Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996. / Includes bibliographical references. / by Garrett Tetsuo Ito. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1994. / Includes bibliographical references (p. 207-213). / by James Sydney Risbey. / Ph.D.

Thesis: S.M., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 97-98). / Understanding the velocity of the compressional waves travelling through rocks is essential for the purposes of applied geophysics in such areas as groundwater and hydrocarbon exploration. The wave velocity is defined theoretically by the Newton-Laplace equation, which relates the wave velocity, V, to the square root of the ratio of the rock's elastic modulus, M, and its density, [rho] (Bourvie et al., 1987). Therefore, the equation indicates that the velocity is inversely proportional to density. However, the in-situ field measurements and laboratory experiments of compressional wave velocity through different rocks show otherwise. In other words, the velocity is directly proportional to approximately the 4th power of density as stated by Gardner (Gardner et al., 1974). This thesis investigates the inconsistency between theory and observations regarding the relationship between velocity and density of saturated porous rocks. The inconsistency is clarified by deriving a new expression for the elastic modulus, M, using Wyllie's time average equation and the Newton-Laplace equation. The new derived expression of the elastic modulus, M, provides dependence of M on density to approximately the 9th power. In addition, Gardner's equation is modified to accurately obtain the velocity over the entire range of densities (from 1.00 g/cm³ to around 3.00 g/cm³) and porosity (from 0% to 100%). The end of this thesis is an application of the previous outcomes with real data sets, where the results validate the derived expression of the elastic modulus as well as the generalized form of Gardner's equation. / by Marwah I. AL Ismail. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996. / Includes bibliographical references (p. 235-242). / by Françoise R. Robe. / Ph.D.

Thesis (S.M. in Geosystems)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002. / Includes bibliographical references (p. 53). / Since 1998, a producing oil field in Oman has been experiencing microearthquake activity. The aim of this project is to compare numerical models of wave propagation using simple source representations to a small subset of these microearthquakes, with three goals in mind: 1) to understand whether the microearthquakes are generated by movement along a known fault system in the field, or by some other mechanism; 2) if the source is fault related, to better understand what kind of movement is occurring on the fault; and 3) to see if this simple modeling method provides useful results, and forms a basis for future work. Synthetic waveforms are generated using a one-dimensional, discrete wavenumber numerical model (Bouchon, 1980) with two simple source representations: an explosive point source and a vertical force. Comparison of the synthetic waveforms to the microearthquake data indicates that the vertical force results in a better match than the explosive point source. In addition, a simple model consisting of the superposition of four vertical forces (representing vertical fault rupture), results in waveforms that are very similar to the recorded events. These results suggest that the source of the microearthquakes is motion along a near-vertical normal fault system that has been mapped in the field. These results are also consistent with work by Sze and Toksoz (2001) in which relocation of the same events imaged a near-vertical normal fault in the field. Further work using fault rupture source modeling may provide additional insight into the amount of fault motion that is occurring in relation to these events. / by Heather J. Hooper. / S.M.in Geosystems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1989. / Includes bibliographical references (p. 227-239). / by Arlindo Moraes da Silva, Jr. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2007. / Cataloged from PDF version of thesis. Pages 81 and 87 missing from original thesis. / Includes bibliographical references (pages 86-91). / Single-origin dark chocolate samples derived from cocoa grown in developing countries from around the world were analyzed to determine their lead concentrations and the isotopic composition of the lead. The lead isotope ratios were compared with published data from aerosols and volcanic rocks nearest to the cocoa growing regions. Samples from different countries and manufacturers were compared, and we conclude that the source of lead depends on the country of origin and not the manufacturer. Chocolates grown in the Northern Hemisphere usually had lead isotope ratios that matched the global atmospheric lead isotopic signature from the Northern Hemisphere. Chocolates grown in the Southern Hemisphere did not match the global signature, but rather more closely matched the lead isotopic signature from volcanic rocks in their respective countries, and had a lower average lead concentration than chocolates from the Northern Hemisphere. Soils from Venezuela were also analyzed, and confirmed the conclusion that atmospheric lead is the predominant source of bioavailable lead. Many of the chocolates also had lead concentrations below the limit of 0.1 ppm set by the FDA; however, one manufacturer, Dagoba, consistently had lead concentrations above the limit. The percent of cocoa in each chocolate bar was also compared with the lead concentrations, concluding that the concentration of lead is not necessarily dependent on the amount of cocoa in the bar. / by Jessica A. Thompson. / S.B.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1990. / Includes bibliographical references (leaves 63-64). / by Eliot F. Young. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996. / Includes bibliographical references. / by Zhou Yu. / M.S.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, February 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Ediacaran and early Cambrian sandy and silty sediments commonly preserve microbial fossils and textures as well as the fossils of soft-bodied organisms. The rarity of similar fossils and textures in coarse-grained siliciclastic strata of the later Phanerozoic suggests that a taphonomic window facilitated this mode of fossil preservation. However, environmental and biological factors that promoted fossilization remain unclear. To experimentally identify mechanisms of preservation in siliciclastic sediments, cyanobacteria and soft tissues (scallop adductor muscles) were incubated in silica sand or clay minerals for up to two months. Clay mineral veneers coated both cyanobacterial filaments and the surfaces of soft tissues and were formed by two main processes: 1) the adhesion of fine particles from the sediment and 2) the precipitation of authigenic minerals. Photosynthetic, sheathed microorganisms were best preserved in the presence of high concentrations of dissolved silica (>0. 1 mM) and suspended fine particles (>5.6 mg/L) in solution. We showed that these organisms could be preserved in oxic environments and that the degradation of cyanobacteria by heterotrophic microorganisms was not necessary for fossilization. In contrast, soft tissues buried in sand/clay were preserved under anaerobic conditions and mineral veneers around them contained reduced iron. All scallops decayed in the presence of endogenous bacteria and the redox cycling of iron that included the microbial reduction of iron(III). We showed that the early precipitation of silica and the formation of microbial death masks is not critical for preservation. Additionally, when the degradation of soft tissues advanced within the first 15 to 30 days, all soft tissues decayed completely and left no morphological imprint. Taken together, these results show that the early microbial reduction of iron(III) present in sedimentary minerals and the formation of mineral veneers are critical for the preservation of organic material. The same processes may have facilitated the formation of exceptionally preserved fossils and textures throughout the Ediacaran and early Cambrian. Future studies should focus on the roles of iron redox cycling, sediment composition and microbial metabolisms in the preservation of soft tissues to better understand mechanisms for Ediacaran-style preservation in siliciclastic environments. / by Sharon Audrey Newman. / Ph. D.