| Over the years,researchers have continued to utilize electromagnetic(EM)methods to explore and prospect for various earth phenomenon ranging from simple to complex scenarios.Electromagnetic applications in geophysics are mostly targeted at measuring the resistivity of the earth materials.There are various forms of EM methods and techniques.Some examples of EM methods that employ natural sources are magnetotelluric(MT)and audio-frequency magnetotelluric(AMT)methods while multi-transient electromagnetic method(MTEM),borehole surface controlled source electromagnetic(BSCSEM)method,borehole controlled source electromagnetic(BSEM),marine and land controlled source electromagnetic method(CSEM))methods are some of the methods that use artificial sources(controlled source).However,in recent years,lots of studies have been focused on the use of CSEM method in geophysical exploration because the source is reliable and predictable unlike natural sources and it makes acquisition of high-quality EM data possible even in regions associated with high EM noise level.However,when CSEM surface measurements are solely used,it becomes difficult to resolve the effect of thin vertical resistivity changes or localized targets of low resistivity contrast but image resolution has been found to increase significantly when sensors and/or transmitters are placed in the observation Wells close to the targets.Hence,the introduction and research on Borehole-surface controlled source electromagnetic method commenced.Therefore,this dissertation,is based on the controlled source electromagnetic method of geophysical exploration precisely,the BSCSEM method because of its potential of producing a CSEM data with better signal to noise ratio,it ability to resolve the effect of thin vertical resistivity changes or localized targets of low resistivity contrast and the drilling cost is relatively low.This study is motivated by the need to understand the behavior of borehole-controlled source electromagnetic responses,and investigate the effects of anisotropy on BSCSEM responses.In this study,accurate and efficient forward modeling algorithms for BSCSEM response modelling using the higher order finite difference frequency domain(FDFD)method were developed.Consequently,stable higher-order accurate codes for the 2D and 3D simulation of diverse scenarios of BSCSEM responses was developed for the total field formulation.Large grid sizes were used for reduced computational cost in the simulation of borehole to surface responses,I also examined the effect of applying perfectly matched layer(PML)on the BSCSEM computational area.Next,I described and verified the accuracy of the higher-order approximations and compared the obtained response with that of lower order approximations.Spatial operators of lower order,first-order accuracy,was employed locally in substantial parts of the computational domain,including but not limited to layer boundary during the solution process.The study also addressed responses from different source orientations and source lengths,using the electric dipole as source.This was done for 2D and 3D models of buried resistive targets and lastly,2D modeling of known scenarios of onshore gas hydrate using BSCSEM configuration was carried out in order to understand the effective techniques for onshore gas hydrate exploration.The sensitivity range was selected based on surface receiver quality,and it reflects the maximum frequency requirement of 100 Hz or below.This was used for the considered measurement range and effective detection of gas hydrate deposit for 200 m borehole dipole with average current of 10 A.General observation indicates that BSCSEM response from vertical transmitters were highly effective in characterizing the resistive layers as well as the measurement of the vertical component of the electric field.By changing the depth of vertical dipole source,BSCSEM has the ability to gain information about subsurface medium from different angles.I conclude that in a complex environment,combining horizontal and vertical transmitters or analyzing both the horizontal and vertical electric responses helps to delineate the subsurface structure more clearly.Therefore,in the case of multiple accumulations as defined in this study,three component(3C)field measurements will be most appropriate.For the development of the anisotropy codes utilized for modelling the effects of anisotropy on BSCSEM responses,the process started from the derivation,implementation and successful validation of the efficient and accurate,2D and 3D higher order finite-difference frequencydomain(FDFD)anisotropy algorithms and codes.The modular approach utilized made it possible to formulate the governing equations on feasible grids with the differential operators.It is defined on several possible discrete grids constructed based on a set of basic higher order difference schemes and averaging operators.All electric and magnetic field components were solved simultaneously,and implemented on a staggered grid.The 2D BSCSEM anisotropy code was developed for simulation of transverse 3D and full 3D anisotropy problems.The simulation experiments considered includes a series of synthetic models starting with full 2D anisotropy formation,complex 2D model example,and then 2D gas hydrate.The 3D BSCSEM anisotropy code was developed for modelling anisotropic effect of 3D BSCSEM problems.The modelling exercise involved 3D anisotropy simulation of the effects of anisotropy on various synthetic geological models of different anisotropy scenarios using various source orientations such as: vertical dipole,horizontal dipole and L shaped orthogonal and azimuth dipole.Simulation results obtained revealed that the codes are highly efficient and suitable for numerical simulation of anisotropic effects on BSCSEM responses especially in gas hydrate exploration and for modeling other complex earth models.More importantly,the numerical exercises indicate that the codes provide general and consistent algorithm formulation to simulate borehole surface electromagnetic measurements in the presence of large conductivity contrasts,dipping wells,electrically anisotropic media,and geometrically complex models of electrical conductivity such as gas hydrate.And also,this method is more straight forward and easy to implement for modeling fully anisotropic media,especially in terms of demonstrating the flexibility of the numerical discretization approach,which can be readily applied to other problems. |
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