provided there exists a dense coverage of sources in the domain and on it’s boundary, the representation was validated in both 1D and 2D.
Approximations to the interferometric representation are investigated by studying the contributions of parts of the domain and boundary integrals. It was found that a dominant spurious event resides in the separate contributions of the domain and boundary integrals, that destructively interferes when both contributions are combined. The role of different source types in the interferometric representation was studied. In a homogeneous medium the measured events have propagated either as an electromagnetic wave or as a shear wave.
The dominant contribution to the reconstruction of an electromagnetic event is by electromagnetic sources. Similarly, can the reconstructed shear wave event be attributed mainly due to seismic sources. In a medium with low electromagnetic and shear wave losses we could ignore the domain integral, this will result in amplitude errors and we will suffer from spurious events.

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15 

A new tool for accurate Sparameters measurements and permittivity reconstruction

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16 

Synthesis of all known analytical permittivity reconstruction techniques of nonmagnetic materials from reflection and transmission measurements

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17 

Comparison of the Different Reconstruction Techniques of Permittivity From SParameters

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18 

Electrical Survey of Peat Deposits

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19 

Merging active and passive surface wave data with interferometry by multidimensional deconvolution
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.

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20 

Scattering of transient diffusive electromagnetic fields

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