Page 22 - \Winter 2015

David L. Bradley
Postal:
Applied Research Laboratory The Pennsylvania State University State College, Pennsylvania 16804
USA
Email:
dlb25@psu.edu
Stephen M. Nichols
Postal:
Applied Research Laboratory The Pennsylvania State University State College, Pennsylvania 16804
USA
Email:
nichols@psu.edu
Worldwide Low-Frequency Ambient Noise
Hydrophone stations deployed for ensuring compliance to the Nuclear-Test-Ban Treaty offer a tremendous tool for monitoring and understanding the underwater acoustic environment.
Introduction
The United Nations Comprehensive Nuclear-Test-Ban Treaty Organization (CT- BTO) has a broad spectrum of sensors to monitor the earth for nuclear explosions. Included in that sensor suite are underwater hydrophone systems that have col- lected and stored acoustic underwater ambient noise. This article discusses that data, its value to the research community, and some potential uses.
Discussion of low frequency ambient noise in the ocean inevitably results in a long list of “sub-topics,” which is NOT the intent of this article. The focus is on three subjects: the change of ambient noise with time, the use of it to monitor meteo- rological conditions in remote locations; and the introduction of a technique to better define the source properties of ambient noise.
At very low frequencies (e.g., less than 100 Hz), the physical conditions in the ocean allow for very efficient sound propagation. Briefly, sufficient energy from the source must make its way to the depth of the minimum sound speed, referred to as the sound channel axis, which, as illustrated in Figure 1 has some variation in each major ocean basin and also varies with latitude in all basins, ranging from the nominal depth values indicated in Table 1, to at, or near, the ocean surface at polar latitudes.
Figure 2 illustrates the fact that acoustic signals, if they reach the sound channel axis at angles of the order 0-15 degrees with respect to the horizontal, will travel without encountering the ocean boundaries, and at low frequencies with low en- ergy losses, have the potential to travel long distances and still retain enough signal strength to be detected by hydrophones in the sound channel. This characteristic provides the potential to monitor the world’s oceans with a few sensors strategi- cally located geographically, and placed at the sound channel axis. This is exactly what the UN’s CTBTO (Auer and Prior, 2014) has done to assure international compliance with the Comprehensive Nuclear-Test Ban Treaty. Three of the hydro- acoustic systems of the International Monitoring System (IMS), and the data col- lected by those sites are the subject of this discussion.
Table 1. Locations and depths of the three hydrophone arrays used
in this discussion
ID Location
HA08 Diego Garcia HA10 Ascension Island HA11 Wake Island
Latitude
7° S
8° S 19° N
Longitude
72° E 14° W 166° E
Depth (m)
1300 850 740
20 | Acoustics Today | Winter 2015, volume 11, issue 1 ©2015 Acoustical Society of America. All rights reserved.