BUCK: ARCTIC ENVIRONMENTAL LF ACOUSTICS MEASUREMENTS, 

 MODELS AND PLANS 



to it. At 20 Hz, the 200-foot hydrophone is 2 dB better than the 

 one at 100 feet and 7.6 dB better than the 30-footer. However, the 

 100-foot depth almost flattens the noise below 30 Hz where the 200-foot 

 depth does not. 



In most Arctic experiments we use vertical or horizontal seis- 

 mometers, or both, frozen into the ice above the hydrophone. In 

 Figure 21 are plotted relative S/N ratios for a 100-foot hydrophone 

 and a vertical seismometer (A) and a horizontal seismometer for various 

 source depths. Below about 30 to 40 Hz, the horizontal seismometer is 

 superior; however, 3 dB of this is due to the directivity of that 

 unit. The vertical seismometer S/N is 6 to 7 dB below that of the 

 100- foot hydrophone. 



Figure 22 compares an ice island with a floe station as an 

 acoustic platform for hydrophones 200 feet below the ice top surface. 

 In all these figures, the floe station (ARLIS 5) 200-foot hydrophone 

 is used as a reference. The top figure shows the median levels for 

 2 months of hourly readings. The T3 phones were installed through 

 3-inch holes 100 feet through the island in 1968 and, of course, froze 

 in so there was no way to check-calibrate the crystal units themselves. 

 However, the preamps were topside and regularly checked for gain. We 

 have shown the April-May ambients for 1968, 1969, and 1970 as the 

 next best thing to a direct crystal calibration. There is seen to be 

 no drastic change in the 2 years, nor was it expected. The noise at 

 T3 was 5 to 9 dB quieter than at ARLIS 5. How much of this differ- 

 ence was due to geographic location and how much to the different 

 types of ice immediately over the phones, is unknown. 



The second data set of Figure 22 is transmission loss for four differ- 

 ent source depths. Reciprocal paths were used for this comparison and 



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