L. Peirlinckx

Parametric modeling and estimation of ultrasonic fields using a system identification technique

A system identification technique is presented for the parametric modeling and estimation of acoustic fields generated by an ultrasonic transducer. In this technique, the ultrasonic fields are modeled by the superposition of Gaussian beams. The model order, i.e., the number of the Gaussian beams used, and the characteristic coefficients of these Gaussian beams for the superposition depend on the source distribution. An experimental system is built to measure the acoustic fields. From the measured amplitude and phase data, a maximum likelihood estimator is utilized to determine the order and characteristic coefficients of the field model. Once the order and coefficients of the field model are estimated, a complete parametric description of the acoustic fields is then obtained. Using the estimated model, the prediction of the acoustic fields can be made in both forward and backward direction. The predicted and measured fields are then compared to validate the model. The experimental results demonstrate the ...

Experimental study of wave propagation models for sediments by calibrated transmission experiments on laboratory scale

Due to the complexity of the seafloor as an acoustical system and the huge degree of difficulty in getting accurate and proper backscattered seafloor data, it is highly advisable to first study and validate acoustic wave propagation models using sediments in laboratory circumstances. A Plexiglass-sediment-Plexiglass configuration is used to carry out a transmission experiment on fine sand. The mechanical properties of the sediment are modelled by a general viscoelastic model. An inverse procedure based on a maximum likelihood estimator is used to estimate the physical parameters of the sediment. In order to validate the model, a cross-examination by a reflection experiment is elaborated.

Identification of parametric models for ultrasonic double transmission experiments on viscoelastic plates

In this paper, an identification method is proposed for the determination of the acoustic properties of viscoelastic plates based on an immersion pulse‐echo technique. A double transmission (DT) configuration is used for the experiments, which facilitates the calibration of the measurement setup. With the DT configuration, parametric models are established for the ultrasonic plane‐wave propagation through a viscoelastic plate at both normal and oblique incidence. The effects of beam spreading due to the finite aperture size of the ultrasonic transducer are taken into account by introducing a diffraction correction factor into the plane‐wave models. A maximum likelihood estimator, which takes into account the measurement noise on both the input and the output signals, is constructed for the estimation of the parameters of the DT models. From the estimated model parameters, the absorption and dispersion properties and velocities for both shear and longitudinal waves, as well as the density and the thickness...

Development and Calibration of Pvdf-hydrophone Sensors for the Remore Sensing of Marine Environments

In this paper the use of the Solef @ piezo film in PVDF (polyvinylidene fluoride) is studied and proposed for the development of a new hydrophone, which can be very useful in echosounding applications. A PVDF sensor is developed to capture reflected echoes in an echosounder experimental set-up. The design and construction, as well as the realisation is discussed. Furthermore, a comparison between the theoretical and experimental characteristics is presented. Also an estimation technique is used for the calibration of the developed sensors and for the equalization of the frequency response of the hydrophones.

Identification of parametric models for ultrasonic reflection experiments performed on sediments at oblique incidence

Reflection of bounded ultrasonic beams at oblique incidence from viscoelastic material in controlled measurement conditions is described using a combination of complex harmonic plane waves (CHW model). Previous work was carried out in this field at normal incidence. The shear properties of materials cannot be determined by the normally incident experiment using an immersion technique. This paper gives an extension by modelling wave propagation at oblique incidence using the magnitude as well as the phase of the incident ultrasonic field. A maximum likelihood estimator (MLE) is constructed for the estimation of the model parameters, which takes the measurement noise on both the input and the output signals into consideration. From the estimated model parameters, the absorption and dispersion properties and velocities for both shear and longitudinal waves as well as the density of the sample medium are obtained simultaneously. In order to guarantee that only the material properties are identified, the calibration of the measurement set-up is incorporated in the estimation approach.

Development of a marine G.I.S. with the view on the monitoring and modelling of the Guayas estuary and the Estero Salado in Ecuador

It is the aim of this research to quantify and correlate in space and time certain actual aspects of the geo-bio-physico-chemical equilibrium in the Guayas estuary and the Estero Salado tidal channel, by means of a functional geographical information system (G.I.S.). The channels are located at a transitional marine environment in the intersection of two important fronts that can be described in terms of the autogenic input of climate, geomorphology, hydrology, tectonics, sedimentary, biological, estuarine, tidal, and ocean dynamics, and the anthropogenic front built up by the extended agricultural, domestic and industrial -including harbour-activities in the Ecuadorian Central-West and South. A major component of this study is the development of a PC-based functional G.I.S. with software packages Intergraph MGE (Modular G.I.S. Environment, versions 6 & 7) and Bentley MicroStation Geographics with as data attributes a set of parameters that describe benthic and pelagic communities, chemical, physical and sedimentological aspects. The graphic component is made of digital navigation charts of the area, topologically verified, and conforming the chart datum. Other supporting aids are the review of historical data and the flexibility of object-oriented development environment tools available with Microsoft Access database to provide trend indicator graphics. This approach of study has resulted in a valuable support for the comparison of events along the seasons and establishing their functional relationship with other variables, among other useful definitions of criteria. Ultimately the production of maps with this criteria is meant for giving the authorities a decision-making tool for sustainable management purposes.

Parametric calibration of electroacoustic transducers

In this paper, an identification approach is introduced for the parametric calibration of electroacoustic transducers. The method is based on the conventional substitution technique for hydrophone calibration and a parametric estimation technique for a linear system. The calibration is carried out in three main steps: (1) measurement of the combined transfer function of an electroacoustic transmitter–hydrophone system; (2) parametric estimation of the measured transfer functions with time delay, and calculation of the zero(s)/pole(s) of the system; (3) common polynomial extraction and reconstruction of the transducer’s characteristics. The calibration approach is illustrated with simulations as well as experiments. This new approach will allow the simultaneous calibration of transmitter and one or more hydrophones, and can be used to calibrate the relative amplitude characteristics as well as the phase characteristics of transducers.

On the design of optimal test signals in the case of time limited excitations

The design of optimal time-limited test signals is discussed for a linear time-invariant system with delay. The design method is twofold. Firstly, the optimal power spectrum is calculated by maximizing the Fisher information matrix. Secondly, a time limited multisine with minimum peak value is generated using a time domain I frequency domain swappiing algorithm. The design procedure is investigated and tested by means of simulations and measurements.

Parametric modelling and estimation of acoustic sediment properties using a system identification approach

The acoustic properties (absorption, dispersion) of marine sediments are measured as a signature of the mechanical characteristics of marine-estuarine sediments. Accounting for this signature, the sediments grain size composition, porosity, permeability, viscosity, mineralogy, and physico-chemical environment of deposition seem the controlling factors. In this paper a parametric modelling and estimation approach is presented for acoustic reflection and transmission experiments on natural sediments performed at normal incidence using a controlled measurement environment (frequency range [300, 700] kHz). Since it has been shown that absorption and dispersion in a nearly linear homogeneous isotropic viscoelastic material can be modelled by means of a rational form, L. Peirlinckx et al. (1993), a similar approach was followed for modelling the sediments. Although marine sediments do not fulfil the constraints of isotropy and homogeneity, it is known that their mechanical behaviour lays in the domain of viscous, elastic, or a combination of both responses to applied stresses of various characteristics, and in L. Bjorno (1977), evidence is given of the nonremarkable acoustic non-linearity of marine sand and silt assemblages, as compared to their high attenuation of energy. By this work it could be demonstrated that the proposed modelling approach is valid, certainly for acoustically identifying fine grained, low viscous mud