1,529 research outputs found
Possible Late Pleistocene pingo development within the Lea Valley: evidence from Temple Mills, Stratford, East London
This report describes and discusses a borehole drilled at the Olympic Park development site at Temple Mills, in the Lea Valley near Stratford in East London. The original borehole number is MBHCZ6A-159. It has been registered as TQ38NE 1366 in the BGS Single Onshore Borehole Index.
The borehole penetrates a 70 metre-thick sequence which, from the top downwards, passes through made ground and Quaternary fluvial deposits before revealing a 43 metre-thick zone of bedrock mélange. This mélange includes material from the lower part of the Lambeth Group, the Thanet Formation, and the Chalk. It is interpreted as being the product of pervasive soft-sediment deformation formed during a series of short-lived elevated pore water events, based upon the presence of diagnostic geological structures characteristic of ductile deformation, probably during rapid ejection of groundwater under artesian pressure. During these events, fragments of chalk were carried to within 17 metres of the surface, some 20 metres above where the top of the Chalk was encountered in nearby undisturbed sequences. Conversely, the presence of glauconitic sand apparently derived from the adjacent Palaeogene bedrock at 69 m depth, more than 30 m below the top of the Chalk nearby, implies that there was also some downwards movement during mélange formation. The presence of such a thick disturbed sequence beneath superficial deposits is extremely unusual although not unique.
A model is proposed where this structure and internal deformation is explained by processes of pingo formation and decay in a part of the Lea Valley where the bedrock aquifer is confined by an aquitard as little as 3 m thick. Alternatively, it is possible that the structure formed during release of artesian groundwater pressure following fluvial scour. In either case, it is very likely that the structure lies above a fault zone including fractured, possibly karstic chalk, with high groundwater conductivity.
The structure is most likely to have formed during Late Devensian times, during the deposition of the Shepperton Gravel, but it might be older, possibly pre-dating part or all of the Devensian Kempton Park Gravel
Measuring the elements of the optical density matrix
Most methods for experimentally reconstructing the quantum state of light
involve determining a quasiprobability distribution such as the Wigner
function. In this paper we present a scheme for measuring individual density
matrix elements in the photon number state representation. Remarkably, the
scheme is simple, involving two beam splitters and a reference field in a
coherent state.Comment: 6 pages and 1 figur
Facets for Art Gallery Problems
The Art Gallery Problem (AGP) asks for placing a minimum number of stationary
guards in a polygonal region P, such that all points in P are guarded. The
problem is known to be NP-hard, and its inherent continuous structure (with
both the set of points that need to be guarded and the set of points that can
be used for guarding being uncountably infinite) makes it difficult to apply a
straightforward formulation as an Integer Linear Program. We use an iterative
primal-dual relaxation approach for solving AGP instances to optimality. At
each stage, a pair of LP relaxations for a finite candidate subset of primal
covering and dual packing constraints and variables is considered; these
correspond to possible guard positions and points that are to be guarded.
Particularly useful are cutting planes for eliminating fractional solutions.
We identify two classes of facets, based on Edge Cover and Set Cover (SC)
inequalities. Solving the separation problem for the latter is NP-complete, but
exploiting the underlying geometric structure, we show that large subclasses of
fractional SC solutions cannot occur for the AGP. This allows us to separate
the relevant subset of facets in polynomial time. We also characterize all
facets for finite AGP relaxations with coefficients in {0, 1, 2}.
Finally, we demonstrate the practical usefulness of our approach. Our cutting
plane technique yields a significant improvement in terms of speed and solution
quality due to considerably reduced integrality gaps as compared to the
approach by Kr\"oller et al.Comment: 29 pages, 18 figures, 1 tabl
Non-deterministic Gates for Photonic Single Rail Quantum Logic
We discuss techniques for producing, manipulating and measureing qubits
encoded optically as vacuum and single photon states. We show that a universal
set of non-deterministic gates can be constructed using linear optics and
photon counting. We investigate the efficacy of a test gate given realistic
detector efficiencies.Comment: 8 pages, 6 figure
Location of a Point in a Planar Subdivision and Its Application
Coordinated Science Laboratory was formerly known as Control Systems LaboratoryJoint Services Electronics Program / DAAB-07-72-C-025
An Optimal Algorithm for Finding the Kernel of a Polygon
Coordinated Science Laboratory was formerly known as Control Systems LaboratoryJoint Services Electronics Program / DAAB-07-72-C-0259 [or DAAB-07-72-C-0592]National Science Foundation / NSF MCS-76-1732
Online Dynamic Power Management with Hard Real-Time Guarantees
We consider the problem of online dynamic power management that provides hard real-time guarantees for multi-processor systems. In this problem, a set of jobs, each associated with an arrival time, a deadline, and an execution time, arrives to the system in an online fashion. The objective is to compute a non-migrative preemptive schedule of the jobs and a sequence of power on/off operations of the processors so as to minimize the total energy consumption while ensuring that all the deadlines of the jobs are met. We assume that we can use as many processors as necessary. In this paper we examine the complexity of this problem and provide online strategies that lead to practical energy-efficient solutions for real-time multi-processor systems.
First, we consider the case for which we know in advance that the set of jobs can be scheduled feasibly on a single processor. We show that, even in this case, the competitive factor of any online algorithm is at least 2.06. On the other hand, we give a 4-competitive online algorithm that uses at most two processors. For jobs with unit execution times, the competitive factor of this algorithm improves to 3.59.
Second, we relax our assumption by considering as input multiple streams of jobs, each of which can be scheduled feasibly on a single processor. We present a trade-off between the energy-efficiency of the schedule and the number of processors to be used. More specifically, for k given job streams and h processors with h>k, we give a scheduling strategy such that the energy usage is at most 4.k/(h-k) times that used by any schedule which schedules each of the k streams on a separate processor. Finally, we drop the assumptions on the input set of jobs. We show that the competitive factor of any online algorithm is at least 2.28, even for the case of unit job execution times for which we further derive an O(1)-competitive algorithm
Improved Implementation of Point Location in General Two-Dimensional Subdivisions
We present a major revamp of the point-location data structure for general
two-dimensional subdivisions via randomized incremental construction,
implemented in CGAL, the Computational Geometry Algorithms Library. We can now
guarantee that the constructed directed acyclic graph G is of linear size and
provides logarithmic query time. Via the construction of the Voronoi diagram
for a given point set S of size n, this also enables nearest-neighbor queries
in guaranteed O(log n) time. Another major innovation is the support of general
unbounded subdivisions as well as subdivisions of two-dimensional parametric
surfaces such as spheres, tori, cylinders. The implementation is exact,
complete, and general, i.e., it can also handle non-linear subdivisions. Like
the previous version, the data structure supports modifications of the
subdivision, such as insertions and deletions of edges, after the initial
preprocessing. A major challenge is to retain the expected O(n log n)
preprocessing time while providing the above (deterministic) space and
query-time guarantees. We describe an efficient preprocessing algorithm, which
explicitly verifies the length L of the longest query path in O(n log n) time.
However, instead of using L, our implementation is based on the depth D of G.
Although we prove that the worst case ratio of D and L is Theta(n/log n), we
conjecture, based on our experimental results, that this solution achieves
expected O(n log n) preprocessing time.Comment: 21 page
Neural Simplex Architecture
We present the Neural Simplex Architecture (NSA), a new approach to runtime
assurance that provides safety guarantees for neural controllers (obtained e.g.
using reinforcement learning) of autonomous and other complex systems without
unduly sacrificing performance. NSA is inspired by the Simplex control
architecture of Sha et al., but with some significant differences. In the
traditional approach, the advanced controller (AC) is treated as a black box;
when the decision module switches control to the baseline controller (BC), the
BC remains in control forever. There is relatively little work on switching
control back to the AC, and there are no techniques for correcting the AC's
behavior after it generates a potentially unsafe control input that causes a
failover to the BC. Our NSA addresses both of these limitations. NSA not only
provides safety assurances in the presence of a possibly unsafe neural
controller, but can also improve the safety of such a controller in an online
setting via retraining, without overly degrading its performance. To
demonstrate NSA's benefits, we have conducted several significant case studies
in the continuous control domain. These include a target-seeking ground rover
navigating an obstacle field, and a neural controller for an artificial
pancreas system.Comment: 12th NASA Formal Methods Symposium (NFM 2020
Effects of Air Entrainment on Fluid Transients in Pumping Systems
In pumping installations, fluid transient computations are necessary to achieve safety, efficiency and
economy in design and operation. In some systems, where air content and air entrainment exist, such
computations become highly inaccurate when constant wave speed is assumed. In this paper, a numerical
model and a computational procedure have been developed to investigate the effects of air entrainment on
the pressure transient in pumping systems. Free gas in the fluid and cavitation at the fluid vapour pressure
were modeled in the form of variable wave speed model, which was numerically solved by the method of
characteristics. This model was tested for the case of pump trips due to power failures. The pressure
transient results obtained by this variable wave speed model were analyzed and compared with those results
obtained by constant wave speed model and with the experimental results of other investigators
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