Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1997. / Includes bibliographical references (leaves 92-95). / by Maria Auxiliadora Pérez. / M.S.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 199-205). / Incorporating overturned waves and multiples in seismic imaging is one of the most plausible means by which imaging results might be improved, particularly in regions of complex subsurface structure such as salt bodies. Existing migration methods, such as Reverse Time Migration, are usually designed to image solely with primaries, and so do not make full use of energy propagating along other wave paths. In this thesis I describe several modifications to existing seismic migration algorithms to enable more effective exploitation of the information contained in these arrivals to improve images of subsurface structure. This is achieved by extending a previously proposed modification of one-way migration so that imaging with overturned waves is possible, in addition to multiples and regular primaries. The benefit of using this extension is displayed with a simple box model and the BP model. In the latter, the proposed method is able to image the underside of a salt overhang when even RTM fails, although substantial artifacts are also present. Progressing to the two-way wave equation, I explain three new ways in which a wavefield may be separated by wave propagation direction, and use these in proposed modifications to the RTM algorithm. With these modifications, overturned waves and multiples can be used more effectively, as they no longer risk subtracting from the image contributions of primaries, their amplitude is boosted to produce greater relative amplitude accuracy, and artifacts usually associated with the use of these arrivals are attenuated. The modifications also provide two means of expressing image uncertainty. Among the results I show are a demonstration of the superior image obtained using the proposed method compared to the source-normalized imaging condition, and an improved image of a salt body in the SEAM model. Finally, I describe another modification to RTM that further reduces artifacts associated with the inclusion of multiples, exhibiting its effectiveness with simple layer models, and on a portion of the SEAM model. / by Alan Richardson. / Ph. D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, February 2000. / Includes bibliographical references (p. 235-244). / Convection in Earth's mantle is driven largely by horizontal density gradients that form when cold, dense, mantle lithosphere descends into the mantle interior, either through subduction for plate-scale flow, or as localized convective instability beneath lithospheric plates. The deformation associated with these processes is resisted by the extreme temperature-dependence of the lithosphere's strength. Ways in which lithosphere deformation affects convection in the mantle are examined here, by comparing both theory and the results of numerical experiments. Convective instability at the base of a cold thermal boundary layer with temperature and strain-rate-dependent viscosity is investigated by defining a quantity, termed here the "available buoyancy," that takes into account the tradeoff between cold temperatures both promoting and resisting convective instability. This quantity can be used to determine approximately whether, and how fast, convective instability grows. Horizontal shortening is also included, which tends to increase gravitational instability, allowing up to 60% of the mantle lithosphere to be convectively removed. The subsequent influx of hot, buoyant, asthenosphere could cause rapid surface uplift. For plate-scale flow, subduction zone deformation may resist convection. This possibility is studied here using a regional finite element model of subduction. This model shows that for sufficiently strong lithosphere, convection is resisted more by the bending deformation of a subducting plate than by shearing of the underlying mantle. Such behavior can be explained by a variation of boundary layer theory that includes an analytic expression for the energy required to bend a viscous plate. For the mantle, the bending resistance should control plate velocities if the effective lithosphere viscosity is greater than about 1023 Pa s. This produces a reasonable distribution of plate velocities for Earth and may reconcile models for its thermal evolution with surface heat flow observations. These results are verified using a new method for implementing subduction that parameterizes plate bending within a small region of a mantle-scale convection model. This model also shows that small-scale convection, by removing the basal part of the oceanic lithosphere, can decrease the bending resistance and thus may be an essential aspect of plate tectonics on Earth. / by Clinton Phillips Conrad. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998. / Includes bibliographical references (p. 65-70). / In this thesis, I study the spatial statistical properties of slow flow through porous media on the pore scale by a combination of numerical simulation and theoretical arguments. I demonstrate that the flow patterns undergo a transition from swirls to strongly focused and channel-like patterns for decreasing porosities. Not only is the flow in low-porosity media strongly focused, but the flow also possesses long-tailed, non-Gaussian velocity probability density distributions. A main result of our simulations is that the statistics of the flow through a single channel captures the entire flow, insofar as the patterns and probability distributions are concerned. I have constructed a simplified, phenomenological model for the fast part of the flow in random porous media. This model yields the desired exponential velocity distributions. For high porosities, I find that the statistical properties of the velocity fluctuations behave in a similar way as those observed in dilute suspensions flows: the swirls have a power-law dependency on the solid volume fraction, the correlation length is finite and has also a power-law dependency. I demonstrate that this scaling behavior is consistent with the predictions of theories. Finally, I study creeping flow through a single rough walled channel by numerical simulation and present a theory that predicts scale dependency of the permeability for tight fractures. / by Olav van Genabeek. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1990. / Includes bibliographical references. / by Eugene M. Lavely. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008. / Includes bibliographical references (p. 29). / Here, we investigate the possibility of a significant atmospheric contribution to the tidal dissipation of the Phobos-Mars system. We apply the classical tidal theory and we find that most of gravitational forcing is projected into the first symmetric Hough mode which has an equivalent depth of about 57 km and it is significantly trapped in the vertical. Therefore, no significant dissipation occurs through the vertical propagation of energy and subsequent breaking of the tidal wave as the wave amplifies with height. Alternatively, from the energy stored in the first trapped mode we estimate that the time scale required for the dissipative mechanisms to account for the total dissipation of the tides is of order 102s. This time scale is unrealistically short, since it would contradict observations of propagating thermal tides in Mars atmosphere. Therefore we conclude that the dissipation of the tidal potential that explains the observed acceleration of Phobos most likely occurs within the solid planet. / by Vid Thayalan. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1995. / Includes bibliographical references (leaves 145-149). / by James Reginald Gunson. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (p. 185-192). / The stratosphere and the troposphere exhibit a strong coupling during the winter months. However, the coupling mechanisms between the respective vertical layers are not fully understood. An idealized spectral core dynamical model is utilized in the present study in order to clarify the coupling timing, location and mechanisms. Since the coupling between the winter stratosphere and troposphere is strongly intensified during times of strong stratospheric variability such as stratospheric warmings, these events are simulated in the described model for the study of stratosphere - troposphere coupling, while for comparison the coupling is also assessed for weaker stratospheric variability. While the upward coupling by planetary-scale Rossby waves in the Northern Hemisphere is well understood, the Southern Hemisphere exhibits traveling wave patterns with a weaker impact on the stratospheric ow. However the tropospheric generation mechanism of these waves is not well understood and is investigated in this study. It is found that in the model atmosphere without a zonally asymmetric wave forcing, traveling waves are unable to induce a significant wave ux into the stratosphere. In the absence of synoptic eddy activity, however, the tropospheric ow is baroclinically unstable to planetary-scale waves, and the generated planetary waves are able to propagate into the stratosphere and induce sudden warmings comparable in frequency and strength to the Northern Hemisphere. While baroclinic instability of long waves may be further strengthened by the addition of moisture, the real atmosphere also exhibits strong synoptic eddy activity, and it will have to be further explored if the atmosphere exhibits periods where synoptic eddies are weak enough to allow for baroclinic instability of long waves. In order to further investigate the coupling between the stratosphere and the troposphere, cases of strong coupling are investigated in the analysis of a Northern Hemisphere - like winter atmosphere. A realistic frequency and strength of sudden warmings is obtained using a zonal wave-2 topographic forcing. An angular momentum budget analysis yields that the Eliassen-Palm (EP) flux is closely balanced by the residual circulation dominated by the Coriolis term on a daily basis, while the change in zonal wind is a small residual between these dominant terms. In the stratosphere, the EP flux term and the Coriolis term balance well in time but not exactly in magnitude, yielding a polar stratospheric weakening of the zonal mean wind as observed during stratospheric warmings. In the troposphere, the loss of angular momentum before a sudden warming induces a weak negative annular mode response, which is amplified by the downward propagating signal about three weeks after the sudden warming. The angular momentum budget does not reveal the mechanism of downward influence, but it nevertheless clarifies the momentum balance of the stratosphere - troposphere system, indicating that the effects of the waves and the residual circulation have to be considered at the same time. Since the annular mode response cannot be directly investigated using the angular momentum budget, the annular mode coupling between the stratosphere and the troposphere is further investigated using a statistical approach. The annular mode response is often framed in terms of Empirical Orthogonal Functions (EOFs), but it is here found that for the stratosphere - troposphere system with its strong vertical pressure gradient, EOFs are strongly dependent on the weighting of the data, while Principal Oscillation Patterns (POPs) are considerably less sensitive to an applied weighting while returning the dominant structures of variability. This encourages further research and application of POP modes for the use of stratosphere - troposphere coupling. These findings represent an improvement of the understanding of stratosphere - troposphere coupling and the results are another step in the direction of finding the mechanism of stratosphere - troposphere coupling and the downward influence after the occurrence of a stratospheric sudden warming, which may influence long-term weather prediction in the troposphere. / by Daniela I. V. Domeisen. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmosphere and Planetary Sciences, 1988. / Includes 1 folded map in pocket. / Includes bibliographical references. / by Joann Miriam Stock. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2006. / Includes bibliographical references. / This thesis seeks to advance our understanding of the dynamic nature, spatial organization and depositional record of topography in sand-bedded rivers. I examine patterns and processes over a wide range of scales, on Earth and Mars. At the smallest scale, ripples and dunes (bedforms) arise spontaneously under most natural flow conditions, acting as the primary agents of sediment transport and flow resistance in sandy rivers. I use physical modeling in a laboratory flume to explore the feedbacks among bedform geometry, fluid flow and sediment transport. Field observations of dunes in the North Loup River, Nebraska, show that bed roughness displays a statistical steady state and robust scaling. Motivated by these data, I develop a nonlinear stochastic surface evolution model for the topography of sandy rivers which captures the essence of bedform evolution in space and time. I then use a simplified kinematic model for bedform evolution to simulate the production of stratigraphy from migrating dunes, allowing a more accurate reconstruction of river flow conditions from preserved bedform remnants in rocks. At the channel scale I examine the conditions that lead to avulsion, the rapid abandonment of a river channel in favor of a new course at lower elevation. / (cont.) Simple scaling arguments and data from 30 natural systems reveal that anastomosing (multi-branch) rivers and distributary deltas are morphologies that arise when avulsion is the dominant mechanism of channel adjustment. I apply these arguments to the Niobrara River, Nebraska, which has experienced rapid in-channel deposition due to base level rise. I show that the planform pattern of the Niobrara is dominated by base-level-driven avulsions, and is decoupled from the smaller-scale sediment transport. At the largest considered scale are depositional fans, which are constructed by avulsing rivers. The evolution of a fan profile may be modeled at long time- and space-averaged scales as a diffusive process. I use such a model to invert topographic and volumetric data from a fluvial fan on Mars, producing an estimate of the time required to build the fan out of channel and overbank deposits. / by Douglas J. Jerolmack. / Ph.D.