• Advancements in seismic tomography with application to tunnel detection and volcano imaging

      Clippard, James Doyle; Christensen, Douglas H.; Pulpan, Hans; Barry, Ronald P.; Harisen, Roger A.; Eichelberger, John C. (1998)
      Practical geotomography is an inverse problem with no unique solution. A priori information must be imposed for a stable solution to exist. Commonly used types of a priori information smooth and attenuate anomalies, resulting in 'blurred' tomographic images. Small or discrete anomalies, such as tunnels, magma conduits, or buried channels are extremely difficult imaging objectives. Composite distribution inversion (CDI) is introduced as a theory seeking physically simple, rather than distributionally simple, solutions of non-unique problems. Parameters are assumed to be members of a composite population, including both well-known and anomalous components. Discrete and large amplitude anomalies are allowed, while a well-conditioned inverse is maintained. Tunnel detection is demonstrated using CDI tomography and data collected near the northern border of South Korea. Accurate source and receiver location information is necessary. Borehole deviation corrections are estimated by minimizing the difference between empirical distributions of apparent parameter values as a function of location correction. Improved images result. Traveltime computation and raytracing are the most computationally intensive components of seismic tomography when imaging structurally complex media. Efficient, accurate, and robust raytracing is possible by first recovering approximate raypaths from traveltime fields, and then refining the raypaths to a desired accuracy level. Dynamically binned queuing is introduced. The approach optimizes graph-theoretic traveltime computation costs. Pseudo-bending is modified to efficiently refine raypaths in general media. Hypocentral location density functions and relative phase arrival population analysis are used to investigate the Spring, 1996, earthquake swarm at Akutan Volcano, Alaska. The main swarm is postulated to have been associated with a 0.2 km$\sp3$ intrusion at a depth of less than four kilometers. Decay sequence seismicity is postulated to be a passive response to the stress transient caused by the intrusion. Tomograms are computed for Mt. Spurr, Augustine, and Redoubt Volcanoes, Alaska. Relatively large amplitude, shallow anomalies explain most of the traveltime residual. No large amplitude anomalies are found at depth, and no magma storage areas are imaged. A large amplitude low-velocity anomaly is coincident with a previously proposed geothermal region on the southeast flank of Mt. Spurr. Mt. St. Augustine is found to have a high velocity core.
    • Characterization and optimization of the magnetron directional amplifier

      Hatfield, Michael Craig; Hawkins, Joseph G. (1999)
      Many applications of microwave wireless power transmission (WPT) are dependent upon a high-powered electronically-steerable phased array composed of many radiating modules. The phase output from the high-gain amplifier in each module must be accurately controlled if the beam is to be properly steered. A highly reliable, rugged, and inexpensive design is essential for making WPT applications practical. A conventional microwave oven magnetron may be combined with a ferrite circulator and other external circuitry to create such a system. By converting it into a two-port amplifier, the magnetron is capable of delivering at least 30 dB of power gain while remaining phase-locked to the input signal over a wide frequency range. The use of the magnetron in this manner is referred to as a MDA (Magnetron Directional Amplifier). The MDA may be integrated with an inexpensive slotted waveguide array (SWA) antenna to form the Electronically-Steerable Phased Array Module (ESPAM). The ESPAM provides a building block approach to creating phased arrays for WPT. The size and shape of the phased array may be tailored to satisfy a diverse range of applications. This study provided an in depth examination into the capabilities of the MDA/ESPAM. The basic behavior of the MDA was already understood, as well as its potential applicability to WPT. The primary objective of this effort was to quantify how well the MDA could perform in this capacity. Subordinate tasks included characterizing the MDA behavior in terms of its system inputs, optimizing its performance, performing sensitivity analyses, and identifying operating limitations. A secondary portion of this study examined the suitability of the ESPAM in satisfying system requirements for the solar power satellite (SPS). Supporting tasks included an analysis of SPS requirements, modeling of the SWA antenna, and the demonstration of a simplified phased array constructed of ESPAM elements. The MDA/ESPAM is well suited for use as an amplifier or an element in a WPT phased array, providing over 75% efficiency and a fractional bandwidth exceeding 1.7% at 2.45 GHz. The results of this effort provide the WPT design engineer with tools to predict the MDA's optimum performance and limitations.
    • Design And Implementation Of A Relative Plasma Density Probe For The Hex Sounding Rocket Mission

      Johnson, Bruce A.; Hawkins, Joseph (2003)
      This thesis describes the design and implementation of a relative plasma density probe for the Horizontal E-region eXperiment (HEX) sounding rocket mission conducted by the University of Alaska Fairbanks (UAF). The purpose of the plasma probe in the HEX experiment is to confirm the anticipated plasma depletion resulting from the upwelling of ionospheric gases theorized to exist poleward of an active auroral arc. The instrument uses a conducting band wrapped around the circumference of the rocket payload to collect and measure positively charged ions in the vicinity of an auroral arc while following a nearly horizontal trajectory. The probe collectors are biased to repel free thermal electrons while the ions are "swept up" by the cross sectional area of the probe's motion. A separate electrode detects the presence of high-energy electrons within the aurora. The resulting collector currents are measured and transmitted to ground equipment for archiving and later analysis.*<p> *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office; QuickTime; Adobe Acrobat; Windows MediaPlayer or RealPlayer; Internet browser.<p>
    • Investigation Of Auroral Hiss Observations On The Ground: Application To Remote Sensing Of Auroral Magnetosphere

      Harikumar, Jayashree; Sonwalkar, Vikas; Hawkins, Joseph; Sentman, Davis; Olson, John (2001)
      Observed both on the ground at high latitudes and on spacecraft in the auroral zone, auroral hiss (AH) emissions (~1 kHz to ~1 MHz) are intense electromagnetic emissions emitted from the auroral region. Standard whistler mode propagation theory in a smooth magnetosphere predicts that AH generated at large wave-normal angles along the auroral field lines by Cerenkov resonance cannot penetrate to the ground. This thesis presents a new mechanism of AH propagation to the ground in which presence of density depletions along the field lines in the auroral zone and meter-scale density irregularities at altitudes <5000 km at high latitude permits the AH propagation to the ground. In the proposed mechanism AH generated at high altitudes (>5000--20,000 km) propagates to lower altitudes (<3000--5000 km) in two modes, the ducted mode and the non-ducted mode, with large wave-normal angles. At altitudes <5000 km meter-scale irregularities scatter the hiss into electrostatic waves with large wave-normal angles that are reflected into the magnetosphere and electromagnetic waves with small wave-normal angles that can penetrate to the ground. The AH propagation model proposed in this thesis also explains the spectral characteristics of AH including the upper and lower frequency cutoffs, the dispersion of AH, the location of ionospheric exit points of AH with respect to visible aurora, and the 2--5 orders of magnitude difference in the power spectral density ratio measured on satellites versus ground. The new understanding of AH permits the determination of AH source region, energetic electron parallel resonance energy, and cold plasma electron concentrations along field lines. Analysis of AH spectra, recorded at South Pole (July 09, 1996 0005 UT), using the model developed in this thesis shows that: (a) AH source region altitude for frequencies 7--9 kHz should be >16,000 km while for frequencies 12--20 kHz it should be <8000 km, (b) parallel resonance energy of the energetic electrons generating the frequencies should be <1 keV, and (c) cold plasma electron concentration along the field line Lambda = 79� should be ~100 el cm-3 at 12,740 km altitude.