http://hdl.handle.net/1721.1/7807 Wed, 19 Jun 2013 07:27:04 GMT 2013-06-19T07:27:04Z http://hdl.handle.net/1721.1/79300 Influence of topographic stress on rock fracture : a two-dimensional numerical model for arbitrary surface topography and comparisons with borehole observations Slim, Mirna I Theoretical calculations indicate that topographic stresses in some landscapes may be large enough to fracture rocks, which in turn could influence slope stability, erosion rates, and bedrock hydrologic properties. These predictions typically have involved idealized topographic profiles, with few direct comparisons of predicted topographic stresses and observed fractures at specific field sites. I use a numerical model to calculate the stresses induced by measured topographic profiles and specified far-field tectonic stress. I compare the calculated stress field and potential shear fracture orientations with fracture abundance and fracture orientations observed in shallow boreholes. The model uses a boundary element method to calculate the stress distribution beneath an arbitrary topographic profile. When applied to topographic profiles extracted from a laser altimetry map of the Susquehanna/Shale Hills Critical Zone Observatory in central Pennsylvania, the model predicts considerable differences in depth profiles of stresses beneath ridgelines and valley floors. Using a representative value for the friction angle of shale, we calculate the minimum cohesion required to prevent shear failure, Cmin, as a proxy for the potential for fracturing or reactivation of existing fractures. We compare depth profiles of Cmin with structural analyses of image logs from four boreholes located on the valley floor, and find that fracture abundance declines sharply with depth in the uppermost 10 m of the boreholes, consistent with the modeled profile of Cm.. In contrast, Cmin increases with depth below ridgetops, suggesting that future analyses of ridgetop wells should observe a different trend in fracture abundance if topographic effects are indeed important. The numerical model used assumes the subsurface to be homogeneous and isotropic. The model-predicted fracture orientations do not reflect the scatter in fracture orientations seen in the wells. Thus, the present results support the hypothesis that topography can influence subsurface rock fracture patterns, suggest the imitation and reactivation of fractures of different generations, and provide a basis for further observational tests. Thesis (S.M. in Geophysics)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 73-75). Tue, 01 Jan 2013 00:00:00 GMT http://hdl.handle.net/1721.1/79300 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79299 Surface deformation analysis over a hydrocarbon reservoir using InSAR with ALOS-PALSAR data Şahin, Sedar Cihan InSAR has been developed to estimate the temporal change on the surface of Earth by combining multiple SAR images acquired over the same area at different times. In the last two decades, in addition to conventional InSAR, numerous multiple acquisition InSAR techniques have been introduced, including permanent scatterer (PS) (Ferretti et al., 2001) and small baseline subset (SBAS) (Berardino et al., 2002). Stanford method for persistent scatterers (StaMPS) (Hooper, 2006) is another multiple acquisition method that has been developed for estimating ground deformation and differs from the permanent scatterer technique through the method used for pixel selection. In this project, we used the SBAS method to detect the surface deformation over a hydrocarbon reservoir in Adiyaman Providence, Turkey. The SBAS technique is performed on combinations of SAR images that are characterized by small orbital distances with large time intervals. By applying singular value decomposition (SVD), the temporal sampling rate is increased and those subsets are connected. We applied the SBAS method to five ALOS-PALSAR fine-beam dual (FBD) mode images, and removed the topographic phase by using a 3 arc-sec SRTM digital elevation model (DEM). The atmospheric artifacts are determined and filtered out based on available spatial and temporal information on processed data. Our analysis has revealed that due to the effective mitigation measures taken by the oil company, the maximum observed LOS displacement velocity in the oil field is 5 mm/yr with a likely uncertainty of a similar magnitude in the period of 2007-2010. The high uncertainty estimate is due to the other spatially correlated signals of similar and larger magnitude seen in regions outside of the oil field. Thesis (S.M. in Geophysics)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 92-97). Tue, 01 Jan 2013 00:00:00 GMT http://hdl.handle.net/1721.1/79299 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79156 Detectability of biosignature gases in the atmospheres of terrestrial exoplanets Messenger, Stephen Joseph Biosignature gases in the atmosphere of an exoplanet provide a means by which we can deduce the possible existence of life on that planet. As the list of possible biosignature gases is ever growing, the need to determine which molecules provide the best opportunities for detection grows as well. One way to explore these systems is through modeling radiative transfer via transmissivity as light travels from the parent star, through the atmosphere of the planet, and then impacts a detector located at Earth. As the light travels through the planetary atmosphere, it acquires molecular features from the planet due to the composition, temperature, and pressure structure of the atmosphere. By adding synthetic noise to the modeled transmissivity spectra, I determine the detectability of a range of atmospheric mixing ratios for ten biosignature gases from the HIgh-resolution TRANsmission molecular absorption (HITRAN) database: oxygen, ozone, methane, nitrous oxide, methyl bromide, methyl chloride, hydrogen sulfide, carbonyl sulfide, phosphine, and sulfur dioxide. The deep investigation of the HITRAN biosignature gases in this study is possible due to the ability to properly map their absorption cross sections to varying temperatures and pressures. For each of the above HITRAN molecules, I analyze alternative spectral features for detection in order to emphasize the importance of and determine the ability for multiple band detection of biosignature gases. Water vapor (though not a biosignature gas) is included in order to study its potential for spectral masking. Though I nd that each of the above HITRAN gases could be detected in exoplanet atmospheres if that molecule has a large enough atmospheric mixing ratio, an Earthsize planet with an Earth-like atmosphere located at 35.45 parsecs would only allow for discernible biosignature features from ozone, nitrous oxide, and methane in the infrared wavelength region. Sixteen additional (and non-standard) biosignature gases included in this study do not have absorption cross sections that are currently mapable to alternative temperatures and pressures. These sixteen biosignature gases are acetaldehyde, acetone, benzene, carbon disulfide, dimethyl disulfide, dimethyl sulfide, dimethyl sulfoxide, ethanol, ethyl mercaptan, fluoroacetone, isoprene, methyl ethyl ketone, methyl mercaptan, methyl vinyl ketone, thioglycol, and toluene. To circumvent the nonmapability of the absorption cross sections to dierent temperatures and pressures, I use the detectivity calculations and the absorption cross sections from ozone, methane, and nitrous oxide to estimate the threshold atmospheric mixing ratios for the detection of the sixteen non-standard biosignature gases with a 35 m telescope, 100 hours of observation, and a target distance of 35.45 parsecs. The combination of the threshold atmospheric mixing ratios calculated for these sixteen non-standard biosignature gases with the results from the HITRAN biosignature gases investigated in this study demonstrate that an atmospheric gas will require a mixing ratio in the tens to hundreds of ppm to be detectable above a 5[sigma] level with a 35 m telescope, an observation time of 100 hours, and a target distance of 35.45 parsecs. Keeping with the theme of multi-wavelength detection, I end the analysis of the sixteen non-standard biosignature gases by proposing potential spectral feature wavelengths for each gas based on their molecular absorption cross section spectral profiles. As many biosignature gases have molecular features at longer wavelengths than the traditional IR region, I investigated the technological requirements for detecting biosignature gas spectral features in one of the low-signal long-wavelength regions, the millimeter. Though the investigation into the millimeter region reveals unrealistic technological demands for the successful detection of the case study, oxygen, I use the analysis as a platform to introduce the theoretical concept of observing future targets with multiple next-generation telescopes stationed in a matrix in order to produce the same observational ability of a larger (and more distant future) telescope. While interferometric investigations into millimeter spectral features are improbable in the near future, the use of interferometry with next generation instruments may allow for investigations in the 10 - 30 [mu]m region, thereby opening alternative wavelengths for biosignature gas detection. Since this theoretical interferometry idea relies on the ability to increase the signal-to-noise ratio (SNR) of the observations, I investigated the interaction between telescope aperture size and observation duration on the detectability (i.e. SNR) of biosignature gases in reference to finding a middle ground between these two system parameters for both a 6 m and 35 m telescope. Unfortunately, a 6 m telescope does not provide a large enough collecting area to increase the SNR sufficiently enough to detect atmospheric gases. For futuristic telescope designs, though a 20 m telescope (or nine JWSTs working together to achieve the same collecting area) would begin to discern some biosignature gas features from the continuum (for high biosignature gas atmospheric abundances), a 35 m class telescope (or equivalent interferometric telescope array) should be the minimum aperture size considered for biosignature gas detection in transmissivity spectroscopy. Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2013.; 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. 179-182). Tue, 01 Jan 2013 00:00:00 GMT http://hdl.handle.net/1721.1/79156 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/78549 Ice shelf-ocean interactions in a general circulation model : melt-rate modulation due to mean flow and tidal currents Dansereau, Véronique Interactions between the ocean circulation in sub-ice shelf cavities and the overlying ice shelf have received considerable attention in the context of observed changes in flow speeds of marine ice sheets around Antarctica. Modeling these interactions requires parameterizing the turbulent boundary layer processes to infer melt rates from the oceanic state at the ice-ocean interface. Here we explore two such parameterizations in the context of the MIT ocean general circulation model coupled to the z-coordinates ice shelf cavity model of Losch (2008). We investigate both idealized ice shelf cavity geometries as well as a realistic cavity under Pine Island Ice Shelf (PIIS), West Antarctica. Our starting point is a three-equation melt rate parameterization implemented by Losch (2008), which is based on the work of Hellmer and Olbers (1989). In this form, the transfer coefficients for calculating heat and freshwater fluxes are independent of frictional turbulence induced by the proximity of the moving ocean to the fixed ice interface. More recently, Holland and Jenkins (1999) have proposed a parameterization in which the transfer coefficients do depend on the ocean-induced turbulence and are directly coupled to the speed of currents in the ocean mixed layer underneath the ice shelf through a quadratic drag formulation and a bulk drag coefficient. The melt rate parameterization in the MITgcm is augmented to account for this velocity dependence. First, the effect of the augmented formulation is investigated in terms of its impact on melt rates as well as on its feedback on the wider sub-ice shelf circulation. We find that, over a wide range of drag coefficients, velocity-dependent melt rates are more strongly constrained by the distribution of mixed layer currents than by the temperature gradient between the shelf base and underlying ocean, as opposed to velocity-independent melt rates. This leads to large differences in melt rate patterns under PIIS when including versus not including the velocity dependence. In a second time, the modulating effects of tidal currents on melting at the base of PIIS are examined. We find that the temporal variability of velocity-dependent melt rates under tidal forcing is greater than that of velocity-independent melt rates. Our experiments suggest that because tidal currents under PIIS are weak and buoyancy fluxes are strong, tidal mixing is negligible and tidal rectification is restricted to very steep bathymetric features, such as the ice shelf front. Nonetheless, strong tidally-rectified currents at the ice shelf front significantly increase ablation rates there when the formulation of the transfer coefficients includes the velocity dependence. The enhanced melting then feedbacks positively on the rectified currents, which are susceptible to insulate the cavity interior from changes in open ocean conditions. Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 121-123). Sun, 01 Jan 2012 00:00:00 GMT http://hdl.handle.net/1721.1/78549 2012-01-01T00:00:00Z