Laurie Linnett

High resolution time delay estimation using sliding discrete Fourier transform

In this paper, we propose a novel time delay estimation approach based on sliding the discrete Fourier transform (DFT) analysis window, sample by sample, over the received short continuous wave (CW) pulse signal with the DFT evaluated successively. This approach uses the maximum magnitude of the spectrum and its corresponding phase offset to estimate the time delay (pulse echo mode) of the signal. We use the corresponding time as the first estimate, which is improved on the basis of the related phase. Examples are given of synthetic signals and simulated delays scenario, with and without added white noise. An underwater application, based on distance and speed of sound measurements using this approach in a water tank is demonstrated. The proposed method is shown to significantly outperform standard correlator-based approaches. Furthermore, the algorithm is simple to use and can be easily implemented, being based on phase detection using the sliding DFT.

A model based approach to object detection in digital mammography

Breast cancer has the highest incidence level of any cancer among women living in developed countries. Early detection of masses, and microcalcification clusters, is presently the best means whereby breast cancer mortality rates may be reduced. To this end, high resolution digital mammograms, generally at the 50 μm resolution, are used with a view to automatically detecting any potential anomalies. This paper presents an original technique for the detection of such microcalcifications. This is done by specifying a parametric model and using this to gain estimates of object positions and sizes directly.

Spatial stochastic models for seabed object detection

We introduce two statistical models designed to detect discrete objects in sidescan SONAR which consider complimentary approaches to the problem. The first considers a complex textural model for the objects and implements detection through a dual hypothesis on texture class presence, while the second implements a complex Gibbs field model of the image and utilizes prior knowledge of typical object morphologies to support its detection rate. The models are demonstrated on examples of different seabed sediments and object types, and are shown to be reliable in operation. The common theme of the two models is use of spatial context in analysis, which, we argue, is a very powerful technique for improving the flexibility and reliability of any analysis system.

Phase-based dispersion analysis for acoustic array borehole logging data

A phase-based dispersion analysis method for velocity (slowness) extraction from guided waves recorded by an acoustic borehole logging tool in a geological formation is presented. The technique consists of acquiring waveforms from an array of receivers distributed along the tool and constructing the dispersion characteristic by processing in the frequency domain and exploiting phase information to measure the travel time for each frequency component. The approach is nonparametric and completely data-driven and provides high resolution estimates that do not rely on velocity guesses or assumptions regarding the type of modes. Results are free of the aliases and spurious modes which are characteristic of some prior approaches. Examples of dispersion estimation curves are presented using synthesized flexural waves and field data from wireline dipole sonic tools; results are compared with those from the weighted spectral semblance (WSS) and amplitude and phase slowness estimation (APES) methods to demonstrate the effectiveness and utility of the proposed method.

Fourier extension and hough transform for multiple component FM signal analysis

Conventional representations in the time or frequency domain are inadequate for non-stationary signals which have statistical properties varying with time. In particular the advent of pulse compression techniques and the use of time varying chirp-type signals widespread in radar, sonar and seismic technologies means there is a need for time-frequency representation. We present a technique which extends the Fourier transform to non-stationary signals. We call the technique Fourier Extension analysis. We show that the analysis extends naturally to a time-frequency representation using the Hough transform projection, and investigate the resolutions obtainable with regard to separation of chirp signals compared with the usual matched filter approach common in radar processing. A visual interpretation of the magnitude and phase of the analytic results is introduced allowing a range of transform orders to be viewed simultaneously. Using frequency modulated signals, we demonstrate significantly higher resolution both in rate and time separation. Examples are given using synthetic and real world chirp signals illustrating improvements in time-frequency resolution using the new approach compared to the commonly used quadratic transforms.

Bandwidth Enhancement: Correcting Magnitude and Phase Distortion in Wideband Piezoelectric Transducer Systems

Acoustic ultrasonic measurements are widespread and commonly use transducers exhibiting resonant behaviour due to the piezoelectric nature of their active elements, being designed to give maximum sensitivity in the bandwidth of interest. We present a characterisation of such transducers that provides both magnitude and phase information describing the way in which the receiver responds to a surface displacement over its frequency range. Consequently, these devices work efficiently and linearly over only a very narrow band of their overall frequency range. In turn, this causes phase and magnitude distortion of linear signals. To correct for this distortion, we introduce a software technique, which considers only the input and the final output signals of the whole systemwhich is therefore generally applicable to any acoustic system. By correcting for the distortion of the magnitude and phase responses, we have ensured the signal seen at the receiver replicates the desired signal. We demonstrate a bandwidth extension on the received signal from 60-130 kHz at -6dB to 40-200 kHz at -1dB in a test system. The linear chirp signal we used to demonstrate this method showed the received signal to be almost identical to the desired linear chirp. Such systemcharacterisation will improve ultrasonic techniques when investigating material properties by maximising the accuracy of magnitude and phase estimations.

An approach for correcting magnitude and phase distortion in wideband piezoelectric transducer systems

Acoustic ultrasonic measurements are widespread and commonly performed using sensitive piezoelectric sensors. An accurate transducer system response to investigate pressure fluctuations in water and their subsequent detection remains a challenge. Typically, these sensors exploit the resonant behaviour of the piezoelectric active element, being designed to give maximum sensitivity in the bandwidth of interest. Calibration of such transducers can provide both magnitude and phase information describing the way in which the sensor responds to a surface displacement over its frequency range. Such resonant sensors are widely used for ultrasonic applications. The resonant nature of the sensors leads to the use of narrowband signals with central frequencies close to the resonant frequency of the piezoelectric element. Consequently, such devices work efficiently and linearly over only a narrow band of their overall frequency range. This causes phase and magnitude distortion of any linear broadband signal being transmitted through such a transmitter-receiver acoustic system. In the present work, we describe a software calibration technique to correct for distortion in a wideband piezoelectric transducer system. We consider only the input and the final output signals of the whole system. Compensating for the distortion of the magnitude and phase responses, we ensured the signal seen at the receiver represents a good replica of the desired signal. A Gaussian, linear, chirp signal was used to demonstrate our approach. This method may be applied to correct system distortion in a wide variety of ultrasonic applications.

Phase-based dispersion analysis of borehole acoustic logs

The dispersive behaviour of acoustic waves in boreholes is of interest in the evaluation of reservoir rocks, particularly from the point of view of near wellbore stress distribution. It is also used as a quality control on dipole sonic calculations that estimate formation shear slowness from the low frequency asymptote of the flexural wave slowness. Multiple methods are available for dispersion analysis; the paper reviews the most commonly used, including the Prony and the spectral semblance methods, and proposes a new phase-based analysis technique that has the benefit of improved slowness resolution. The methods are applied to synthetic and real data sets, and results compared. The new method is show to have lower slowness uncertainty for any given frequency, and the upper and lower frequency limits for which dispersion can be calculated is also extended.

A Novel Bio-Inspired Acoustic Ranging Approach for a Better Resolution Achievement

Bat and dolphin use sound to survive and have greatly superior capabilities to current technology with regard to resolution, object identification and material characterisation. Some bats can resolve some acoustic pulses thousands of times more efficiently than current technology (Thomas & Moss, 2004 ). Dolphins are capable of discriminating different materials based on acoustic energy, again significantly out-performing current detection systems. Not only are these animals supreme in their detection and discrimination capabilities, they also demonstrate excellent acoustic focusing characteristics - both in transmission and reception. If it could approach the efficiencies of bat and cetacean systems, the enormous potential for acoustic engineering, has been widely recognised. Whilst some elements of animal systems have been applied successfully in engineered systems, the latter have come nowhere near the capabilities of the natural world. Recognizing that engineered acoustic systems that emulate bat and cetacean systems have enormous potential, we present in this chapter a breakthrough in high-resolution acoustic imaging and physical characterization based on bio-inspired time delay estimation approach.

Extended Fourier for multi‐component chirp analysis: Application to bat signals.

Much progress has been made recently for improving the time‐frequency analysis. Typically, signals have frequencies that change with time. Even, in bio‐acoustic, creatures such as bats and dolphins rely on time varying signals for echolocation and target detection. With the advent of pulse compression techniques in radar and sonar technologies, the need for time‐frequency representations has become more important. The underlying basis of many of the techniques is the Fourier transform. In this paper, we present a technique, which relies on Fourier transform, exhibiting both greater resolution and conceptual simplicity. We have termed the technique Fourier extension analysis since it extends the use of Fourier analysis to signals with time varying components. Furthermore, we demonstrate how the analysis extends naturally to a time‐frequency representation using the image processing Hough transform tool without the need for windowing, as it is the case for short time Fourier transform, for example, and we a...