Earth Resources Laboratory
http://hdl.handle.net/1721.1/67704
2014-08-13T23:42:47ZNonlinear Conjugate Gradients Algorithm For 2-D Magnetotelluric Inversion
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.
2000-01-01T00:00:00ZSimulation Of An Acoustically Induced Electromagnetic Field In A Borehole Embedded In A Porous Formation
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.
2000-01-01T00:00:00ZAeolian And Fluvial Depositional Systems Discrimination In Wireline Logs: Unayzah Formation, Central Saudi Arabia
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.
2000-01-01T00:00:00ZThe Effect Of Image Resolution On Fluid Flow Simulations In Porous Media
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