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Nov 1976

Volume 60, Issue S1, pp. S1-S125

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back to top Session FF. Underwater Acoustics V: Impulse Sources
Invited Papers
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Survey of nonexplosive sources for acoustic propagation measurements (A)

J. H. Cawley

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S70-S70 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Since the mid 1940's explosive charges have been widely used by offshore seismic exploration companies, by the U. S. Navy, and by other groups to generate high‐intensity acoustic pulses for seismic and sonar‐related activities. In the late 1950's offshore exploration groups began to develop alternate energy sources, and in the mid 1960's a number of non‐explosive impulse and cw marine sound sources were introduced and soon dominated the offshore exploration field. Despite their relatively low‐energy levels, these sources have since been tailored to reproducibly generate acoustic spectra concentrated in the range below 100 Hz at efficiency levels that have allowed them to completely replace shallow‐detonated explosives. This report briefly describes important characteristics of four classes of nonexplosive acoustic impulse sources developed for offshore seismic exploration purposes and compares their acoustic signatures with those of explosive SUS charges used in sonar propagation studies. The total energy levels of SUS and several nonexplosive impulse sources are compared and acoustic levels, bubble pulse, and the spectral characteristics of the signatures are discussed and methods for defining and controlling these characteristics are briefly described. [Work supported by ONR/LRAPP.]
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Hydroacoustic impulse sound source (A)

J. V. Bouyoucos

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S71-S71 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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An impulse source concept is described in which the potential energy of the pressure head of an underwater environment is converted to kinetic energy of an imploding flow which, in turn, is converted to an acoustic impulse free of after‐shot clutter. This energy conversion concept has been realized in various hardware forms under the designation HYDROSHOCK.® It is characteristic of this class of source that considerable latitude exists in specifying the spectral level and spectral shape of the transmitted signal. The conversion mechanisms will be outlined, and the properties of several of the existing HYDROSHOCK® devices will be described. One recently developed source exhibits a peak source level of 255 dB re 1 μPa at 1 m, a pulse duration of 40 μsec, and a useful spectrum extending from 20 Hz to 100 kHz. The trade offs in source configuration between source level and bandwidth will be described, and comparisons will be made to spark sources, air guns, and various explosive devices. Applications for such impulse sources will also be reviewed. [This research has been supported by the Office of Naval Research, Code 222.]
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Airgun array design techniques (A)

T. L. Teer

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S71-S71 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Since airguns were introduced to the oil exploration industry in the late 1960's, they have replaced dynamite as the primary acoustic source for marine seismic profiling. Arrays of airguns compare favorably in the seismic bandwidth to dynamite for depth of penetration and resolution. Multi‐element arrays capable of peak‐to‐peak pressures exceeding 50 bars (at 1 meter) are presently being used in the industry. Independent control of chamber pressure and volume, operating depth and distance between airguns allows the design of airgun arrays which can meet particular resolution or depth of penetration objectives. Array design techniques are reviewed and data from a Shell array is presented to illustrate the degree to which success in meeting the design objectives of a high peak pressure, a high primary‐to‐bubble ratio and broad bandwidth is achieved. Far‐field signatures from the array are shown to possess the necessary shot‐to‐shot stability required of modern signal enhancement techniques when proper care is taken in deploying and towing the array. Boundary effects (ghosting) are shown to reduce shot stability. Some mechanical deployment and towing considerations are discussed.
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Underwater explosion sources (A)

Ermine A. Christian

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S71-S71 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Explosions have been used as underwater acoustic sources for many years, sometimes at rates exceeding 50 000 charges a year. During the past decade the types of studies undertaken with explosion sources have changed in character, and the requirements on charge levels have become more demanding. For example, an accuracy of ±1 dB in narrow‐band levels is now desired for propagation‐loss measurements in the 25 to 300 Hz frequency range. Fixed explosion source levels of such accuracy can only be accomplished if the charge depth is closely controlled; but strict depth control severely limits the type of experiment that is feasible. Compromise techniques for effective utilization of explosion sources are being developed. Recent progress along two avenues of attack on the problem—improved methods of describing the explosion sound field, and of taking the source characteristics into account during farfield data analysis—will be reviewed.
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A computer model of the explosive acoustic source (A)

R. B. Lauer and L. C. Maples

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S71-S71 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Explosives are widely used as broadband acoustic sources, but their spectrum characteristics are not adequately defined. A comprehensive experimental evaluation is not feasible economically and the problem can be solved only by a model, supported by a limited critical experiment. A computer model of the source waveform, using only analytic functions, is at last available. The model, known as DIAL‐A‐SUS, readily provides the desired spectrum for any charge size and detonation depth by Fourier transform techniques. Certain amplitude and time characteristics of the waveform, which are defined in terms of the source weight and depth, are used to evaluate the parameters of the model's analytic functions. The model is ideally suited to study the sensitivity of the spectrum characteristics to variations in the critical parameters of the waveform and determine the important factors requiring further experimental evaluation. The results and significance of such studies are discussed, with experimental implications, and model predictions are compared with measurements. An exciting spinoff is the characterization of the explosive as a multifrequency, narrow‐band (e.g., 1 Hz) source, with important potential applications. [Work supported by NUSC.]
Contributed Papers
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Low‐frequency underwater explsoive source levels (A)

R. C. Hughes

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S72-S72 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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New experimental measurements of underwater source levels for explosive charges at depths ranging from 20 to 200 m are reported. The explosives used include 45 g (1.6 oz) TETRYL primer charges, 454 g (1.0 lb) bare TNT charges, 31 g (1.1 oz) TETRYL SUS charges and 816 g (1.8 lb) TNT SUS charges. Underwater pressure measurements were made at a nominal distance of 1500 m. For frequencies below approximately twice the bubble frequency, the experimentally measured acoustic energy spectra show a remarkable correspondence when they are appropriately scaled for detonation depth and for charge weight. A universal model of the low frequency energy spectra of TNT and similar explosives, which can be scaled to different charge weights and different detonation depths, shows considerable promise.
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Source level measurements of 1.8 lb and 1.1 oz underwater sound signals (A)

J. M. Thorleifson and P. D. Boyle

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S72-S72 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The source levels of underwater sound signals (SUS), containing 1.8 lb and 1.1 oz of TNT detonated at 60 and 600 ft, were measured experimentally at Jervis Inlet in November 1974. The measurements were made by recording the source pressure‐time waveforms received on hydrophones placed about 1000 ft below the detonation depth. The measured source levels are compared with levels computed using a model based on Weston's analytical model [D. E. Weston, Proc. Phys. Soc. 76, 233–249 (1960)] and with Gaspin and Shuler's results [J. B. Gaspin and V. K. Shuler, Naval Ordnance Lab TR 71‐160, Oct. 1971] for 1.8 lb of TNT detonated at 60 ft. For the 1.8‐lb sources, the measured results are up to 5 dB different from the levels computed using the Weston model over the band 10–1600 Hz. However, the measured levels are in good agreement with those of Gaspin and Shuler. For the 1.1‐oz source the measured results are up to 20 dB lower than the levels computed using the Weston model. This large reduction in level is likely due to energy lost in breaking through the SUS container.
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Nearfield data from small and large explosions. I. Time‐domain analysis (A)

Jean A. Goertner and Ira M. Blatstein

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S72-S72 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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In a joint sea test of the Naval Research Laboratory, the White Oak Laboratory has obtained nearfield explosion data from 8‐, 48‐, and 1000‐lb TNT charges. Pressure‐time measurements at constant range were made, with multigain recordings at two depths. Recordings were made using both a ceramic hydrophone and a tourmaline piezoelectric gage. The experiment will be described and experimental data presented. These shallow depth‐constant range data validate previous results for underwater explosion similitude expressions for pressure, decay constant, etc., that were largely obtained from data where range and depth effects were intertwined. The data also extends to shallower depths the reduced parameter curves currently employed for predicting underwater explosion effects and for modeling acoustic source levels. Discussion will focus on how comparison of various records are useful for comparing different sensor outputs and for studying nonlinear effects—factors that influence source level determination. [This work was funded by DNA.]
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Nearfield data from small and large explosions II: Spectral analysis (A)

Joel B. Gaspin and Jean A. Goertner

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S72-S72 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The problem of the definition of the spectral source energy levels for the joint NSWC‐NRL experiment, discussed in the previous paper, will be treated. Since surface reflections occur in the pressure records, the removal of surface reflection effects is necessary in order to obtain the source spectra. The spectral modulation pattern due to the surface reflections are different at the two sensor depths. This makes these data well suited for the examination of different techniques for demodulating the spectrum. Both time and frequency domain deconvolution and various smoothing techniques are examined, and their effects on the spectral energy in ⅓‐octave bands are demonstrated. Certain computational difficulties arising in the frequency‐domain deconvolution are avoided by use of time‐domain deconvolution. In situations where the surface reflection is not acceptably coherent, or if cavitation occurs, simple deconvolution is not applicable, and special techniques must be used. The dependence of the source spectra on sensor type is also examined. The source energy levels derived from the 1000‐lb‐charge data are found in good agreement with the scaled‐octave band source level curves of Christian [J. Acoust. Soc. Am. 42, 905(L) (1967)], which were derived from much smaller charges. [This work was funded by DNA.]
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Weak‐shock solution for explosive shock waves (A)

Peter H. Rogers

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S72-S72 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The initial pressure wave measured at modest distances from an underwater explosion is often modeled as a spherical shock wave with an exponential decay. A closed‐form analytical “weak‐shock” solution for the subsequent propagation of such a wave has been obtained. Simple formulas expressing the peak pressure and initial decay constant as function of reduced range are presented. These allow the prediction of the amplitude and initial slope of the wave given only the amplitude and decay constant of the original exponential shock and the density, sound speed and parameter of nonlinearity of the water. The results are in good agreement with the Kirkwood‐Bethe theory, measurements, and the widely used experimentally based semiempirical similarity formulas. An expression which gives a close approximation to the waveform as a function of distance is also derived.
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