TOEFL Listening Practice: Lecture14

In 2006, the USA National Oceanic and Atmospheric Administration has picked up some extraordinary sounds using its Equatorial Pacific Ocean autonomous hydrophone array, and a few of them remain unidentified today. Of course, we have some theories and none of them involve little green men from outer space setting up home in the depths, but they remain mysterious and intriguing all the same. Let’s look at a few of them.

The first, we call Upsweep. It was detected on the American National Oceanic and Atmospheric Administration’s equatorial autonomous hydrophone arrays, and was present when the Pacific Marine Environmental Laboratory first started recording sound using its surveillance system back in August, 1991. It consists of a long series of rising sounds. Each one several seconds in duration and is continuous. The level is high enough that we can record it on arrays throughout the Pacific. What makes it even more unusual is that it appears to be seasonal, reaching peaks in the Spring and Autumn. Whether that’s because of the changes at the source or environmental changes, it’s not entirely clear. Scientists have narrowed down the source of the sound to roughly 54°S by 140°W, and this is a site where we believe there to be volcanic seismicity, but we haven’t pinpointed it down to anything specific. The sound level has actually been declining over the years since 1991, but they can still be detected on the arrays.

Another interesting and unidentified sound that we’ve captured on our surveillance system is Bloop. That’s the name given to an ultra-low-frequency and extremely powerful underwater sound that was detected by the NOAA in…a… 1997. Using triangulation techniques, they narrowed down the source to a remote point in the South Pacific Ocean, just west of the southern tip of South America. The sound was detected several times by the Equatorial Pacific Ocean autonomous hydrophone array. The sound rose rapidly in frequency over the course of about one minute, and the audio profile closely resembled that of a living creature – a whale perhaps. The sound, however, was of sufficient amplitude to be heard on multiple sensors, and, so was several times louder than the loudest recorded animal, the blue whale. The NOAA Vents program has attributed the sound to that of a large icequake, as numerous icequakes share similar spectrograms with Bloop, and they can exhibit the necessary amplitude for them to be recorded at ranges exceeding 5000 km. Since NOAA’s Dr. Christopher Fox does not believe the sound was man-made, a bomb perhaps, nor are the sounds consistent with those from geological events like volcanoes or earthquakes, this seems to be the most likely reason for the sound, and if this is the case, the most probable location was between Bransfield Straits and the Ross Sea.

A couple of others, whose sources have not been formally identified, but are slightly less mysterious, are Julia and Train. Julia was recorded on March 1, 1999, again by the NOAA. The sound could be head over the entire Equatorial Pacific Ocean autonomous hydrophone array and lasted for about 15 seconds. The sound is consistent with that of an iceberg running aground. The source hasn’t been pinpointed precisely, due to the uncertainty of the arrival azimuth, but was likely between Bransfield Straits and Cape Adare. Train was probably caused by the same thing, even though the sound itself was quite different. A rising sound which reached a quasi-steady frequency. It was recorded on March 5, 1997 on the same array.