The absorption of sound in seawater is considered as the sum of three contributions: those from pure water, magnesium sulfate, and boric acid. Contributions from other reactions are small and are not included. The pure water and magnesium sulfate contributions obtained from analyses of extensive oceanic measurements, including many in the Arctic, were discussed in Part I. In Part II, an analysis is made of all reported measurements in the low-frequency region (0.2–10 kHz) to evaluate the contribution of boric acid. This is done by subtracting the pure water and magnesium sulfate contributions determined in Part I from the total absorption to give a more accurate estimate of the boric acid contribution than previously obtained. The three contributions are then combined to form an equation with both a theoretical basis and a satisfactory empirical fit that will be useful to researchers and engineers in the field of underwater sound. The equation applies to all oceanic conditions and frequencies from 200 Hz to 1 MHz.

Between 10 and 1000 kHz the absorption of sound in sea water can be considered as the sum of contributions from pure water and magnesium sulfate. Near 10 kHz there is also a small contribution from boric acid. In this paper, the contribution from MgSo4 is treated extensively using the authors’ measurements of absorption in the ocean to construct a quantitative equation for absorption as a function of temperature, salinity, and depth. The frequency region below 10 kHz, where the boric acid contribution predominates, will be reviewed later in Part II which includes an equation for the total absorption in the frequency range 400 Hz to 1 MHz.

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Underwater acoustic systems operating in the Arctic are generally used to detect ice or objects in an ice background. In either case, knowledge of the reflections from ice, both undisturbed and ridged, is necessary. For the past few years, the authors have been measuring monostatic reflections near normal incidence from arctic ice to advance this knowledge. In 1988, reflections at 20-80 kHz were measured from the ends of four cylindrical blocks of flat arctic ice with diameters of 27-84 cm. The blocks were individually depressed below the surface so that their reflections were separable from those off the underside of the ice canopy. The transducer was moved horizontally beneath the block to measure the angular response pattern.Examination of the bottom of the block after it was removed showed that the surface was flat except for some roughness with a standard deviation less than 3 mm. Compared with solid ice, the skeletal layer reduced normal-incidence reflection by 8-11 dB. The results were consistent for all four blocks, confirming that the results were valid. Measurements of reflection from the underside of the ice canopy at 15-300 kHz gave similar results and extended the frequency range. Measurements of the physical properties of the ice were helpful in analyzing the nature of the reflection.

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