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Figure 1. Sound speed profiles for the three hydrophone stations, re- trieved from the World Ocean Atlas (NOAA, 2001). The depth of the sound channel, located at the sound speed minimum, is marked in red.
The ambient noise data discussed were obtained from the hydroacoustic stations located at Diego Garcia (Chagos Ar- chipelago), Ascension Island, and Wake Island, shown in Figure 3 and described more precisely in Table 1 (Lawrence, 2004). Each of these stations consists of a pair of triangular hydrophone arrays (three hydrophones) at the sound chan- nel axis, situated on opposite sides of a host island. This ar- rangement helps to limit the area of detection due to the acoustic shadow zone produced by the host island and also provides some degree of directionality of the sound field. Each hydrophone is equipped with a preamplifier/filter and a 250-Hz digitizer. Buried fiber optic cables carry the digi- tized ambient noise signals from the moored hydrophones to a station on the surface of the host island, where they are transmitted via satellite to the CTBTO headquarters in Vienna for monitoring (Auer and Prior, 2014). The data is also available from the host country (of the site) via con- tact with the appropriate government agency. While these stations were built with the primary purpose of monitoring the world’s oceans for unsanctioned nuclear weapons test- ing, the nearly continuous recordings produced by this net- work, spanning over 10 years in duration at some locations (data logs received from CTBTO), constitute one of the best collections of low-frequency ambient noise data currently available.
The Data: Details and Some Characteristics and Comparisons
with Other Ambient Noise Data
The requirements of the CTBTO monitoring system limit the acoustic bandwidth of the data recorded to range from approximately 1 to 100 Hz. This region is where the domi-
Figure 2. Propagation paths of sound from a source at the sound channel depth, in the sound speed profile reported near Wake Island. Ray launch angles range from -15 (Below the horizontal) to +15 de- grees (Above), with an interval of 1 degree. Every fifth ray is drawn with a bold line for ease of viewing.
Figure 3. Map of the three hydrophone arrays providing the data for this discussion. The three island stations are: Diego Garcia in the Indian Ocean, Ascension Island in the Atlantic Ocean, and Wake Island in the Pacific Ocean.
nant sources of acoustic signals are non-linear sea surface wave interaction; seismic activity; some biologic; and final- ly, commercial shipping. More about each of these sources later. The underwater acoustics community, since 1962, have used what are casually called “The Wenz Curves” as the “standard” for ocean ambient noise, so the normal pro- cedure is for experimentalists at sea to collect and plot the data under consideration on those curves to provide a “san- ity check.” Since the Wenz Curves are accepted by research- ers to be typical of ambient noise for various environments encountered at sea, the comparison gives confidence that the current data collected is good data. In keeping with that process, first consider some long term average levels from the Indian, Pacific, and Atlantic Oceans in the context of the Wenz curves (Wenz, 1962) (Figure 4):
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