b'Acoustic methods in geophysicsFeaturecontinuous monitoring of the climatically critical thermohaline circulation. Jackson etal. (2022) have provided a review of what has happened to the Atlantic Meridional Overturning Circulation (AMOC), a key component of the global thermohaline system since 1980. Buoyancy changes that perturb this system may have an important anthropogenic component via reduced production of cold, dense brines because of rising temperatures and consequent reductions in the extent of sea ice growth. The possible effects of changed acoustic properties linked to thermohaline changes remain to be explored.InfrasoundWhilst pioneering studies such as those just mentioned relied primarily on audible sound and active acoustic methods, infrasound provides many opportunities to study phenomena in the solid earth using passive methods. A global network of Figure 1.Colladon and his father measuring the speed of sound in water oninfrasound monitoring stations has been established under Lake Geneva in 1826. Source: Wirgin (2017). the auspices of the CTBTO (Comprehensive Test Ban Treaty Organisation), in order to be able to detect and identify the arrival of the sound with the aid of a large, thin-walled metal earsource of nuclear detonations (see Figure 2).tube, the sound-collecting flared opening of which, sealed by a thin metal membrane, was held at 2 m below the water surfaceThis network is commonly referred to simply as the IMS (see Figure 1). Colladon measured the time taken from the(International Monitoring System). Where possible, IMS visible flash set off from his fathers boat until the detection ofstations are in locations including forests (see Figure 3), that the underwater sound with his ear pressed to the listening tubeare sheltered from the effects of wind and other infrasound (an early hydrophone). sources, so as to reduce the occurrence of false alarms. Australia hosts five of the ~ 60 operational IMS stations, including one In the tradition of 19th century scientific experimentation,at Warramunga, near Tennant Creek, in the Northern Territory. this work was carefully executed, including the most exactThere are two others on the mainland, one in Tasmania, one in determination of the distance between the two boats thenAntarctica, and a fifth is on the Cocos Islands.possible, as well as making allowance for other factors such as the lagged response of the triggering of the stopwatch afterWhilst being designed to listen for infrasound from nuclear the flash was seen (Colladon estimated his response time todetonations, IMS stations can detect signals from environmental be 0.25 s). The submerged bell was actually struck three timessources also. Fee and Matoza (2013) reported that in 1883, in each replication of the experiment, to avoid confusion withbarometers around the world recorded low-frequency pressure any other possible local sound sources. In the final analysis,signals from the eruption of the Krakatau (Krakatoa) volcano the mean travel time of the sound through the water, at 8.1C,in Indonesia, whilst cannon-like sounds were audible up to was found to be 9.4 s. The distance between the bell and the5000 km away. Current IMS stations use purpose-built detectors hydrophone was determined to be 13.487 km, yielding anable to operate in the range 0.01 - 8 Hz (Matoza etal. 2017), estimate of the speed of sound at 2 m depth of 143524 mand the back-azimuth of the signal detected at multiple IMS s1. A detailed account of these pioneering experiments wasstations can then be used to identify the region from which provided by Wirgin (2017). the infrasound originated. This approach can however also be harnessed to locate geological processes such as episodes Subsequent research, again using active acoustic methods,of explosive volcanism, free for instance from the limitations was extended to oceanic sound transmission (with higherthat cloud cover can impose on satellite-based detection. As a transmission speeds related to the salinity), including theresult, catalogues of volcanic events can be created from the pioneering experiments of Ewing and Worzel (1948) in thearchived data from the IMS. There are many other potential Atlantic. This work employed the detonation of 4 lb (~2 kg) of TNT as a signal source, at a depth of 4000 feet (~1316 m). The results led to the identification of the sound-carrying capacity of the SOFAR channel, which is centred on ~ 1 km depth, and includes the zone where a minimum in sound velocity (~ 1500 m s1) is reached. This results from the presence of the thermocline and the increase in velocity related to increasing pressure with depth in the lower, almost isothermal, water. Given the long-distance transmission that could be achieved via repeated refraction along the SOFAR channel, the US explored this in WWII as a means to locate downed aircrew, a small charge intended to be set off by survivors as a signal. Given the dependence of velocity on density, oceanic sound transmission was employed subsequently to estimate temperatures in the deep ocean, and ocean acoustics has grown considerably since. Major concerns apart from oceanic noise and its possible effectsFigure 2.Map of the International Monitoring System Infrasound Network. on marine organisms include the need for better and moreSource Hupe etal. 2022)43 PREVIEW JUNE 2022'