Determination of source depth from the spectra of small explosions observed at long ranges

Abstract

The acoustic pressure versus time signature of an underwater explosive source is a sensitive function of the charge weight and the depth of the explosion. If those parameters are known, then the amplitude of the primary shock pulse and the amplitudes and spacings of the following bubble pulses may be accurately predicted. The power spectrum of the signal is characterized by a scalloped form, which also is determined by the charge weight and depth. Even though the signal observed at long range is distorted due to propagation effects, the scalloped spectral structure is preserved. By using power cepstrum techniques, it is possible to determine the characteristic period of the spectrum. From the relation between the spectral period and the bubble pulse period, the exact detonation depth may be determined from a received shot signal if the charge weight is known. An example of such a measurement is presented in this paper. A series of signals from 1.8 lb Mk 61 SUS charges detonated at nominal depths of 60 ft was examined. The bubble pulse period of each source was determined from recordings made onboard the ship from which the shots were deployed. Shots were received at ranges of 250–310 NM at one location and at ranges of 625–700 NM at another location. The depths, as estimated at the remote receiver, were distributed about the onboard measurements with standard deviations of 0.49 ft at the shorter ranges and 0.52 ft at the longer ranges. Subject Classification: [43]30.75.