Browsing by Subject "Soils--Analysis"
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Item Determination of soil properties from earthquake data(2002-05) Huerta López, Carlos Isidro; Powers, Edward J.; Pulliam, Robert J.; Stokoe, Kenneth H.Soil damping and site (system) dominant vibration frequency estimations were obtained by means of the Random Decrement Method (RDM) using numerically simulated time series of soil model responses upon random excitations and real earthquake records. Highly reliable estimations were obtained when the system response was dominated by a distinctive or preferential vibration mode. Different damping mechanisms did not play a significant role in the variability of damping estimations; however, the excitation type did. The damping estimations were highly dependent on how well the Randomdec signature was defined. The alternate methods to measure the decay of the Randomdec signature may produce large variability in the damping estimations. The most consistent and reliable estimations were obtained using the average of the decay every halfcycle of the Randomdec signature. Hurley’s method consistently underestimated the damping values by an average of 50%. The frequency estimations were highly consistent when the Randomdec signature is well defined. Horizontal-to-vertical (H/V) spectral ratios were used to characterize local sediment response, and 1-D wave propagation modeling was used to estimate soil properties and theoretical amplification factors of shallow marine sediment layers in an experimental site in the Gulf of Mexico (GOM). Relative to the vertical spectral amplitude, the horizontal spectral amplitudes increased by an order of magnitude at 0.35 Hz, and by at least two orders of magnitude at 1.9 Hz. A 50-mthick soil system parameterized as three solid layers resting over a half-space with a water layer at the top produced theoretical H/V spectral ratios largely consistent with the observed H/V spectral ratios. The modeling results were consistent with both earthquake and background noise records. The use of background noise offers the advantage of better defining the spectral characteristics of the signal when, during the averaging process, a large ensemble is used. Time-frequency signal processing techniques were used to characterize the time-frequency variability of earthquake transient signals and to quantify the frequency-temporal variability of the seismic phases that constitute the multicomponent earthquake records. The state-of-the-art time-frequency distributions when applied to earthquake records were studied, and the conceptual bases for using this technique for seismic wave parameters estimations were set. In order to overcome various shortcomings associated with the Short Time Fourier Transform (STFT), a class, known as Cohen's class, of time-frequency energy distributions has been developed in recent years. This class includes the WignerVille Distribution (WVD), Choi-Williams Distribution (CWD), Reduced Interference Distribution (RID), Radially Gaussian Kernel Distribution (RGKD), and the Adaptive Optimal Kernel Distribution (AOKD). Each one of these processes was applied to controlled signals numerically generated as well as to seismic records. These time-frequency distributions possess advantages and disadvantages, which were discussed in this dissertation. On the basis of this study, I conclude that the RGKD and the AOKD are the most suitable for the analysis of the time-frequency characteristics of the waves (multi-components) that constitute an earthquake record.Item Simulation of an INS soil analysis system(2007-12) Doron, Oded, 1979-; Biegalski, Steven R.Global climate change in either the form of global warming or global cooling is occurring relatively rapidly today. Studies have shown that increased levels of greenhouse gases, especially atmospheric carbon dioxide (CO₂) are the dominate component contributing to the change. A reduction in CO₂ may be influenced by making larger efforts to sequester carbon in soil and therefore to not only keep soil organic carbon (SOC) levels steady but by possibly increasing them through human assistance. Soil sequestration of carbon has been estimated to have one of the largest potentials to sequester carbon in the world. By some estimation up to 2 billion tons of carbon can be sequestered terrestrially. Therefore the efficient and repetitive monitoring of SOC on a local and global scale is a critical issue. The current soil measurement technique utilized around the world is chemical analysis of one form or another. Chemical analysis of soil is a well studied technique that returns relatively accurate results of density, moisture content, and elemental breakdown of a soil. However, chemical analysis is costly, time consuming, and destructive. As a result of the destructive nature of soil chemical analysis, repeated measurements of the same soil site is impossible. Also, due to time constraints, it would be difficult to analyze a large area utilizing chemical analysis. To surmount the inherent issues with chemical analysis a system based on inelastic neutron scattering (INS) is under development for non-destructive monitoring of carbon in soil. It is based on spectroscopy of gamma rays induced by fast (14 MeV) neutrons emanating isotropically from a D-T neutron generator (NG). The calibration of the INS system is a remains a challenge. Calibration of the system is necessary for relating the carbon gamma ray counts from the detectors to a carbon concentration in the soil volume measured. Utilizing a benchmarked Monte Carlo model of the INS system it is possible to create many calibration curves. The advantages of the model are that the calculations require a relatively short amount of time, and that all the soil variables are defined by the user.