Vertical Source Array Research Articles

On the transmission line model for lightning return stroke representation

The widely used transmission‐line (TL) model of lightning return stroke in a vertical channel is most rigorously represented by a vertical phased array of current sources that produce a spherical transverse electromagnetic (TEM) wave in the case of return‐stroke speed v equal to the speed of light c. If the radius of the lightning channel were equal to zero, the equivalent representation could be obtained by applying a hypothetical infinitesimal source of pure spherical TEM wave at the bottom of the channel. A non‐zero‐radius vertical wire above ground excited by a practical source at its bottom end cannot support unattenuated current waves, and the associated electromagnetic field structure is non‐TEM.

The Geoclutter experiment: Remotely imaging sub-bottom geomorphology in shallow water

Preliminary results of the Geoclutter experiment of April–May 2001 are presented. This bistatic reverberation experiment was conducted at the New Jersey Strataform site in continental shelf waters. Roughly 3000 waveforms were transmitted from vertical source arrays in the 390–440 Hz band and received by a horizontally towed array. On average roughly 10–100 discrete clutter events were registered per transmission for a total of at least 30<th>000 geoclutter events that could be confused with a discrete target over the period of the experiment. Bathymetry throughout the New Jersey Strataform area is extremely benign with few discrete features. The vast majority of clutter returns appeared in level areas where absolutely no bathymetric features are present. The clutter events often registered well with buried river channel networks previously characterized in the Geoclutter program. Extensive probing of river-channel-infested areas revealed that the number and level of clutter events is highly dependent on bistatic orientation. The registrations suggest that bistatic aspects that produce ‘‘glints’’ from large projected areas along river channel axes yield the strongest and most frequent targetlike returns. These results are consistent with those predicted by a unified model for reverberation and submerged-target scattering [J. Acoust. Soc. Am. 109, 909–941 (2001)].

Geoacoustic inversion using global search

The purpose of an experiment in ocean acoustics is often to estimate a representative set of parameters for a given environmental model and forward propagation model. Global search is a systematic approach to obtain these parameters relative to manual comparisons of the observed and modeled fields. The SAGA-code provides an engine for doing this comparison. The array data type can be transmission loss, complex amplitude, or reverberation. Several choices of state-of-the-art forward models are available as well as several choices of objective functions (least squares, Bartlett, broadband cross correlation, etc.). Finally, the optimization can be carried out either by exhaustive search, genetic algorithms, or simulated annealing. In a Bayesian approach, the result of an inversion is the a posteriori probability density for the estimated parameters from which all information such as mean, higher moments, and marginal distributions can be extracted. These distributions are useful in assessing data, environmental model, and obtained estimates. In a recent time reversal mirror experiment [Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)], a vertical source array was transmitting to a vertical receive array at range 6.3 km in a 125-m waveguide. It will be investigated how a multisource inversion performs relative to a single source inversion for one way transmission.

The extraction of a target scattering response from measurements made in a range-dependent shallow-water environment

The free-field scattering response of a target may be extracted from measurements made over long ranges in a range-independent shallow-water environment by the use of vertical array of sources and vertical array of receivers. The procedure requires the generation of a low-order shallow-water mode by the source array and the extraction from the received echo at the receiver array of a low-order shallow-water mode. The algorithm is generalized to range-dependent environments where additional complications exist due to the mode coupling produced by the range dependence in the shallow-water environment, which exists in addition to the mode coupling produced by the scatterer. Results of numerical simulations are presented for a ribbed cylindrical shell with hemispherical end caps. The target scattering response is computed using the finite elements and infinite elements method, and the propagation is computed using the parabolic equations method. [Work supported by ONR.]

Measurements and modeling of signal spread in the Barents Sea using time-forward and time-reversed signals

Low-frequency (350-450 Hz), shallow-water propagation measurements were made in several sites in the southern Barents Sea in June 1991 using combinations of broadband signals. The experiment used a two-element vertical source array to transmit the signals, and an omnidirectional source/receiver echo-repeater system to retransmit the signals back to a horizontal line array near the original source location. (All platforms moved at 3 kt.) Measures of two-way signal spreads were collected over a variety of ranges (5 to 50 km) and environmental conditions. Considerable time spreads (up to 1 s) were observed with a characteristic constancy of peak values over the length of the spreads, while analysis of PRN and CW data showed that spectral spreads were confined to very low Dopplers. Furthermore, selected sets of signals were time reversed prior to echo repeating to measure the ability of this highly-spread environment to refocus acoustic energy back to the original source location. Simulations of two-way temporal signal dispersion using the OASES and ORCA models for both time-forward and time-reversed retransmissions are presented and compared to data results for several representative cases. [Work supported by ONR and NAWC.]

Extraction of target characteristics from the response of a ribbed cylindrical shell in a shallow water environment

Bistatic and monostatic target strength values of a ribbed cylindrical shell contain various scattering phenomena such as Bloch waves, rib resonances, specular reflections and other effects due to waves traveling on the surface of the shell. When the target is placed in a shallow water environment, the response is further cluttered by the presence of the various multipaths that a signal travels in a bounded medium. By using a vertical line array of sources and receivers, the target response may be extracted. A vertical array of sources is used to suppress certain shallow water modes in the incident field. A receiver array is used to remove certain shallow water modes from the scattered field. Time-frequency analysis is a final filter for selecting the modes of interest for which the target response is extracted. The extracted target response is correlated against the free field target characteristics previously determined by time-frequency synthesis techniques.

Low-frequency sea surface scatter and horizontal bistatic angle dependence utilizing short coherent pulses

A comprehensive low-frequency (<1000-Hz) low grazing angle (<30°) sea surface acoustic scatter experiment was conducted during February/March 1992 in the Gulf of Alaska. During data collection, wind speeds ranged from 3–17.5 m/s and sea states from 2–6. cw waveforms less than 500 ms in length at 250, 380, 900, and 935 Hz were transmitted by three collinear vertical towed source arrays. Surface scattered energy was received in 17 cosine-spaced beams by a horizontal source-ship-towed line array. This experimental geometry allowed measurement of surface scattering strength as a function of mean grazing angle, frequency, and horizontal bistatic angle. Analysis of these measurements in terms of these dependencies is presented along with comparison to current surface scattering strength model predictions.

Shallow-water backscatter reduction through selective mode excitation

In shallow-water areas the utilization of active sonar is complicated by the fact that in these environments the performance of active sonar systems is degraded by backscatter generated by source signal interaction with channel boundaries. In this paper knowledge about the shallow-water acoustic channel, namely the normal-mode structure, is used to design vertical source arrays which minimize source signal boundary interaction. The boundary signal interaction is minimized for many environments by exciting only the first mode. Mode one excitation is accomplished by weighting the array sources by the amplitude and polarity of the mode one eigenfunction. Simulations across the frequency range from 200 to 700 Hz using a two-way PE backscatter model were conducted. It was shown that a vertical array of sources, containing only 15 elements, can reduce bottom generated backscatter by as much as 15–20 dB.

Bistatic reverberation from an ocean ridge

Bistatic reverberation from a highly lineated ridge in the mid-Atlantic was measured by two research vessels during the ARSRP Main Acoustics Experiment of 1993. Each vessel used a vertical source array and a horizontal receiving array to collect bistatic and monostatic data over a full range of incident and scattered angles with respect to the ridge axis. High-resolution bathymetry reveals that the ridge protrudes above conjugate depth for roughly 50 km, and is characterized by a number of extended scarps running parallel to the ridge axis. Sufficient excess depth exists around the ridge to isolate it with waterborne propagation paths from 1/2 convergence zone (CZ) range (∼33 km) where low-frequency returns register precisely with steep scarps. Significant variations in received level from a given scarp occur as a function of incident and scattered angle. The most prominent returns register with scarps facing the receiving array. Registration between returns from high-resolution waveforms (∼10 m in range) and fine-scale bathymetry is used to elucidate the fundamental scattering process. Comparisons with monostatic measurements of the same ridge from 1/2 CZ and 2 1/2 CZ indicate how fine-scale structure from a given scarp is smeared in long-range reverberation.

Scattering from an object in a stratified medium. II. Extraction of scattering signature

This paper investigates methods to extract and/or characterize the scattering signature of an object in a stratified medium based on data received on a vertical array. Scattering by an object in free space transforms the incident (plane-wave) field into a scattered field with a characteristic angular distribution. The scattering function can be estimated by correlating the scattered field with the incident field over a domain of incident and scattered angles. In an ocean waveguide, the acoustic field incident on the (submerged) object involves many multipath arrivals. Likewise, the scattered field from the object is modified by multipath propagation from the scatterer to the receiver array. Hence, extraction of the scattering function requires a method to deconvolve the channel transfer function from the received data. As the scattering function is a function of both the incident and scattered angles, a comprehensive characterization requires data covering a range of incident and scattered angles. For a distant target, a vertical array of sources and receivers is considered in order to invert for the scattering function. But the data are limited to shallow incident and scattering angles since high angle arrivals are severely attenuated in a shallow water waveguide. For practical problems, a holographic image of the scatterer is proposed for target classification.

Seafloor backscattering from explosive sources during the 1993 Mid-Atlantic Ridge experiment: Preliminary results

During the Acoustic Reverberation Special Research Program (ARSRP) acoustic experiment of July 1993, an experiment was performed to identify seafloor scattering mechanisms. Explosive SUS sources were deployed on the western flank of the Mid-Atlantic Ridge to provide omnidirectional broadband sound for long-range reverberation measurements. In addition, pulsed cw tones in the frequency band 200 to 280 Hz from a vertical source array were used. The signals scattered by the seafloor were received by two 64-wavelength subapertures of a towed array, processed to form beam-time series, and then displayed as reverberation images on map projections. In order to obtain good signal-to-noise ratios, this analysis was performed in frequency bands of 4 Hz (time resolution 0.25 s) and then averaged to form beam-time series of 20-Hz bandwidth at harmonics of the bubble-pulse frequency. Initial results show that the cw measurements are consistent with the broadband results and that scattering strengths of specific seafloor features are generally constant over the frequency band 100 to 1000 Hz. [Work supported by ONR.]

Acoustic color of the Mid-Atlantic Ridge

Narrow-band waveforms in the frequency range 200–300 Hz were transmitted from a vertical source array during the Main Acoustics Experiment (SRP) of July 1993. The resulting backscatter from the ocean bottom was received on a horizontal array. Two-way travel times are computed using a ray trace that includes bottom and surface interacting paths. Combining these results with beamforming, backscatter returns are charted to respective scattering sites on the western flank of the Mid-Atlantic Ridge (MAR). A histogram of the backscatter level as a function of frequency is obtained for each grid point in the survey area. Each histogram is then assigned a color to represent that particular frequency response. A chart of the frequency responses or acoustic colors of the survey area is then composed by charting the acoustic color derived for each grid point. The acoustic color chart is compared with charts of two-way transmission loss, geomorphology, and mean acoustic intensity or brightness to elucidate the observed spatial characteristics. For example, deterministic facets much larger than a wavelength backscatter sound uniformly with frequency and are acoustically white. Surfaces with roughness scales comparable to an acoustic wavelength are characterized by acoustic colors that vary with the incident wavelength.

A model interference method for shallow water reverberation suppression

A method of boundary reverberation reduction for shallow water ducts has been devised that makes use of a vertical array of sound sources. By controlling the waveforms of the sources, control over the phase and amplitudes of a selected set of modes can be made. This in turn allows formation of nulls in the field at specified range/depth pairs. At a given range, the nulls are chosen to be on the bottom of the duct where, for low and mid frequencies, most of the scattering is generated. However, at that same range, maxima in the field at other depths are present, thus enhancing the echo-to-reverberation ratio for reflectors at certain depths in the water column. Presented is the formalism in terms of mode theory, and field calculations for several environments. [Work supported by ONT.]

Low-frequency single interaction acoustic scatter from the sea surface using short pulses

During February/March 1992 a comprehensive low-frequency (<1000 Hz) and low-grazing angle (<30 deg) acoustic sea surface scatter and air/sea interaction experiment was conducted in the Gulf of Alaska. For comparison with acoustic models, measurements were made of air/sea boundary descriptions such as near-surface bubble characteristics, wind speed, wave height, whitecap coverage, volume scatterers (fish), wind stress, etc. A variety of waveforms of short duration (<500 ms) were scattered from a surface path simultaneously isonified by three collinear vertical towed source arrays at frequencies around 250, 400, and 1000 Hz. Surface-scattered energy was received on a horizontal line array towed by the source ship. During the measurement wind speeds from 4 to 19 m/s and rms wave heights from 0.9 to 2.3 m occurred associated with sea states 2 to 6. Surface grazing angles of 4 to 32 deg and horizontal bistatic angles of 15 to 82 deg were observed. Scattering strength analysis includes frequency, grazing angle, waveform, and wind speed dependence as well as correlation with air/sea boundary descriptors. Current scattering strength model predictions utilizing the appropriate measured environmental parameters will be compared to measured data.

Seismic wave generator

A seismic wave generator and method of using it, which comprises a vertical array of sources at known spacings, for detonation in a seismic wave propagating medium of known seismic velocity; means to initiate the sources in time sequence in a selected series of known different time intervals and recording the seismic waves at a distant location to provide a first electrical signal; and correlating the first electrical signal with a second electrical signal derived from the pattern of initiation, as modified by the travel times of the waves between the several separate charges.

Efficient numerical solution technique for wave propagation in horizontally stratified environments

A new solution technique for wave propagation in horizontally stratified viscoelastic media is presented. The model provides a full-wave solution for the field generated by a single source as well for that generated by a vertical source array. It allows the spatial distribution of the acoustic field to be evaluated at least one order of magnitude faster than with existing models based on the Thomson—Haskell solution technique. Computational examples are given that demonstrate the model's versatility in providing exact solutions to a wide range of guided and nonguided propagation problems in underwater acoustics.

A full wave solution for propagation in multilayered viscoelastic media with application to Gaussian beam reflection at fluid–solid interfaces

A new solution technique for wave propagation in horizontally stratified viscoelastic media is presented. The model provides a full wave solution for the field generated by a single source as well as for that generated by a vertical source array. It allows the spatial distribution of the acoustic field to be evaluated at least one order of magnitude faster than with existing models based on the Thomson–Haskell solution technique. The computational efficiency of the numerical code is demonstrated by providing exact numerical solutions for the reflectivity pattern associated with narrow ultrasonic beams incident on a fluid–solid interface near the Rayleigh angle.

Reflection and transmission of narrow beams at a water/bottom interface

The reflection and transmission of narrow sound beams at the interface between two fluid media was studied experimentally by Muir et al. Sound Vib. 64, 539–551 (1979)] who found that narrow beams impinging on a sedimentary bottom at grazing angles below the critical angle will not be totally reflected as predicted by Snell's law. Here a numerical model yielding an exact solution to the wave equation in horizontally stratified environments is used to analyze the observed phenomenon. A beam of any realistic width is generated by introducing a vertical source array and properly phasing the single sources. It is shown that the deviation from Snell's law is due to the finite width of the angular spectrum of narrow beams, and the results given are in good qualitative agreement with the experimental results.

A new fast‐field solution to propagation in multilayered solid environments

During the last decade the fast field program (FFP) has been an important tool for modeling wave propagation in stratified media. The earlier models of this type were based on a solution of the depth‐separated wave equation by means of the Thomson‐Haskell method. In this new FFP model a more direct and faster solution technique is employed. The field is expressed in terms of unknown scalar potentials. At each interface the boundary conditions yield a linear system of equations in the Hankel transforms of the potentials. Using a technique similar to the one used in finite‐element programs, these equations are mapped into a global system which is solved with an efficient band‐solver. Contrary to the Thomson‐Haskell method, the new technique can treat several sources and receivers with one single solution. Hence, the field produced by vertical source arrays can be determined. Even with one source and receiver the computational speed has been improved by an order of magnitude, and pulse calculations by means of Fourier synthesis have become feasible. The features of the model are illustrated through a few computational examples.

Experiments on single mode excitation in shallow water propagation

The normal mode theory for shallow water propagation is used to develop a method permitting the excitation of a single mode by means of a vertical array of sources. The experiments were carried out in a laboratory tank, in a thin layer of water lying on a thick sand bed. Single mode excitation was performed by a vertical array of 15 piezoelectric transducers, whose gain and polarity depend on the mode to be excited. Single mode propagation between a source array and a receiving array is also examined. The receiving array can simultaneously resolve five modes. In these conditions crosstalk between mode and mode conversion can be studied.