Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998. / Includes bibliographical references (p. 12). / This paper discusses how changes in the orientation of a Ground Penetrating Radar's receiver with respect to its transmitter can affect the detection of radar pulses by the receiver. The orientation changes discussed in this paper are elevating the receiver, rotating the receiver in the surface plane, translating the receiver sideways, tilting the receiver backwards, tilting the receiver forwards, and tilting the receiver sideways. The effect of these movements will be measured by changes in the arrival times and amplitudes of the ground wave and the first reflection. From the data taken, it seems that, except for the change in amplitude from elevating the receiver, the change in orientation required to significantly effect the data is greater than the change that would occur during most uses of GPR and should not be a serious problem. / by Robert A. Roffman. / S.B.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1992. / Includes bibliographical references (leaves 89-91). / by Richard Sven Patterson. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1991. / Includes bibliographical references (p. 207-215). / by Shuntai Zhou. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1995. / Includes bibliographical references. / Sang-Mook Lee. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998. / Includes bibliographical references (p. 78-85). / In this study a -plane quasi-geostrophic channel model is used to study the dynamics of African waves. In part I the structure and energetics of the African easterly waves are well replicated compared to observations. Highly simplified physical processes, including Ekman damping and relaxation, are included to study the dynamic equilibrium problem. Similar to the results of observation, the barotropic process is important during the linear stage of the life cycle. The structure during the nonlinear stage is similar to observations, especially the upper-level meridional wind field. During the nonlinear stage, baroclinic processes dominate. Afterwards, the system reaches saturation and stays in an oscillatory state due to the nonlinear wave-mean flow interaction. Simple bulk aerodynamic formulae are used to parameterize the surface heat fluxes. Ekman damping works to spin down the circulation. Relaxation is used to restore the zonal mean field back to its initial condition and works a simple way to mimic the radiative processes which maintains the strength of the environment. The experiment with Ekman damping and relaxation shows that the system reaches a state of dynamic equilibrium. The relaxation restores the strength of the jet and the kinetic energy is dissipated due to Ekman damping. The results of different sensitivity experiments are compared to the control run to see the effects of parameterized processes. In part II we deal with the quasi-equilibrium problem. Wind-induced heat and water vapor fluxes from the ocean surface are also calculated using the bulk aerodynamic formulae. By adopting the Emanuel convection scheme, we put together different diabatic processes including radiative cooling, Ekman damping and convection. Although physical processes are idealized, the results demonstrate strong interaction between the circulation of African disturbances and convection. The scenario is that the African waves works as the synoptic-scale forcing to modify the entropy of the subcloud layer. Convection, in response to the large-scale forcing, redistributes the heat and water vapor upward. The interaction of convection and African easterly waves ultimately reaches a state of quasi-equilibrium. Two-dimensional perturbations are introduced at the lower boundary as the system reaches a two-dimensional equilibrium state. The structure and evolution of the African easterly waves are well replicated. The results show that wave disturbances grow in the expense of zonal mean available potential energy and kinetic energy trough barotropic and baroclinic energy conversions. The trough has a cold core below and warm core above structure. The geographic distribution of precipitation rate shows that the convection is enhanced by the dynamic forcing of the approaching mid-level trough of the African waves. The structure and evolution of the African disturbances is modified by the convective processes. The results show much resemblance to the observations. Sensitivity experiments show that higher SST with fixed temperature gradient results in higher and weaker jet and weaker vertical wind shear below the jet. According to Miller and Lindzen's results, the effect of higher SST on jet properties is a possible cause that leads to later organization of convections over the Atlantic. Larger temperature gradient with fixed SST gives rise to a stronger jet. The results are insensitive to the initial background humidity but sensitive to the surface water vapor flux over the land. / by Bruce, Ting-Hsin Kuo. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996. / Includes bibliographical references (p. 248-259). / by Wenwei Pan. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1990. / Includes bibliographical references (p. 85-87). / by Volkmar Wirth. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, September 1999. / "August 6, 1999." / Includes bibliographical references (leaves 188-190). / We have developed a two-dimensional viscoelastic finite-difference modeling method for highly complex surface topography and subsurface structures. Realistic modeling of seismic wave propagation in the near surface region is complicated by many factors, such as strong heterogeneity, topographic relief and large attenuation. In order to account for these complications, we use a velocity-stress staggered grid and employ an 0(2,4) accurate viscoelastic finite-difference scheme. The implementation includes an irregular free surface condition for topographic relief and a variable grid technique in the shallow parts of the model. Several methods of free surface condition are bench marked, and an accurate and simple condition is proposed. In the proposed free surface condition, stresses are calculated so that the shear and normal stresses perpendicular to the boundary are zero. The calculation of particle velocities does not involve any specific calculations, and the particle velocities are set to zero above the free surface. A stable variable grid method is introduced, where we use a three times finer grid in the near surface or low velocity region compared to the rest of the model. In order to reduce instability, we apply averaging or weighting to the replacement of the coarse grid components within the fine grid. The method allows us to avoid any limitation of the shape of the grid size boundary. Numerical tests indicate that approximately ten grid-points per shortest wavelength with the variable grid method results in accurate calculations. The method requires a stair-shaped discretization of a free surface. We investigated the stair-shaped structures, and found that the cause of the dispersion from irregular free surface is mainly a numerical error due to the large grid sizes rather than the Rayleigh waves scattering due to the stair-shaped boundary. The finite-difference modeling is applied to the investigation of near surface wave propagation. Several numerical simulations are performed to show the characters of wave propagation in the near surface region. The simulations show that the low velocity thin layers just below the surface and anelastic attenuation have significant effect on surface seismic record. The 2-D modeling of near surface structure beneath a 2-D refraction survey line is carried out. The comparison of the observed data with theoretical waveforms is performed. The characters in the observed data can be explained by a subsurface model constructed by P-wave traveltime tomography. / by Koichi Hayashi. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 163-178). / Transiting exoplanets provide an amazing sample through which transmission spectroscopy observations, combined with atmospheric retrieval, can characterize the atmospheres of those planets. Out of that sample, super Earth exoplanets are particularly interesting because it is expected that their atmospheres will have a large diversity - from terrestrial-like to mini-Neptune-like. Discovering and understanding this large diversity will lead to fundamental progress forward in categorizing exoplanets and planet formation theory. Due to this intrigue of investigating super- Earth-sized exoplanets, the following thesis is separated into two projects involving the characterization of super Earth atmospheres. TESS will provide an unprecedented sample of nearby transiting super Earth exoplanets. Due to the large number of anticipated detections, developing prescreening techniques is especially important in order to determine which objects are the most desirable for in-depth investigations. The first study focuses on how we can use transmission spectroscopy to separate low-cloud H2-dominated super Earth atmospheres from other types of super Earth atmospheres. To do such, I define a metric called the "relative amplitude" of the spectral features in transmission. I find that spectral features in low-cloud H₂-dominated super Earth atmospheres will have a relative amplitude approximately 2.5 times larger than both high-cloud H2-dominated super Earth atmospheres and low-mean-molecular-mass super Earth atmospheres. I use this metric to predict the number of planets in which we could make the low-cloud H₂-dominated characterization for three different categories of exoplanets [currently detected exoplanets, anticipated TESS-planets, and anticipated transiting planets on the sky] and three different telescope options [0.3 m aperture microsatellites, a 0.76 m aperture telescope (same photon collecting area as FINESSE), and a 6.5 m aperture telescope (same photon collecting area as JWST)]. For the anticipated TESS-planets, a 6.5 m telescope would be able to characterize approximately 14 planets in 100 hours of observation time, 50 planets in 550 hours of observation time, 100 planets in 1600 hours of observation time, and 200 planets in 5300 hours of observation time. In comparison, the smaller telescopes (0.76 m and 0.3 m) require much longer amounts of time to build up the SNR required to make the low-cloud H2-dominated characterization. Specifically, the 0.76 m telescope (or the 6 microsatellite configuration) would characterize between 15 - 26 planets within the first three years of observation. A four microsatellite configuration would characterize 5 - 11 planets within its first three years of observation. In summary, if the field is unwilling to spend more than 500 hours of 6.5 m telescope time on prescreening exoplanets, then we must launch a characterization mission in the near future. In the second study, I focus specifically on the super Earth GJ 1214b, the most studied super Earth to date. GJ 1214b is particularly interesting due to the difficulty in characterizing its atmosphere because of observed flat transmission spectra. The currently accepted characterization for the flatness in the observed spectra is that GJ 1214b's atmosphere is dominated by high-altitude clouds. In this study, I investigated, using a Monte Carlo Markov Chain atmospheric retrieval algorithm, whether planetary parameters could compensate for each other to create the apparent flatness in the observed transmission spectrum of GJ 1214b. My analysis confirms the conclusions of previous studies that high-altitude cloud deck characterizations are consistent with the GJ 1214b transmission spectrum. However, by probing how different planetary parameters can compensate for each other, I have uncovered a small portion of phase space (approximately five percent of the MCMC solution set phase space) with low-altitude clouds (cloud tops at pressures greater than 100 Pa) and realistic atmospheric temperatures (temperatures greater than 400 Kelvin). Further investigation of this space is required to determine if such atmospheric compositions could exist in chemical equilibrium. Ultimately, future work may definitively prove that the only realistic characterization for GJ 1214b is high-altitude clouds. To this end, important future work for GJ 1214b includes determining its effective temperature, constraining the pressure level probed in transmission, and estimating its albedo. All three of these investigations will provide for better constraints on the possible characterizations for GJ 1214b. In conjunction, future higher signal-to-noise ratio, broader wavelength coverage, and higher spectral resolution observations between 1 - 5 [tm are needed at an SNR greater than ten times the current observations in order to better constrain the atmospheric composition of GJ 1214b. The James Webb Space Telescope is the optimal facility for these measurements. / by Stephen Joseph Messenger. / Ph. D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, February 1999. / Includes bibliographical references. / Simple thermal models of collisional orogenesis generally predict metamorphic temperatures that are much cooler than those revealed by thermo barometric studies of real metamorphic terrains. This thesis focuses on a more realistic model that accounts for the redistribution of crust enriched in heat-producing elements by accretion and erosion. In collisional settings, these processes lead to the development and growth of a wedge of heat producing crust within the overriding plate. Maximum temperatures in an orogenic setting occur within this wedge, and inverted thermal gradients occur beneath the zone of maximum temperatures, a characteristic not displayed by simple models that ignore the combined effects of accretion and erosion. Synthetic metamorphic field gradients generated by tracking the pressure temperature history of rocks advected through a model orogen are generally similar to those observed in the field. Specific aspects of these metamorphic patterns can be related to the relative rates of accretion, erosion, and plate convergence. In particular, peak metamorphic temperatures within the core of an orogen are related to ratios of accretion rate to convergence velocity; and the distance from the toe of an orogen to the metamorphic core is controlled by the ratio of erosion rate to accretion rate. In addition, results of this model challenge two prevailing paradigms in metamorphic petrology: 1) that the metamorphic record does not closely reflect the geothermal gradients within an orogen; and 2) that metamorphic pressure-temperature paths characterized by isothermal decompression require rapid un roofing. Under special (but nonetheless realistic) conditions, metamorphic field gradients closely mimic actual geotherms. The model predicts that pressure-temperature paths for rocks from the metamorphic cores of mountain ranges will display a component of isothermal decompression, even at low exhumation rates. The sensitivity of the metamorphic history of a synthetic collisional orogen to the rate and geometry of accretionary and erosional processes implies that it should be possible to use the metamorphic record of a real orogen to extract information about rates of deformation and denudation. Tests of this hypothesis with data from the Himalayan orogen confirm that most critical parameters can be constrained to within about 30%. / by Audrey Dean Huerta. / Ph.D.