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CEAREX Noise Data

The CEAREX noise data include acoustic noise measurements made from a large horizontal hydrophone array, multiple vertical hydrophone arrays, and geophone arrays, and ambient noise observations made from the POLARBJORN using omni-directional hydrophones tethered beneath the ice cover.

Information in this document has been derived from documentation files on NSIDC's CD-ROM titled Eastern Arctic Ice, Ocean and Atmosphere Data, Volume 1: CEAREX-1, version 1.0, 8/91.


Contents

Data Description
Data Acquisition and Equipment
Data Organization and Format - Notes
Investigators | References | Acronyms
CD-ROM Documentation Files: Acoustic Data - Ambient Data

Data Description:

Acoustic data descriptions are included in the following papers and reports that resulted from analysis of the acoustic data: Fricke (1991), Gerstoft and Schmidt (1991), Miller and Schmidt (1991), Peal (1990), and Seong (1991.) The ambient noise measurements are described in Pritchard (1989).

Data Acquisition and Equipment:

Acoustic Noise Data

Direct acoustic measurements were made at the CEAREX Acoustics Camp (A-Camp) using a large horizontal hydrophone array, multiple vertical hydrophone arrays and geophone arrays.

Acoustic data were recorded by a multichannel digital acquisition system, capable of greater than 120 dB dynamic range, developed at Woods Hole Oceanographic Institution (WHOI) and described in von der Heydt (1991). The MIT/WHOI digital data set consisted of 40 channels devoted to hydrophone sensors of the horizontal array and 12 channels from four 3-axis geophones. Channels were selectively recorded on a 14-channel FM tape recorder. All data included on the CEAREX CD-ROM were acquired on the digital system.

In support of coherent processing techniques for the low frequency data (1 to 250 Hz) recorded from the hydrophone arrays, an independent system operating in the 10 kHz region was used to continuously measure delays between six tone burst sources and the array sensors, all at a common depth. These travel times were later used to estimate relative locations of sensors. Nominal location estimates are tabulated as supporting data on the CEAREX CD-ROM.

Ambient Noise Data

Ambient noise was measured using omni-directional hydrophones tethered beneath the ice cover, (Pritchard 1989). 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 September 27; nearly continuous measurements began October 10, and observations ended November 18 near 81 degrees north 32 minutes east when severe ice deformations made it impossible to maintain instrumentation cable continuity. A PC-based LOFAR system measured the hydrophone output (Prada 1986).

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 entered 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 entered about 50 percent of time and FFT calculations required about 50 percent 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 enter 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 on the CD-ROM. 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 (1990). The hydrophone indicated by H2 was tethered at 60 m depth. It was located near the upper left corner of the same photograph. Please refer to the table in the CD-ROM documentation file that shows when data were recorded at each site.

Data Organization:

The noise data are presented in four data files in two subdirectories on the CEAREX CD-ROM. One subdirectory contains acoustic noise data and one holds ambient noise data.

Data Format:

Acoustic Noise Data

Each file begins with an ASCII header of 512 bytes with information concerning the experiment, the number of sensors, and the number of data values in the file. A data stream for the first sensor follows this header. The stream end is indicated by a "new line" character. Second sensor data follows in the same way, with the sequence repeated for all sensors. The last record in the file may be padded to 512 characters with blanks. All data were sampled at 1 kHz. A listing of sensor locations relative to the nominal origin at sensor 9 is given in the description of file 3.

The FORTRAN data format is E8.6; the C language format is 13.6e.

Ambient Noise Data File

This file contains 700416 data values (700.416 seconds) from each of four sensors. These data were acquired as an ambient noise experiment during a particularly windy day in which the wind speed exceeded twenty knots. The initial start time of the data is 1534 GMT, April 11. In order, the sensors recorded on the file are NE320, NW7000, G4Y, and G4Z. The first two are hydrophones with a sensitivity of -160 dB re: 1 volt per micropascal. The first hydrophone was "hard wired" to the acquisition equipment at base camp. The second hydrophone, NW7000, communicated with base camp via a radio link with 10dB of gain. A gain normalization was applied to the data from this sensor to make the hydrophone gains consistent. The last two sensors, G4Y and G4Z, include the horizontal and the vertical component, respectively, of geophone G4.

Total file size is 39223813 bytes, consisting of a 512-character header, four groups of 700416 values each (14 characters per value), a new line (0D hexadecimal) following each group and an additional new line at the end of the file. For the CD-ROM, the files were converted into 512-character records.

For cross-referencing with original data held by the investigators, this file contains data copied from WHOI optical disk CRX22.dat. Seventy-two records starting at record 157 are included for channels 24, 36, 50, and 51. Channel 36 is scaled by 0.31623 to normalize for preamp gain, as described above.

Plate Wave Experiment Data Files

This file contains 450001 values (450.001 seconds) from each of six sensors: NW20, APEX, NE40, G1X, G1Y, and G1Z. The data were acquired during a "Plate Wave Experiment" in which primer cord and SUS explosives were set off near the A-Camp. The data were collected at 1631 GMT, April 16, 1989.

Three shot events are recorded in this file: a primer cord shot at 32 feet depth; a second one at 64 feet depth; and finally a SUS shot at 800 feet. The first three sensors in the file are ordinary hardwired hydrophones with sensitivity of -160 dB re: 1V/uPa. The latter three include both horizontal components and the vertical component of geophone G1.

Total file size is 37800603 bytes. This consists of a 512-character header, four groups of 450001 values each (14 characters per value), a new line following each group and an additional new line at the end of the file.

For cross-referencing with original data held by the investigators, this file contains data copied from WHOI optical disk CRX52.dat. Fifty-four records starting at record 556 are included for channels 5, 9, 11, 40, 41 and 42.

Remote Air Guns/Ambient Noise Data File

This file contains 55001 data values from 50 sensors, collected at 0848 GMT, April 17, 1989. Because of the large number of sensors, only 55 seconds of data are included. During this interval, an air gun was fired at 48-minute intervals from the Oceanography Camp (O-Camp), approximately 300 km to the northeast. With signal processing, it is possible to detect these signals. Otherwise, the data can also serve as a sample of ambient noise on a fairly quiet day in the Arctic.

The first 38 channels are hydrophones with a sensitivity of -160 dB re: one volt per micropascal. The last twelve channels are geophones. The last eight hydrophones on the file were radio-linked phones with an extra 10 dB of gain, for which no normalization is made.

A list of "X-Y" sensor position estimates is available in the CD-ROM documentation file. The estimates are in meters relative to a Y-axis baseline between the apex sensor, channel 9 and channel 7.

Total file size is 38501263 bytes. This consists of a 512-character header, 50 groups of 55001 values each (14 characters per value), a new line following each group and an additional new line at the end of the file.

For cross-referencing with original data held by the investigators, this file contains data copied from WHOI optical disk CRX58.dat. Six records starting at record 26 are included. Because of excessive noise, channel 16 was deleted.

Ambient Noise Data

The ambient noise data can be read from the archive using the FORTRAN statements presented in the CD-ROM documentation file.

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, seven-minute average FFTs were given, in contrast with the one- to three-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.

Notes:

For a discussion of the motivation for the CEAREX acoustic noise work, please refer to the CD-ROM documentation file.

For a discussion of the results of the CEAREX ambient noise work, please refer to the CD-ROM documentation file.

Investigators:

Arthur M. Baggeroer, Massachusetts Institute of Technology - acoustic noise data
Robert S. Pritchard, IceCasting, Inc. - ambient noise data
Keith von der Heydt, Woods Hole Oceanographic Institution - acoustic noise data

For a complete list of all CEAREX investigators, please refer to the CEAREX Investigator Address List.

References:

Cousins, J. D. 1991. CEAREX Ambient Noise Data Measured Northeast of Svalbard. Thesis, Naval Postgraduate School, Monterey, CA.

Fricke, R. 1991. Finite difference solution for acoustic scattering from ice keels and rough ice. Ph.D. diss., MIT-WHOI Joint Program.

Gerstoft, P., and H. Schmidt. 1991. Boundary element approach to ocean seismo-acoustic facet reverberation. Journal of the Acoustical Society of America 89(4):1629-1642.

Miller, B., and H. Schmidt. 1991. Observation and inversion of seismo-acoustic waves in a complex arctic ice environment. Journal of the Acoustical Society of America 89(4): 1668-1685.

Peal, K. R. 1990. Arctic Remote Autonomous Measurement Platform - Post CEAREX Engineering Report. Woods Hole Oceanographic Institution. Technical Report no. 90-46.

Prada, K. E. 1986. Real time ambient noise spectra acquisition and display. In Fourth Working Symposium on Oceanographic Data Systems. Proceedings, 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):2830-2842.

Pritchard, R. S. 1989. Eastern Arctic ambient noise. In Oceans '89 (4):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):1115-1118.

Seong, W. 1991. Hybrid Galerkin boundary element wavenumber integration method for acoustic propagation in laterally inhomogenous media. Ph.D. diss., Department of Oceanographic Engineering, Massachusetts Institute of Technology.

von der Heydt, K. 1991. High Continuous Bandwidth Multichannel Acquisition System. Woods Hole Oceanographic Institution. Technical Report no. 91-14.

von der Heydt, K., N. R. Galbraith, A. B. Baggeroer, R. Meunch, P. S. Guest, K. L. Davidson. 1991. CEAREX "A" - Camp: Navigation, Bathymetry, CTD, Meteorology, and LOFAR Data Report. Woods Hole Oceanographic Institution. Technical Memorandum no. WHOI-1-91, 152 p.

Acronyms:

A/D - analog/digital
ASCII - American Standard for Computer Information Interchange
CD-ROM - Compact Disc-Read Only Memory
CEAREX - Coordinated Eastern Arctic Experiment
CTD - conductivity-temperature-depth
dB - decibel
FFT - fast Fourier transform
FM - frequency modulation
Hz - hertz
kHz - kilohertz
km - kilometer
LOFAR - not defined
Mb - megabyte
MIT - Massachusetts Institute of Technology
NSIDC - National Snow and Ice Data Center
RC - resistance capacitance
SUS - not defined
VGA - video graphics array
V/uPa - not defined
WHOI - Woods Hole Oceanographic Institution