Discussion on the physical significance of the acoustic target strength of an underwater object in shallow water and modal-filtering measurement

The acoustic target strength (TS) of an underwater platform is usually tested in shallow water. Utilizing normal mode theory, the analytical formula of the TS of an underwater object in shallow water is derived, based on the average-measurement method, and its physical significance is analyzed deeply. Analysis shows that, as for as a certain azimuth, TS is dependent on the target scattering function values at a few vertical incident angles, θm, and scattering angles, θn. θm and θn are discrete, their values depending on normal-mode order, m, n. Furthermore, TS is concerned with both the marine environment and the testing range, and it is approximately equal to the free-field target strength, TSfree, when the testing range is large enough. A new TS measurement method put forward, namely Modal-Filtering Measurement (MFM), based on a vertical source array and a vertical hydrophone array. According to theoretical analysis, MFM method performs well in both interface reverberation resistance and testing efficiency, compared with the average-measurement method and the classical vertical array method, its result is closer to TSfree. ©, 2015, China Ship Scientific Research Center. All right reserved.