TRANSARCTIC ACOUSTIC PROPAGATION

Sea Ice Mechanics Initiative

Principal Investigators

Prof. A.B. Baggeroer, MIT Dept. Ocean Engineering
Prof. Henrik Schmidt , MIT Dept. Ocean Engineering

Students

Brian Tracey

Collaborating Institutions

Woods Hole Oceanographic Institution Science Applications International
Andreiev Institute, Moscow
AcoustInform Ltd. Moscow

Objectives

The objective of the Transarctic Acoustic Propagation (TAP) experiment was to test the feasibility of using long-range, low frequency acoustic thermometry for climate study work in the Arctic. Since trans-basin propagation had never been attempted before in the Arctic there were several fundamental issues that needed to be resolved; (1) how much propagation loss would there be at 20 Hz, and would there be enough useful received energy, (2) would the continuous wave (CW) and maximal length sequences (MLS) remain coherent enough to achieve sufficient narrowband processing gain, and pulse compression gain respectively, and (3) would there be a repeatable and predictable mode/ray arrival structure? All of these issues were resolved successfully in the affirmative. In addition the measured modal arrival times indicate faster propagation speeds than predicted using historical climatology. The reason for this discrepancy could be warmer water in the Arctic as reported during recent icebreaker CTD measurements in the Arctic. This will be one of the important issues of the on-going research.

TURPAN Camp Source

Ice camp Turpan was established on April 9, 1994 at 83x 26~ N and 27x 10~ E by a joint U.S. and Russian team. An acoustic source built by the Institute of Applied Physics in Nizhny Novgorod for this experiment was deployed at Turpan. The acoustic source was based on a well tested electromagnetic design. The source level transmitted was 195 dB re 1 fPa or 250 watts acoustic. Acoustic transmissions were started at 0900Z on April 17 after a day of testing and continued through 1400Z on April 22. Forty-three (31 CW and 12 MLS) transmissions centered at 19.6 Hz each of one hour duration (except for two half-hour transmissions at 17.6 Hz and 21.6 Hz) were successfully completed. Receiving acoustic arrays were located in the Lincoln Sea at ice camp Narwhal, and in the Beaufort Sea at ice camp SIMI (Sea Ice Mechanics Initiative) (see following section). A 19 element vertical array was deployed at Narwhal with the elements spaced at approximately every 30m. The locations of the source- and receiver camps are shown in the map above.

TURPAN transmissions.

All transmissions on "regular schedule" except where noted. Start times shown, one hour transmission.

Date - 0900z, 1100z, 1300z, 1500z, 1700z (day schedule)
1900z, 2200z, 0100z, 0400z, 0700z (night schedule)

4/15 - Source testing in air and @ 6m depth
4/16 - Source testing at 6m, 20m, 30m, 40m, 50m, 60m depth
1930-2030z 19.60746Hz @ 190 dB
4/17 - CW, CW, M255(note 1), CW, M511(note 1)
CW, CW, CW, CW, CW

All M-sequences on and after 4/18 are 12.5 cycles/digit

4/18 - CW(note 2), CW(note 2), M255

- Secured transmissions -

4/19 - 1500z-1630z intermittent source testing
1630z-1700z CW @ 21.6 Hz
1900z-2000z CW @ 19.6 Hz
2040z-2100z CW @ 17.6 Hz
2200z-2300z M255

CW(note 3), M255, CW (regular night schedule starting @0100z)

4/20 - CW, CW, M255, CW, M511(note 4)
CW, CW(note 5), CW, CW(note 7), CW
4/21 - M127, CW, M1023, CW(note 6), M511(note 8)
CW, CW, CW, CW, CW
4/22 - M127, CW, M255(note 9)

- End of experiment -

Notes: 1: 25 cycles/digit +/-pi/4 modulation
2: generator (and rubidium clock) turned off after transmission
3: this CW and all following CW's @ 19.6Hz
4: this sequence had problems during transmission, source
unable to make phase transitions cleanly
5: CW signal went off and then on at ~2226z
6: tone went off and on again twice around 1514z
7: signal went off @ 0458z erratic until 0500z
8: pi/2 phase shifts observed ~1730z
9: waveform distorted possible mechanical problem

SIMI Camp Receiving Arrays

Seismo-Acoustic arrays deployed by MIT/WHOI on and around the East Camp floe. A total of 32 hydrophones were deployed in the horizontal array in a combined circular/cross configuration. A 32 element vertical array was deployed at the horizontal array apex. Two J-9 acoustic sources (J9A and J9B) were deployed for sensor tracking.

With the primary objective of monitoring and localizing ice activity during the Spring SIMI field experiment, MIT/WHOI deployed 2 hydrophone arrays, approximately 300m from the main East Camp area, both of which were also used for recording the TAP transmissions in the period Apr.17--22, 1995. A vertical line array (VLA) of hydrophones, having 32 sensors linearly spaced from a depth 62 to 279 meters was deployed at the APEX site. Surrounding this APEX was a horizontal array consisting of a 12 hydrophone circular deployment augmented by 12 additional sensors on nominal north-south and east-west legs 240m in length. Prior to the Transarctic Acoustic Propagation receptions, 8 more hydrophones were deployed for a total of 32 in the horizontal array (HA), all at a depth of 60 m. With the exception of breaks for generator maintenance, data were acquired and stored continuously on 8mm tape (Exabyte), from a set of these sensors according to the table below.

DATE(Apr) #data ch BW(Hz) time/tape tape #s QTY data

4 - 13 24 HA 300 26 hr 1,3 - 14 42 GB
14 32 HA,16 VLA 300 10 hr 15 3.5
15 32 HA,18 VLA 300 10 hr 16 3.5
16 - 18 32 HA,25 VLA 300 10 hr 17 - 22 23
18 - 23 32 HA,32 VLA 300 10 hr 23 - 33 45
23 32 HA 2700 2.5 hr 34 - 37 16

Two USRD J9 acoustic sources were deployed and each driven with a dual tone signal for use in localizing the VLA channels. Consequently, during the period 15-23 April, tonals in the 64 to 76 Hz region are evident.