http://hdl.handle.net/1721.1/67705 2013-06-20T01:38:14Z http://hdl.handle.net/1721.1/75725 Nonlinear Conjugate Gradients Algorithm For 2-D Magnetotelluric Inversion Rodi, William L.; Mackie, Randall L. We investigate a new algorithm for computing regularized solutions of the two-dimensional magnetotelluric inverse problem. The algorithm employs a nonlinear conjugate gradients (NLCG) scheme to minimize an objective function that penalizes data residuals and second spatial derivatives of resistivity. We compare this algorithm theoretically and numerically to two previous algorithms for constructing such 'minimum-structure' models: the Gauss-Newton method, which solves a sequence of linearized inverse problems and has been the standard approach to nonlinear inversion in geophysics, and an algorithm due to Mackie and Madden, which solves a sequence of linearized inverse problems incompletely using a (linear) conjugate gradients technique. Numerical experiments involving synthetic and field data indicate that the two algorithms based on conjugate gradients (NLCG and Mackie-Madden) are more efficient than the GaussNewton algorithm in terms of both computer memory requirements and CPU time needed to find accurate solutions to problems of realistic size. This owes largely to the fact that the conjugate gradients-based algorithms avoid two computationally intensive tasks that are performed at each step of a Gauss-Newton iteration: calculation of the full Jacobian matrix of the forward modeling operator, and complete solution of a linear system on the model space. The numerical tests also show that the Mackie-Madden algorithm reduces the objective function more quickly than our new NLCG algorithm in the early stages of minimization, but NLCG is more effective in the later computations. To help understand these results, we describe the Mackie-Madden and new NLCG algorithms in detail and couch each as a special case of a more general conjugate gradients scheme for nonlinear inversion. 2000-01-01T00:00:00Z http://hdl.handle.net/1721.1/75724 Simulation Of An Acoustically Induced Electromagnetic Field In A Borehole Embedded In A Porous Formation Hu, Hengshan; Wang, Kexie; Wang, Jingnong When an acoustic point source located on the borehole axis emits an acoustic wave, an electric field, as well as an acoustic field, is generated in the porous formation around the borehole due to an electrokinetic effect. The coupled acoustic and electromagnetic wavefields were formulated by applying Pride's (1994) governing equations and boundary conditions at the borehole wall. Numerical examples show that two kinds of electric fields can be received on the borehole axis. The propagating electromagnetic wave arrives at different receivers almost simultaneously, and appears as the first wave packet in the full electric field waveform. Another kind of electric field accompanies the acoustic pressures, and consists of the same component waves as that of the acoustic waveforms. On the borehole axis, the coupled electric field vector is in the axial direction on the borehole axis. A study of the ratio of the magnitude of electric field strength to the magnitude of pressure, or REP, shows that the compressional wave has the largest REP value, the Stoneley wave the next, and the shear wave the smallest. The peak value of the electric field strength at 1 kHz is about 100 times larger than that at 10 kHz, while the REP at 1 kHz is about five times larger than REP at 10 kHz. Off the borehole axis, the electric field has a radial and axial component, and one Can also receive a circumferential magnetic field. When the interdependence between porosity, tortuosity and permeability is ignored, the REP increases rapidly with porosity, decreases with tortuosity, and changes little with permeability. The electric field strength decreases with borehole fluid salinity and formation water salinity. 2000-01-01T00:00:00Z http://hdl.handle.net/1721.1/75723 Aeolian And Fluvial Depositional Systems Discrimination In Wireline Logs: Unayzah Formation, Central Saudi Arabia Al-Dajani, AbdulFattah; Burns, Daniel; Toksoz, M. Nafi The objective of tills study is to discriminate between aeolian and fluvial deposits of the Permian Unayzah formation in Central Saudi Arabia by using wireline logs. The analysis is conducted on wire-line logs (field data): Density, sonic, gamma, and neutron, from two vertical wells (U1 and U2) in Central Saudi Arabia. Core data are available at well location U1 but not at U2. We apply an automated neural-network method to the wireline data for facies discrimination. Our analysis has been applied to the logs of well U2 after training the method on U1 logs using available core information. Results indicate that the Unayzah formation at well location U2 consists mainly of fluvial deposits (about 90%), which is consistent with previous studies and is supported by surface seismic images. We also investigate an analysis method based On the Fourier transform. We study the decay of the energy spectrum in the frequency domain and estimate the associated power-law exponent (i.e., the slope of the decay) for each depositional system. Analysis on the porosity logs (density, neutron, sonic, and shear), which are highly influenced by deposition composition and texture, has shown that the exponent is about the same for fluvial deposits at both well locations, while it is different for aeolian deposits. 2000-01-01T00:00:00Z http://hdl.handle.net/1721.1/75722 The Effect Of Image Resolution On Fluid Flow Simulations In Porous Media Edie, Margaret S.; Olson, John F.; Burns, Daniel R.; Toksoz, M. Nafi Realistic simulations of flow in porous media are dependent upon having a three-dimensional, high resolution image of pore structure which is difficult to obtain. So, we ask the question, "How fine a resolution is necessary to adequately model flow in porous media?" To find the answer, we take a 7.5 p,m resolution image and coarsen it to five different resolutions. Lattice gas simulations are performed on each image. From the simulation results, we observe changes in permeability and velocity fields as the resolution is altered. The results show permeability varies by a factor of 5 over the resolution range. Flow paths change as the resolution is changed. We also find that the image processing has a large impact on the outcome of the simulations. 2000-01-01T00:00:00Z