Reassessing the Issue of Pulse-Length Dependence of Bottom Scattering Strengthsin Critical Sea Test Data

摘要

During the course of the Critical Sea Test (CST) program, a substantial number ofmeasurements have been made of low frequency, direct path bottom scattering using both SUS charges and controlled waveforms. An unexpected pulselength dependence (PLD) of low frequency, bottom scattering strengths (BSS) was reported to have been observed in some coherent source data collected in several CST-5 and CST-7 Phase-3 locations. This dependence was basically independent of grazing angle and almost exclusively seen at frequencies between 875 and 1010 Hz. (Different signals were used across the tests. In a given run, the two signals with different pulse lengths also had different center frequencies.) However, such a dependence of scattering strength on pulse length contradicts basic theory, barring an unlikely sharp resonance in the dominant bottom scattering mechanism. A review of the original processing and results, as well as independent reprocessing of selected CST-5 data revealed the reported PLDs to be due primarily to two effects: (1) a misapplication of intended source levels into the sonar equation based programs that compute scattering strengths (a 4 dB, time independent error); and (2) an error in how time was converted to grazing angle in these programs (up to 2 dB, mildly time dependent error). These adjustments are generally consistent with the observed grazing angle independence of the BSS's. Additionally, differences were found between the original and reprocessed absolute reverberation levels and the range of valid times (grazing-angle coverage). The bottom line is that pulse length dependence of bottom scattering strengths is no longer an issue.