Model analysis of energy spreading loss off the Carolina Coast for tactical active sonars

Abstract

Energy spreading loss (ESL) is the reduction of the transmitted pulse energy level by spreading of the pulse in time due to multipath propagation. This energy spreading will reduce the effectiveness of mid-frequency tactical sonars. The U.S. Navy training areas of Long Bay and Onslow Bay off the Carolina Coast were chosen for the study of ESL to provide contrasts in many of the geoacoustic properties that can change ESL. Inputs were varied by source depth, receiver depth, sound speed profile(SSP), bathymetry, and geoacoustic properties. The computer model FEPE_SYN calculated the ocean transfer function (OTF) for the modeled environment in the frequency domain. The time domain output pulse was calculated using the OTF, an input pulse, and an inverse discrete Fourier transform. Using the same energy as the output pulse, a compressed pulse was created with the same shape as the input pulse. ESL was determined by comparing the peak level of the output pulse to the peak level of the compressed pulse. A mismatch loss (MML) was calculated by comparing the maximum values from the correlation of the input pulse with the output pulse and compressed pulse. The ESL of the output pulse was dependent on several factors. Absorptive (silt/clay) sediment sea beds had average ESL values 3 dB less than that of compacted sand. The compacted sand bottom was also compared to an even more reflective sediment, a limestone sediment layer. ESL values were higher by an additional 3 dB for the limestone bottom. Minimum ESL levels were found when the source and target were at the same depth. Changing source and target depths (e.g., cross layer) could increase ESL levels up to 8 dB from the minimum ESL level. The impact of using a range-dependent SSP vice constant SSP was inconclusive in that ESL values could be larger or smaller by 3 dB compared to range-independent runs. Similar inconclusive results were obtained when actual bottom depths were employed vice a flat-bottom run.