Controlled source electromagnetic (CSEM) geophysics has been used with a fair amount of success in near surface hydrogeological studies. Recently, these investigations have been conducted frequently in human impacted field sites containing cultural conductors such as metal fences and buried pipes. Cultural noise adds an element of complexity to the geological interpretation of this type of data. This research investigates the influence of mutual induction between two buried targets in a CSEM experiment. In particular, it looks at the mutual coupling between a buried cultural conductor and a geological heterogeneity. We attempt to isolate the Hz field induced by tertiary currents in targets caused by mutual coupling. This is achieved with a Texas A&M 3D CSEM finite element code, which calculates the secondary Hz fields emanating from a target buried in a halfspace. Buried geological targets and cultural conductors are modeled as volumetric slabs embedded in a halfspace. A series of models have been simulated to study the effect of varying parameters such as target conductivity, transmitter location and shape of a target on the mutual inductance. In each case, the secondary Hz field is calculated for a model with two slabs, and two models with individual slabs. The mutual coupling is calculated by removing the secondary fields from the individual slab models from the response of a two slab model. The calculations of mutual inductance from a variety of such models suggests a complicated interaction of EM fields between the two targets. However, we can explain most of these complexities by adapting a simple approach to Maxwell’s equations. Although the tertiary Hz field is complicated, it may be useful in the characterization and delineation of electrical heterogeneities in the subsurface, which can then be related to geological features such as fractures or joints. It is seen that the most important factor affecting the mutual coupling is the host conductivity. The results have also shown that mutual coupling is very sensitive to transmitter (TX) location, especially when the TX is positioned near one of the targets.

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<p>Controlled Source Tensor Magnetotelluric (CSTMT) ground measurements were executed on the Hallandsåsen horst where a major tunnel is under construction. The instrument system EnviroMT are used for this purpose. The major research aspect of this thesis has been to form an opinion of the effectiveness of the method by comparing the results from the CSTMT survey with a prior investigation performed with the DC resistivity method. Another important part of this thesis has been to compile the basic and fundamental CSTMT and RMT theory, in a way that people outside the EM community easily can be introduced to the subject.</p><p>When comparing the different inversion models from the CSTMT and DC resistivity surveys one can see differences in the depth at which the conductors are resolved. In the CSTMT inversion models (TE+TM) there are two conductors that possibly can reach the depth of the tunnel in construction. These conductors are not resolved at the deeper structures in the DC resistivity models. Whether the conductors in the CSTMT inversions (TE+TM) truly extend to the depth at which they are modeled, or if they in deeper parts are artificial effects of regularisation in the inversion cannot be said for sure. Accounting for the low frequencies utilised in the TE mode, one has very strong arguments that the deep conductors seen in the CSTMT model are true.</p><p>The TE-mode models have shown to be much less affected by the complex problems of near field effects in comparison with the TM-mode models. The evidence of the near field effects is very prominent in the TM-mode phase, but in the phase of the TE-mode one can not see any such tendencies. However, one can see a discontinuity in the same part of three profile lines which shows that the data is disturbed but not nearly as much as in the TM-mode. The apparent resistivity seems to be over all less affected by the near field effects. In the apparent resistivity of the TE-mode, one can not discern any near field effects at all.</p><p>In the TM-mode, the apparent resistivity shows higher apparent resistivity than the real apparent resistivity in the near field. To receive more information about the deeper structures, lower controlled source frequencies were allowed in the TE-mode than in the TM-mode inversion models. The RMS in the TE-mode inversions has not been deteriorated, which is an another indication that the TE mode is not very disturbed by the near field effects.</p><p>The RMT inversion models are shown to be heavily biased in the deeper parts to which the RMT data are insensitive and regularization determining the outcome of the inversion. One can also see that regularisation is influencing the whole inversion model. In the shallow subsurface the inversion models should be same for CSTMT and RMT, but one can see differences in resistivity between the models.</p><p>The real induction arrows show features that are not as clearly displayed in either the phase or apparent resistivity. It seems that the real induction arrows are better at detecting lateral differences in conductivity in a more resistive media, than the phase and apparent resistivity.</p>

Controlled Source Tensor Magnetotelluric (CSTMT) ground measurements were executed on the Hallandsåsen horst where a major tunnel is under construction. The instrument system EnviroMT are used for this purpose. The major research aspect of this thesis has been to form an opinion of the effectiveness of the method by comparing the results from the CSTMT survey with a prior investigation performed with the DC resistivity method. Another important part of this thesis has been to compile the basic and fundamental CSTMT and RMT theory, in a way that people outside the EM community easily can be introduced to the subject. When comparing the different inversion models from the CSTMT and DC resistivity surveys one can see differences in the depth at which the conductors are resolved. In the CSTMT inversion models (TE+TM) there are two conductors that possibly can reach the depth of the tunnel in construction. These conductors are not resolved at the deeper structures in the DC resistivity models. Whether the conductors in the CSTMT inversions (TE+TM) truly extend to the depth at which they are modeled, or if they in deeper parts are artificial effects of regularisation in the inversion cannot be said for sure. Accounting for the low frequencies utilised in the TE mode, one has very strong arguments that the deep conductors seen in the CSTMT model are true. The TE-mode models have shown to be much less affected by the complex problems of near field effects in comparison with the TM-mode models. The evidence of the near field effects is very prominent in the TM-mode phase, but in the phase of the TE-mode one can not see any such tendencies. However, one can see a discontinuity in the same part of three profile lines which shows that the data is disturbed but not nearly as much as in the TM-mode. The apparent resistivity seems to be over all less affected by the near field effects. In the apparent resistivity of the TE-mode, one can not discern any near field effects at all. In the TM-mode, the apparent resistivity shows higher apparent resistivity than the real apparent resistivity in the near field. To receive more information about the deeper structures, lower controlled source frequencies were allowed in the TE-mode than in the TM-mode inversion models. The RMS in the TE-mode inversions has not been deteriorated, which is an another indication that the TE mode is not very disturbed by the near field effects. The RMT inversion models are shown to be heavily biased in the deeper parts to which the RMT data are insensitive and regularization determining the outcome of the inversion. One can also see that regularisation is influencing the whole inversion model. In the shallow subsurface the inversion models should be same for CSTMT and RMT, but one can see differences in resistivity between the models. The real induction arrows show features that are not as clearly displayed in either the phase or apparent resistivity. It seems that the real induction arrows are better at detecting lateral differences in conductivity in a more resistive media, than the phase and apparent resistivity.

The ability of marine controlled-source electromagnetic (MCSEM) methods to help image electrical conductivity contrasts below the Earth’s surface makes them useful for both initial reconnaissance surveying for hydrocarbons and for delineating prospective regions of high resistivity in development drilling. A 3-D finite-element MCSEM Fortran algorithm used for forward modeling was developed by Badea. Additional code was written and used for this thesis, with the goal of enforcing more realistic electromagnetic (EM) Dirichlet boundary value conditions. The results of the new boundary conditions on a MCSEM survey model, with a hydrocarbon-saturated region in the subsurface, show that the method does not work as hoped. Constant boundary values were applied to gauge the transmitter-receiver (TXRX) range at which results are not boundary influenced, using a hydrate/hydrocarbon model of the subsurface, at each of the three transmitter frequencies used in this study (1 Hz, 3 Hz, and 10 Hz). Results showed that electric field data were reliable to roughly 5000 m of TX-RX offset for the 1 Hz and 3 Hz cases, and to 6500 m offset for 10 Hz. The gas hydrate/hydrocarbon model was then run with zero-value boundary conditions. The goal was to determine what effect changing parameters of the gas hydrate, including hydrate radius, thickness, and depth, have on the EXEXS (xcomponent of secondary electric field inline with the transmitter dipole axis) curves at various offset, particularly in relation to a hydrocarbon-only model of the subsurface response, so as to evaluate the EM masking effect the hydrate has on the hydrocarbon. The results showed that the x-component of electric field in an inline survey is dominated by the hydrate response, in all cases studied, with a couple of exceptions. One exception is 1 Hz transmitter frequency at 2500 m to 3000 m offset when depth to top of the massive gas hydrate zone was greater or equal to 250 m. Receivers at these offsets would successfully detect the hydrocarbon target.

Presence of cultural refuse has long posed a serious challenge to meaningful geological interpretation of near surface controlled–source electromagnetic data (CSEM). Cultural refuse, such as buried pipes, underground storage tanks, unexploded ordnance, is often highly conductive and magnetically permeable. Interpretation of the CSEM response in the presence of cultural noise requires an understanding of electromagnetic field diffusion and the effects of anomalous highly conductive and permeable structures embedded in geologic media. While many numerical techniques have been used to evaluate the response of three dimensional subsurface conductivity distributions, there is a lack of approaches for modeling the EM response incorporating variations in both subsurface conductivity σ and relative permeability μr. In this dissertation, I present a new three dimensional edge–based finite element (FE) algorithm capable of modeling the CSEM response of buried conductive and permeable targets. A coupled potential formulation for variable μ using the vector magnetic potential A and scalar electric potential V gives rise to an ungauged curl–curl equation. Using reluctivity (v=1/mu ), a new term in geophysical applications instead of traditional magnetic susceptibility, facilitates a separation of primary and secondary potentials. The resulting differential equation is solved using the finite element method (FEM) on a tetrahedral mesh with local refinement capabilities. The secondary A and V potentials are expressed in terms of the vector edge basis vectors and the scalar nodal basis functions respectively. The finite element matrix is solved using a Jacobi preconditioned QMR solver. Post processing steps to interpolate the vector potentials on the nodes of the mesh are described. The algorithm is validated against a number of analytic and multi dimensional numeric solutions. The code has been deployed to estimate the influence of magnetic permeability on the mutual coupling between multiple geological and cultural targets. Some limitations of the code with regards to speed and performance at high frequency, conductivity and permeability values have been noted. Directions for further improvement and expanding the range of applicability have been proposed.

While geophysical reservoir characterization has been an area of research for the last three decades, geophysical reservoir monitoring, time-lapse studies, have recently become an important geophysical application. Generally speaking, the main target is to detect, estimate, and discriminate the changes in subsurface rock properties due to production. This research develops various sensitivity and feasibility analyses to investigate the effects of production-induced time-lapse changes on geophysical measurements including seismic and controlled-source electromagnetic (CSEM) data. For doing so, a realistic reservoir model is numerically simulated based on a prograding near-shore sandstone reservoir. To account for the spatial distribution of petrophysical properties, an effective porosity model is first simulated by Gaussian geostatistics. Dispersed clay and dual water models are then efficiently combined with other well-known theoretical and experimental petrophysical correlations to consistently simulate reservoir model parameters. Next, the constructed reservoir model is subjected to numerical simulation of multi-phase fluid flow to replicate a waterflooding scenario of a black oil reservoir and to predict the spatial distributions of fluid pressure and saturation. A modified Archie’s equation for shaly sandstones is utilized to simulate rock resistivity. Finally, a geologically consistent stress-sensitive rock physics model, combined with the modified Gassmann theory for shaly sandstones, is utilized to simulate seismic elastic parameters. As a result, the comprehensive petro-electro-elastic model developed in this dissertation can be efficiently utilized in sensitivity and feasibility analyses of seismic/CSEM data with respect to petrophysical properties and, ultimately, applied to reservoir characterization and monitoring research. Using the resistivity models, a base and two monitor time-lapse CSEM surveys are simulated via accurate numerical algorithms. 2.5D CSEM modeling demonstrates that a detectable time-lapse signal after 5 years and a strong time-lapse signal after 10 years of waterflooding are attainable with the careful application of currently available CSEM technology. To simulate seismic waves, I employ different seismic modeling algorithms, one-dimensional (1D) acoustic and elastic ray tracing, 1D full elastic reflectivity, 2D split-step Fourier plane-wave (SFPW), and 2D stagger grid explicit finite difference (FD). My analyses demonstrate that acoustic modeling of an elastic medium is a good approximation up to ray parameter (p) equal to 0.2 sec/km. However, at p=0.3 sec/km, differences between elastic and acoustic wave propagation is the more dominant effect compared to internal multiples. Here, converted waves are also generated with significant amplitudes compared to primaries and internal multiples. I also show that time-lapse modeling of the reservoir using SFPW approach is very fast compared to FD, 100 times faster for my case here. It is capable of handling higher frequencies than FD. It provides an accurate image of the waterflooding process comparable to FD. Consequently, it is a powerful alternative for time-lapse seismic modeling. I conclude that both seismic and CSEM data have adequate but different sensitivities to changes in reservoir properties and therefore have the potential to quantitatively map production-induced time-lapse changes. / text

Audiomagnetotelluric measurements with (CSAMT) and without (AMT) a controlled source were performed near Gällivare and Kiruna in Swedish Lapland in order to retrieve representative conductivity models of the subsurfaces. Magnetotelluric transfer functions were gained from processed time series’ and subsequently inverted to generate the sought models successfully. Additionally, a strike angle analysis was performed to determine the dimension of the ground structures. That information was used to justify the approaches of 1D and 2D inversions of the data sets and to judge their applicability. In Kiruna, two profiles were installed. One profile is considered to be in line with the strike direction, the other profile was oriented rather orthogonal to the strike direction. In Gällivare, only one profile was installed orthogonally with respect to the strike direction. The strike analysis showed a preferentially 2-dimensional structure for Kiruna’s parallel profile. For the orthogonal oriented profiles from Kiruna and Gällivare, the analysis revealed a 2D (with distortions) to 3D dimension of the ground structures. For the AMT method, it was possible to generate 1-dimensional and 2-dimensional models. Regarding the CSAMT data, it was only possible to generate a 1D conductivitymodel for the subsurface. Due to a significant transmitter overprint, no undistorted start model for the 2-dimensional CSAMT data inversion could be produced. The models from Kiruna had a sufficient data quality and thus resulted in reliable 2D AMT resistivity models with, locally, 2 to 3 layers. However, in combination with the 1D models for AMT and CSAMT, a 3-layer structure was predicted, where a resistive layer is covered by a thin conductive layer and underlaid by a rather conductive basement. For Gällivare’s profile, the data quality was good such that for all inversion methods good results were achieved. The predicted 2-layer models were resolved for for depths between 10m and 10,000m and coincide with the at hand geological maps and cross sections.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22423号 / 工博第4684号 / 新制||工||1731(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 小池 克明, 教授 三ｹ田 均, 准教授 柏谷 公希, 教授 後藤 忠徳 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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