275 research outputs found
Velocity ambiguity mitigation of off-grid range migrating targets via Bayesian sparse recovery
Within the scope of sparse signal representation, we consider the problem of velocity ambiguity mitigation for wideband radar signal. We present a Bayesian robust algorithm based on a new sparsifying dictionary suited for range-migrating targets possibly straddling range-velocity bins. Numerical simulations on experimental data demonstrate the ability of the proposed algorithm in mitigating velocity ambiguity
Unambiguous Sparse Recovery of Migrating Targets with a Robustified Bayesian Model
The problem considered is that of estimating unambiguously migrating targets observed with a wideband radar.
We extend a previously described sparse Bayesian algorithm to the presence of diffuse clutter and off-grid targets. A hybrid-Gibbs sampler is formulated to jointly estimate the sparse target amplitude vector, the grid mismatch and the (assumed) autoregressive noise. Results on synthetic and fully experimental data show that targets can be actually unambiguously estimated even if located in blind speeds
The occurrence and origin of salinity in non-coastal groundwater in the Waikato region
Aims
The aims of this project are to describe the occurrence, and determine the origin of non-coastal saline groundwater in the Waikato region. High salinity limits the use of the water for supply and agricultural use.
Understanding the origin and distribution of non-coastal salinity will assist with development and management of groundwater resources in the Waikato.
Method
The occurrence of non-coastal groundwater salinity was investigated by examining driller’s records and regional council groundwater quality information. Selected wells were sampled for water quality analyses and temperatures were profiled where possible. Water quality analyses include halogens such as chloride, fluoride, iodide and bromide. Ratios of these ions are useful to differentiate between geothermal and seawater origins of salinity (Hem, 1992). Other ionic ratio approaches for differentiating sources and influences on salinity such as those developed by Alcala and Emilio (2008) and Sanchez-Martos et al.,
(2002), may also be applied. Potential sources of salinity include seawater, connate water, geothermal and anthropogenic influences. The hydrogeologic settings of saline occurrence were also investigated, to explore the potential to predict further occurrence.
Results
Numerous occurrences of non-coastal saline groundwater have been observed in the Waikato region.
Where possible, wells with relatively high total dissolved solids (TDS) were selected for further investigation.
Several groundwater samples are moderately saline and exceed the TDS drinking water aesthetic guideline
of 1,000 g m-3 (Ministry of Health, 2008).
Selected ion ratios (predominantly halogens) were used to assist in differentiating between influences on salinity such as seawater and geothermal. Bromide to iodide ratios, in particular, infer a greater geothermal influence on salinity, although other ratios are not definitive.
The anomalously elevated salinity observed appears natural but nevertheless has constrained localised groundwater resource development for dairy factory, industrial and prison water supply use. Further work may show some relationship with geology or tectonics, which could assist prediction of inland saline groundwater occurrence
From the bottom to the top: Assessing the potential subsurface migration of fracking-related fluids along natural geological pathways
Over the last century humans have increasingly gone from just extracting fluids from the subsurface to also injecting them, for example hydraulic fracturing (fracking) for shale gas. Concurrently, there has been increased environmental interest in how injected fluids might interact with natural groundwater systems. One potential environmental impact of fracking is the contamination of shallow groundwater from the upward migration of injected and formation fluids along natural geological pathways. High-permeability fault and fracture zones have generally been regarded as the highest-risk pathway. Therefore, the first aim of this thesis was to determine a horizontal respect distance between fluid injection locations and known faults. Using published microseismic data a horizontal respect distance of ~900 m was proposed. The second aim of this thesis was to consider complex geological structure in numerical models to investigate hydrogeological factors which might increase the vulnerability of shallow groundwater resources to the potential upward migration of fracking-related fluids. Using 2D and 3D numerical models four hydrogeological factors increased the vulnerability of a shallow aquifer to upward fluid migration from an underlying shale reservoir: increased stimulated fracture extent; greater amounts of overpressure; the presence of low-permeability faults; and the absence of deep high-permeability formations. Low-permeability faults acted to compartmentalise groundwater, thereby discouraging regional horizontal flow whilst encouraging upward flow through strata. The integration of groundwater quality and seismic reflection data provided a novel method for identifying compartmentalisation, but compartmentalisation could not be identified from surface water quality data in the studied basin. However, analysis of surface water quality data could demonstrate regional-scale groundwater-surface water interactions. Importantly, this thesis highlights the need to understand and model interactions between deep groundwater, shallow groundwater, and surface waters by integrating data from a variety of industry and environmental regulatory sources
Three Dimensional Bistatic Tomography Using HDTV
The thesis begins with a review of the principles of diffraction and reflection tomography; starting with the analytic solution to the inhomogeneous Helmholtz equation, after linearization by the Born approximation (the weak scatterer solution), and arriving at the Filtered Back Projection (Propagation) method of reconstruction. This is followed by a heuristic derivation more directly couched in the radar imaging context, without the rigor of the general inverse problem solution and more closely resembling an imaging turntable or inverse synthetic aperture radar. The heuristic derivation leads into the concept of the line integral and projections (the Radon Transform), followed by more general geometries where the plane wave approximation is invalid. We proceed next to study of the dependency of reconstruction on the space-frequency trajectory, combining the spatial aperture and waveform. Two and three dimensional apertures, monostatic and bistatic, fully and sparsely sampled and including partial apertures, with controlled waveforms (CW and pulsed, with and without modulation) define the filling of k-space and concomitant reconstruction performance. Theoretical developments in the first half of the thesis are applied to the specific example of bistatic tomographic imaging using High Definition Television (HDTV); the United States version of DVB-T. Modeling of the HDTV waveform using pseudonoise modulation to represent the hybrid 8VSB HDTV scheme and the move-stop-move approximation established the imaging potential, employing an idealized, isotropic 18 scatterer. As the move-stop-move approximation places a limitation on integration time (in cross correlation/pulse compression) due to transmitter/receiver motion, an exact solution for compensation of Doppler distortion is derived. The concept is tested with the assembly and flight test of a bistatic radar system employing software-defined radios (SDR). A three dimensional, bistatic collection aperture, exploiting an elevated commercial HDTV transmitter, is focused to demonstrate the principle. This work, to the best of our knowledge, represents a first in the formation of three dimensional images using bistatically-exploited television transmitters
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