| Abstrakt: |
Acoustic volume-scattering measurements using 3.5 and 12 kHz pulses and broad-band explosive signals were made from the USNS CHAS H. DAVIS during an oceanographic cruise in the Indian Ocean in May-July 1969. Seven acoustic stations were occupied, three in the Java Trench area and four in the Bay of Bengal. Downward-looking directional transducers were used in the pulse-type measurements. Strong scattering layers (maximum scattering strength, -55 dB to -70 dB re 1 yd) were observed during daytime measurements at 12 kHz. Layer depths at 12 kHz ranged from 325 to 550 yd and layer thickness (at 6 dB down) varied from 80 to 300 yd. Layers were less well defined at 3.5 kHz and maximum scattering strengths in the layer are less than at 12 kHz by from 10 to 30 dB. In general, layers are thicker and extend to greater depths than at 12 kHz. Column strengths obtained from pulse measurements (column lengths 700-900 yd) range from -35 to -50 dB at 12 kHz and from -45 to -65 dB at 3.5 kHz. With two exceptions, column strengths at both 3.5 and 12 kHz are higher at night than for daylight observations. The exceptions, both at 12 kHz, have day column strengths which exceed the night values by 4 and 12 dB, respectively. Results obtained using explosive signals (1.8 lb TNT) and omnidirectional hydrophones cover 13-octave bands from 1 to 6.4 kHz. Overall values of column-strength range from about -75 dB at 1 kHz to -40 dB at 5 or 6 kHz. There is good agreement between the pulse results at 3.5 kHz and those found at 3.2 kHz by the explosive procedure. Our results, from both the pulse and explosive procedures, show fair to good agreement with the extensive column-strength results obtained by Hall [M. Hall, J. Acoust. Soc. Am. 54, 473-477 (1973)] in some of these same Indian Ocean areas. [ABSTRACT FROM AUTHOR] |
