53 research outputs found
Basin-scale tidal measurements using acoustic tomography
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1990Travel-times of acoustic signals were measured between a bottom-mounted source near
Oahu and four bottom-mounted receivers located near Washington, Oregon, and California
in 1988 and 1989. This paper discusses the observed tidal signals. At three out
of four receivers, observed travel times at M2 and S2 periods agree with predictions
from barotropic tide models to within ±30° in phase and a factor of 1.6 in amplitude.
The discrepancy at the fourth receiver can be removed by including predicted effects of
phase-locked baroclinic tides generated by seamounts.
Our estimates of barotropic M2 tidal dissipation by seamounts vary between 2 x 1016
and 1 X 1018 erg·s-1. The variation by two orders of magnitude is due to uncertainties in
the numbers and sizes of seamounts. The larger dissipation (1 x 1018 erg·s-1) is the same
order as previous estimates and amounts to 4% of the total dissipation at M2
Analysis of internal wave induced mode coupling effects on the 1995 SWARM experiment acoustic transmissions
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1997As part of the Shallow Water Acoustics in a Random Medium (SWARM) experiment
[1], a sixteen element WHOI vertical line array (WVLA) was moored in 70 meters
of water off the New Jersey coast. This array was sampled at 1395 Hz or higher for
the seven days it was deployed. Tomography sources with carrier frequencies of 224
and 400 Hz were moored about 32 km shoreward, such that the acoustic path was
anti-parallel to the primary propagation direction for shelf generated internal wave
solitons. Two models for the propagation of normal modes through a 2-D waveguide
with solitary internal wave (soliton) scattering included are developed to help
in understanding the very complicated mode arrivals seen at the WVLA. The simplest
model uses the Preisig and Duda [2] sharp interface approximation for solitons,
allowing for rapid analysis of the effects of various numbers of solitons on mode arrival
statistics. The second model, using SWARM thermistor string data to simulate
the actual SWARM waveguides, is more realistic, but much slower. The analysis of
the actual WVLA data yields spread, bias, wander, and intensity fluctuation signals
that are modulated at tidal frequencies. The signals are consistent with predicted
relationships to the internal wave distributions in the waveguides.The funds for my education were
provided by the Office of Naval Research through an ONR Fellowship (MIT award
002734-001); the funds for SWARM were also provided by the Office of Naval Research
through ONR Grant N00014-95-0051
Entanglement Entropy for Singular Surfaces
We study entanglement entropy for regions with a singular boundary in higher
dimensions using the AdS/CFT correspondence and find that various singularities
make new universal contributions. When the boundary CFT has an even spacetime
dimension, we find that the entanglement entropy of a conical surface contains
a term quadratic in the logarithm of the UV cut-off. In four dimensions, the
coefficient of this contribution is proportional to the central charge 'c'. A
conical singularity in an odd number of spacetime dimensions contributes a term
proportional to the logarithm of the UV cut-off. We also study the entanglement
entropy for various boundary surfaces with extended singularities. In these
cases, similar universal terms may appear depending on the dimension and
curvature of the singular locus.Comment: 66 pages,4 figures. Some typos are removed and a reference is adde
Comments on Holographic Entanglement Entropy and RG Flows
Using holographic entanglement entropy for strip geometry, we construct a
candidate for a c-function in arbitrary dimensions. For holographic theories
dual to Einstein gravity, this c-function is shown to decrease monotonically
along RG flows. A sufficient condition required for this monotonic flow is that
the stress tensor of the matter fields driving the holographic RG flow must
satisfy the null energy condition over the holographic surface used to
calculate the entanglement entropy. In the case where the bulk theory is
described by Gauss-Bonnet gravity, the latter condition alone is not sufficient
to establish the monotonic flow of the c-function. We also observe that for
certain holographic RG flows, the entanglement entropy undergoes a 'phase
transition' as the size of the system grows and as a result, evolution of the
c-function may exhibit a discontinuous drop.Comment: References adde
Some Calculable Contributions to Holographic Entanglement Entropy
Using the AdS/CFT correspondence, we examine entanglement entropy for a
boundary theory deformed by a relevant operator and establish two results. The
first is that if there is a contribution which is logarithmic in the UV
cut-off, then the coefficient of this term is independent of the state of the
boundary theory. In fact, the same is true of all of the coefficients of
contributions which diverge as some power of the UV cut-off. Secondly, we show
that the relevant deformation introduces new logarithmic contributions to the
entanglement entropy. The form of some of these new contributions is similar to
that found recently in an investigation of entanglement entropy in a free
massive scalar field theory [1].Comment: 52 pages, no figure
Predictors of early recurrence after resection of colorectal liver metastases
BACKGROUND: Early recurrence after resection of colorectal liver metastases (CLM) is common. Patients at risk of early recurrence may be candidates for enhanced preoperative staging and/or earlier postoperative imaging. The aim of this study was to determine if there are any risk factors that specifically predict early liver-only and systemic recurrence. METHODS: Retrospective analysis of prospective database of patients undergoing liver resection (LR) for CLM from 2004 to 2006 was undertaken. Early recurrence was defined as occurring within 18 months of LR. Patients were classified into three groups: early liver-only recurrence, early systemic recurrence and recurrence-free. Preoperative factors were compared between patients with and without early recurrence. RESULTS: Two hundred and forty-three consecutive patients underwent LR for CLM. Twenty-seven patients (11%) developed early liver-only recurrence. Dukes C stage and male sex were significantly associated with early liver-only recurrence (P < 0.05). Sixty-six patients (27%) developed early systemic recurrence. Tumour size ≥3.6 cm and tumour number (>2) were significantly associated with early systemic recurrence (P < 0.001). CONCLUSIONS: It is possible to stratify patients according to the risk of early liver-only or systemic recurrence after resection of CLM. High-risk patients may be candidates for preoperative MRI and/or computed tomography-positron emission tomography (CT-PET) scan and should receive intensive postoperative surveillance
Holographic Calculations of Renyi Entropy
We extend the approach of Casini, Huerta and Myers to a new calculation of
the Renyi entropy of a general CFT in d dimensions with a spherical entangling
surface, in terms of certain thermal partition functions. We apply this
approach to calculate the Renyi entropy in various holographic models. Our
results indicate that in general, the Renyi entropy will be a complicated
nonlinear function of the central charges and other parameters which
characterize the CFT. We also exhibit the relation between this new thermal
calculation and a conventional calculation of the Renyi entropy where a twist
operator is inserted on the spherical entangling surface. The latter insight
also allows us to calculate the scaling dimension of the twist operators in the
holographic models.Comment: 71 pages, 6 figure
Towards a derivation of holographic entanglement entropy
We provide a derivation of holographic entanglement entropy for spherical
entangling surfaces. Our construction relies on conformally mapping the
boundary CFT to a hyperbolic geometry and observing that the vacuum state is
mapped to a thermal state in the latter geometry. Hence the conformal
transformation maps the entanglement entropy to the thermodynamic entropy of
this thermal state. The AdS/CFT dictionary allows us to calculate this
thermodynamic entropy as the horizon entropy of a certain topological black
hole. In even dimensions, we also demonstrate that the universal contribution
to the entanglement entropy is given by A-type trace anomaly for any CFT,
without reference to holography.Comment: 42 pages, 2 figures, few new ref's and comments adde
The afterlife of colonial radio in Christian missionary broadcasting of the Philippines
The article explores Christian missionary radio broadcasting as part of a wider sonic colonization of the Philippines under US colonial rule. Specifically, I explore how some post-Second World War faith-based broadcasters shaped the listening practices of Filipino audiences through programming tactics such as blocktiming. Furthermore, I consider how missionary broadcasters cultivated direct relationships with listeners through the imagined ‘shared experiences’ of aural space. As a case study, I explore the activities of the US-based Far East Broadcasting Company (FEBC), which began its operations in the Philippines in 1948. Since then, the organization has used the country as a hub for its expanding domestic and international radio network, which now includes broadcasts to South East Asia, China and other parts of the world. In addition to exploring how FEBC’s localized approach to programming has cultivated specific listening audiences, I explore how programmes have been received by listeners in the Philippines, many of whom continue to tune in via terrestrial radio
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