Source levels of shallow explosive charges

Abstract

Experimental measurements are reported in this article for the source levels of 0.82-kg signals underwater sound (SUS) charges detonated at nominal depths of 18.3, 91, and 183 m. The data were recorded in the farfield using a high-resolution digital data acquisition system that provided good reproduction of both the high- and low-frequency components of the signal from the explosion. The results are presented in 1/3 octave bands from 5–630 Hz, and represent the mean values of samples of at least 30 charges for each depth group. The standard deviations were within 1 dB for most frequency bands. Analysis of the distribution of explosion depths indicated that the average detonation depths were consistently deeper than the nominal depths. This result was attributed to the effect of the relatively cold water temperatures at the experimental site. In addition to the measured source levels, estimates for 0.82-kg SUS charges at 244 m were scaled from the experimental data using a method suggested by Hughes [R. Hughes, ‘‘Underwater Explosives:Scaling of Source Spectra,’’ in Underwater Acoustics and Signal Processing, edited by Leif Bjorno (Reidel, Dordrecht, The Netherlands, 1981), pp. 87–91]. Comparisons were made with the values for 0.82-kg SUS source levels provided by Gaspin and Shuler [J.B. Gaspin and V.K. Shuler, ‘‘Source levels of shallow underwater explosives,’’ Naval Ordnance Laboratory NOLTR 67–87 (1971)]. Their predictions are generally higher at high frequencies owing to the relatively high value of the shock wave decay constant that they used to generate their waveforms. At low frequencies, the Gaspin and Shuler predictions were not in good agreement with the scaled values since their analysis did not specifically account for the impulse of the waveform.