Peter G. Cable
William J. Marshall
BBN Systems and Technol., Union Station, New London, CT 06320-6147
Broadband underwater acoustic measurements are often performed using explosive charges as sources. For moderate weight charges detonated at shallow depths the bubble pulse oscillation period can exceed the separation between direct and surface reflected arrivals for any source-to-receiver geometry. Because of the overlap of multipath arrivals with the time signature of the charge, a direct determination of free-field source levels and spectral characteristics for such a situation can be problematical. In order to perform acoustic calibrations of explosive charges in shallow water, a method has been developed that uses the multiple arrivals on a vertical receiving array of hydrophones separated from the source detonation by one or two water depths. The technique involves averaging the time series from the vertical array phones after time aligning the direct shock arrivals on all the phones. This focused beamforming method has been used to obtain source calibrations from data obtained during area characterization test II (ACT II) in water 50 fathoms deep using 4 lb Comp B charges detonated at 25 fathoms. High levels of consistency and repeatability were obtained from several sources detonated at different ranges from the array. The calibration technique will be described and illustrated, and the achievable precision of the method discussed. [Work supported by ARPA under the Adverse Environments Program.]