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CEAREX Drift Experiment Ambient Noise Observations Robert S. Pritchard IceCasting, Inc. 11042 Sand Point Way NE Seattle, WA 98125-5846 USA 1. Introduction CEAREX Drift Operations used the ship POLARBJORN as a scientific base. The ship steamed into the polar pack ice during early September 1988 with icebreaker support from the USCG NORTHWIND. POLARBJORN was allowed to freeze into the ice on 16 September at 82 degrees 41 minutes N, 32 degrees 26 minutes E. The ship drifted southeastward toward Viktoriya Island, then southwestward past Kvitoya Island, and finally into the Barents Sea. All drift operations were completed by mid-January 1989. The drift experiment was designed to observe simultaneous atmosphere, ice, and ocean behavior, including wind, air temperature, current, large scale ice motion, local ice motion, ice stress, and under-ice noise ["CEAREX drift experiment", Pritchard, R.S. and twenty-eight others, EOS, 71(40), 1990, p. 1115-1118]. Ambient noise was measured using omni-directional hydrophones tethered beneath the ice cover ["Eastern Arctic ambient noise", Pritchard, R.S. (1989) In: Oceans '89, vol. 4, p. 1246-1251. IEEE Pub. no. 89CH2780-5]. Two omni-directional, calibrated hydrophones were deployed at 60 and 90 meters beneath the pack ice at two sites roughly 1 km away from the ship and from each other. A few ambient noise observations began 27 September, nearly continuous measurements began 10 October, and observations ended on 18 November near 81 degrees N 32 minutes E when severe ice deformations made it impossible to maintain instrumentation cable continuity. A PC-based LOFAR system measured the hydrophone output ["Real time ambient noise spectra acquisition and display", Prada, K.E. (1986) In: 4th Working Symposium on Oceanographic Data Systems. Proceedings, pp.199-207. Sponsored by IEEE Computer Society and Scripps Institution of Oceanography.] 2. Data Collection Methods Shielded instrumentation cable carried amplified hydrophone output into a Frequency Devices low-pass Butterworth filter with adjustable corner frequency (Model 901). Data were filtered by an RC circuit to minimize signals below 1 Hz. The acquisition system was a Compaq 386/20 computer on the ship. Data were input through a Metrabyte A/D interface board (Model DAS-16G). Computer peripherals included VGA monitor, 80387 numeric co-processor, 130 Mb hard disk and 135 Mb tape cartridge backup, and NEC color printer. The LOFAR computer program provided interactive control of data acquisition. Digitized input signals were sampled at the chosen maximum frequency. The anti-aliasing filter corner frequency was set between half and two-thirds of the Nyquist frequency. After collecting 1024 data points, an FFT was performed, and the power spectral density estimated. One hundred twenty-eight individual spectral densities were averaged, and the averages were recorded. Narrow-band harmonics were removed, data were calibrated, and the resulting power spectral densities collected into one-third octave bands. The first output bin was discarded from all spectra because of possible contamination by DC and other low frequency signals. For a frequency bandwidth of 512 Hz, data were input about 50% of time and FFT calculations required about 50% of time. For these parameters, an average spectral estimate was obtained at roughly 2-minute intervals. For all frequency bandwidths, the number of data points in the FFT was held constant at 1024. The time required to input data decreased inversely with bandwidth. The average spectral estimate was obtained at roughly 1-minute intervals at a bandwidth greater than 2000 Hz. Since 1024 data values were used in each FFT, resolution decreased as bandwidth increased (for a maximum frequency of 512 Hz, frequency bins were 1 Hz, but for a frequency maximum of 8192 Hz, frequency bins were 16 Hz). The system 'noise floor' was estimated by removing the hydrophone from the system and connecting all cables at the hydrophone location. The system contained the shielded instrumentation cable, anti-aliasing filter, A/D interface board, and acquisition computer. The noise floor was about 72 dB re 1 microPa**2/Hz across the spectrum, a threshold that remained rather steady throughout the experiment. This noise floor probably resulted from the 12-bit A/D interface board, which limited the dynamic range to about 39 dB. All observed values are included here. The user must eliminate those values contaminated by the noise floor. Data were recorded from only one hydrophone at a time. The hydrophone indicated by H1 was tethered at 90 m depth. Its location appears on the cover photograph presented by Pritchard and twenty-eight others . The hydrophone indicated by H2 was tethered at 60 m depth. It was located near the upper left corner of the same photograph. The following table shows when data were recorded at each site. Hydrophone Time when data began on hydrophone (UT) H1 10/08/88 1452 UT H2 10/20/88 2101 H1 10/21/88 0817 H2 10/21/88 0952 H1 10/21/88 1139 H2 10/21/88 1420 H1 10/21/88 2208 H2 10/22/88 0727 H1 10/22/88 1008 H2 10/22/88 1432 H1 10/22/88 1900 H2 10/22/88 2246 H1 10/23/88 0718 H2 10/23/88 1119 H1 10/23/88 1614 H2 10/24/00 0922 H1 10/24/88 1421 H2 10/24/88 2106 H1 10/25/88 0903 H2 10/25/88 1657 H1 10/25/88 2038 H2 10/26/88 0855 H1 10/26/88 1435 H2 10/26/88 1720 H1 10/27/88 0655 H2 10/27/88 1330 H1 10/27/88 1947 H2 10/28/88 0839 H1 10/28/88 1310 H2 10/28/88 2311 H1 10/29/88 1718 H2 10/30/88 0958 H1 10/31/88 0411 H2 11/01/88 1017 H1 11/01/88 1706 H2 11/02/88 2258 None 11/03/88 1655 No hydrophone, noise floor tests H2 11/05/88 0213 3. Data Format Data are in the file AMBIENT.DAT in the directory \NOISE\AMBIENT on the CD-ROM "CEAREX-1". The data can be read from the archive using the following set of FORTRAN statements: PROGRAM ICINOISE DIMENSION FREQ(28), DATA(28) CHARACTER*80 FILENAME WRITE(*,*)' Enter name of ambient noise data file' READ(*,800) FILENAME 800 FORMAT( A80) OPEN(UNIT=1,FILE=FILENAME,FORM='FORMATTED') READ(1,910) READ(1,910) (FREQ(I),I=1,28) 910 FORMAT( 10X,28F10.2) c loop on time 1000 CONTINUE READ(1,920,END=2000) TIME, (DATA(I),I=1,28) 920 FORMAT( F10.4,28E10.4) c c do something with the data here c c end time loop GO TO 1000 2000 CONTINUE c c STOP END Noise intensity is presented for one-third octave bands from 2 Hz to 1000 Hz. Units are microPascal**2/Hz. Average values over each one-third octave band have been divided by bandwidth and therefore describe spectrum level. Missing values are indicated by data values of 1.E-9. Time (Greenwich Mean Time) is presented in consecutive days of the year (decimal fractions), where 1 January 1988 at 0000 UT is TIME = 1.0000. Prior to day 272.5219, eight FFTs were averaged, giving roughly 7-second averages, in contrast to the 1-3 minute averages calculated later. The times of these closely-spaced data cannot be resolved by the resolution of 1.E-4 day, with the result that several consecutive records have identical times. This limitation occurs only for the first 988 data points. 3. Results Ambient noise data were energetic at frequencies below 500 Hz at the outset, but the frequency band increased to over 5000 Hz as nearby ice ridged and rubbled. Data at frequencies above 1000 Hz were contaminated by the noise floor, and have not been archived. A visual comparison between the ambient noise and environmental variables showed that correlations exist. Noise increased during high winds, deformations, and when stress levels increased in the ice. A strong semi-diurnal tidal and/or inertial signal was also observed. The concurrent wind, air temperature, current, large scale ice motion, local ice motion, ice stress, and under-ice noise data should provide opportunities to identify and describe noise generated by different processes. Two different types of analyses have been pursued: statistical analysis of the data, and modeling. Cousins [CEAREX Ambient Noise Data Measured Northeast of Svalbard. Naval Postgraduate School thesis, 1991] has estimated the 5, 50, and 95 percent noise levels, and the correlations between noise and local environmental variables. Pritchard [Sea ice noise-generating processes. Journal of the Acoustical Society of America, 88(6), pp.2830-2842, 1990] has developed a model that accounts for local and distant noise sources generated by pressure and shear ridging, microcracking, and mixed layer shearing. 4. References Cousins, J. D. (1991) CEAREX Ambient Noise Data Measured Northeast of Svalbard. Monterey, CA, Naval Postgraduate School, Thesis. Prada, K. E. (1986) Real time ambient noise spectra acquisition and display. In: Fourth Working Symposium on Oceanographic Data Systems. Proceedings, pp.199-207. Sponsored by IEEE Computer Society and Scripps Institution of Oceanography, University of California, San Diego, CA. IEEE Catalog No. 86CH2269-9. Pritchard, R. S. (1990) Sea ice noise-generating processes. Journal of the Acoustical Society of America, 88(6), pp. 2830-2842. Pritchard, R. S. (1989) Eastern Arctic ambient noise. In: Oceans '89, vol.4, p. 1246-1251. New York, IEEE. IEEE Pub. No.89CH2780-5. Pritchard, R.S. and twenty-eight others. (1990) CEAREX Drift Experiment, EOS, Transactions of the American Geophysical Union, 71(40), (October 2), pp.1115-1118. 6. Contact Information Robert S. Pritchard IceCasting, Inc. 11042 Sand Point Way NE Seattle, WA 98125-5846 (206) 363-3394 Telemail: R.PRITCHARD/Omnet 7. Acknowledgments Acquisition of this ambient noise data set required the co-operation and help of many individuals, and I am grateful to all. A.B. Baggeroer (MIT), K.E. Prada and K. von der Heydt (WHOI) identified necessary hardware, and provided hydrophones and LOFAR software. R.H. Bourke (NPS) provided the data acquisition computer. T. Stanton (NPS) guided me in designing, assembling, and testing the data acquisition system. M.D. Coon (BDM Corp.), Chief Scientist of the first leg of the drift experiment, helped acquire and deploy the equipment. A. Heiberg (UW) provided logistical support to the entire expedition. J. Ardai and A. Gill provided on-ice help and expertise that eased the workload. T.A. Jakobsen and crew of the POLARBJORN provided an efficient and pleasant ship that made it possible to concentrate on the scientific aspects of the expedition. The entire scientific team worked diligently throughout the 10-week drift. They showed remarkable resilience and good humor, even during breakup, when all our plans changed without notice. This work was funded by the Office of Naval Research under contract N00014-88-C-0580 to IceCasting, Inc. I thank T.B. Curtin, Manager of the Arctic Sciences Program, for his encouragement and support. February 1991 (revised April 1991)