Ocean Transfer Function Research Articles

Broadband acoustic transmission intensity fluctuations in the Tyrrhenian Sea

An acoustic propagation experiment was carried out in the Tyrrhenian Sea in October 1985 in which signals from a broadband source were recorded at a range of 5 km with a vertical 62-m hydrophone array over a period of 5 days. The experiment, named ‘‘NAPOLI 85,’’ was designed to investigate the transfer function of the ocean medium over an acoustic frequency range from 250–2000 Hz as a function of time and position down the vertical array. This paper treats the ocean transfer function in both the time and frequency domains for the lower refracted path. The intensity fluctuations down the array at an acoustic frequency of 1 kHz are presented for the 5 days. Large fluctuations occur simultaneously over the whole extent of the array, and these can be explained by refractive effects in an intrusive layer. There are also weak spatial variations down the array arising from diffractive effects in the layer. The physical mechanisms causing the time variations in the layer are not yet understood.

Forward coupled-mode propagation modeling for coastal acoustic tomography

A coupled normal-mode approach for the prediction of pulse propagation in a complex coastal environment is presented. In this broadband approach, the received time-domain sound-pressure signal is generated by Fourier synthesis using the product of the source signal spectrum and the source-to-receiver ocean transfer function. For a given frequency, the basic formulation involves decomposing the acoustic pressure into slowly varying complex envelopes that modulate (mode by mode) analytic, rapidly varying, adiabatic-mode solutions. A coupled set of differential equations governing these complex modal envelopes is derived and the numerical solution techniques used are discussed. This coupled-mode approach has been used to model the modal arrival structure in the 1992 Barents Sea Coastal Acoustic Tomography Test. Model results and comparison to the data measured by a vertical hydrophone array are discussed.

A note on identification of the acoustic ocean transfer function in the Tyrrhenian Sea

Recently, exploratory analysis of signatures and responses collected during the NAPOLI 85 trial was conducted [L. Ju. Fradkin, J. Acoust. Soc. Am. 87, 1569 and 1577 (1990)]. It was concluded that an ocean transfer function (OTF) relating the two was a non‐negative function of time consisting of two narrow peaks. The second peak was included to compensate for the mismatch in the signature and response shapes. In the absence of an alternative explanation, the peak was attributed to a lenslike feature in the ocean. It transpires now that the responses were preprocessed with a different hardware filter to signatures. Recalibration removes most of the mismatch and therefore a need for a second peak. Significantly, other conclusions on the OTF behavior at high frequencies still stand.

Identification of the acoustic ocean transfer function in the Tyrrhenian Sea. I: Statistical considerations

In the process of statistical analysis of signature/response pairs collected in the Tyrrhenian Sea during the Napoli 85 Trial, a novel method has been developed for identifying the ocean transfer function (OTF). This method allows for the first-order diffraction effects, multipath propagation, and the signal bandwidth narrow compared to the bandwidth of the OTF. It is based on deconvolution of the received pulse with the signature of the emitted broadband pulse. The deconvolution is carried out under several physically reasonable constraints. One of them restricts the method to situations when no phase reversal takes place along any of the paths or, alternatively, when all the paths deliver the signal with a reversed phase. When applied to the Napoli 85 data, the method appears to indicate the presence of two direct (deep sea) paths between the experimental source and receiver with quite distinct times of propagation. In this paper, exploratory statistical data analysis is used to validate both the method and the underlying constraints.

Identification of the acoustic ocean transfer function in the Tyrrhenian Sea. II: Physical considerations

In the process of statistical analysis of signature/response pairs collected in the Tyrrhenian Sea during the Napoli 85 Trial, a novel deconvolution method has been developed for identifying the ocean transfer function (OTF) when its bandwidth is larger than the bandwidth of the emitted pulse. The analysis itself is described in the preceding paper [L. Ju. Fradkin, J. Acoust. Soc. Am. 87, XXXX (1990)]. Here, the underlying convolution formula relating the received signal to the signature as well as other important physical constraints are derived under the assumptions (reasonable in the analyzed case) that the emitted pulse contains no low frequencies, and that it travels along the paths none of which reverses the phase (i.e., no reflections or caustics are involved). The results of processing a representative signature/response pair with the proposed method are discussed briefly. It is shown that for the given source/receiver configuration at least two deep sea (and hence non-phase-reversing) paths exist. It is also shown that the ‘‘internal waves’’ model, rejected by other investigators in the course of their analysis of the environmental data, would not allow for existence of the second macropath. It is concluded that in certain circumstances, the proposed deconvolution method could be used to assess models of medium fluctuations.

Effective sensor placement and intersensor processing parameter selection for covergence zone acoustic source localization

The performance of an intersensor processing algorithm which seeks to localize a moving vessel radiating a narrow‐band acoustic signal is dependent upon the placement of the sensors relative to the source, as well as on the cross‐correlation algorithm and its processing parameters. Since the localization uncertainty is dependent upon the accuracy of the measurements of intersensor differential time of arrival, the processing bandwidth and coherent integration time of the correlator must be chosen in such a way as to maximize the output signal to noise ratio, while simultaneously avoiding decorrelation effects caused by source‐sensor motion and nonidentical ocean transfer function (multipath) effects. In this paper we will demonstrate how a‐priori estimates of the acoustic source spectrum, the source kinematics, and the propagation characteristics of the ocean medium were used to specify a reasonable deployment geometry for a set of distributed sensors and an effective set of correlation processing parameters for the over‐the‐horizon localization of a towed projector. [Work supported by NADC.]

Impact of the ocean acoustic transfer function on the coherence of undersea propagations

Multipath propagation, in conjunction with source motion, creates a splitting and spreading of the received source-signal energy over the time register time scale-factor (ambiguity) plane. This splitting of the received signal energy is manifested as correlation interactions between both; the set of eigenray signal arrivals (comprising the multipaths) at each receiving sensor, and the set of eigenray signal correlation pairs comprising two (or more) sensors. The more tightly the sets of eigenray signals are clustered in the ambiguity plane, the stronger will be the signal component interactions at each sensor and the less will be the expected correlation degradation between sensors. However, the greater will be the variance. Conversely, the more diffuse the clustering, the weaker will be the signal component interactions and the greater will be the expected intersensor coherence degradation. Also, the less will be the variance. Criteria which define the degree of signal component interactions in terms of the signal and the ocean transfer function parameters have been defined and are displayed in graphical form.