b'Acoustic methods in geophysicsFeatureFigure 3.The Canadian IMS station (station I10CA) in Manitoba. The main microbaromter is located in the concrete structure in the background. Four rosettes of additional sensors are used to monitor and reduce the effects of local turbulent air movements. Source: https://can-ndc.nrcan.gc.ca/is_infrasound-en.phpapplications of the sensing capabilities of the IMS. For instance, there is considerable interest in advancing understanding of the behaviour of dust devils (willy willies) on earth, given theFigure 4.The infrasound signal (panel A) and the frequencies involved (panel detection of their tracks in imagery of the Martian surface,B) that preceded an earthquake (M = 7.5) located in the East China Sea (after confirmed by their direct observation by cameras on theWang etal. 2021).NASA Curiosity rover (e.g. see https://www.youtube.com/watch?v = k8lfJ0c7WQ8) (Lorenz etal. 2016). It is possible thatOngoing developmentsat least larger dust devil vortex flows can be detected in the vicinity of IMS stations located in suitable environments, suchAn area of application of acoustic methods that is showing as the Warramunga site (Lorenz and Christie 2015). In this way,constant development is the field of distributed acoustic more can be learned about the relationship to conditions at thesensing (DAS). DAS uses fibre optic cables, either installed surface and in the lower atmosphere, including temperaturespecifically for research purposes, or existing dark fibre gradients above the surface. networks (unused fibres that were originally established for communications). A recent review of optical fibre seismology The prediction of damaging earthquakes has remainedwas presented by Lindsey and Martin (2021). Laser radiation challenging. Precursor infrasound signals arising as cracktransmitted along an optical fibre can result in a reflected signal propagation occurs may provide one path to achieving this,generated at a location where the fibre properties or geometry and may usefully supplement the search for seismic precursors.change, as can occur when the fibre is affected by local vibrations The acoustic energy emitted coincident with the growth ofin its surroundings. Detecting the position along the fibre at fracture systems has been detected well in advance of the mainwhich such reflections occur relies on an interrogator device. earthquake shock. Wang etal. (2021) reported that in almostEach source of reflected signal provides, in effect, a microphone 76% of a large sample of major earthquakes worldwide (M7.0)or geophone. Sensor locations can be resolved with m spacing in the period 2002-2009, anomalous infrasound was recordedalong a fibre, providing a potentially very large array of virtual 1 - 9 days prior, and in a number of cases, more than two weeksdetectors of acoustic energy, and the extensive network of ahead of the main shock. Figure 4 shows the infrasound signaloptical fibres worldwide presents an opportunity for the wide (panel A) and the frequencies involved (panel B) that precededapplication of DAS. For instance, Li etal. (2021) reported the rapid an earthquake (M = 7.5) located in the East China Sea. Thecommissioning of a monitoring network relying on four existing earthquake was recorded on 31 March, but the infrasound datatelecommunications cables following the 2019 Ridgecrest shown were recorded two days earlier, on 29 March. earthquake (M 7.1) in California, USA. They report that this system permitted the detection of about six times as many aftershocks The use of the IMS network to catalogue explosive volcanicduring a three month period than did the standard catalogue eruptions is a further instance of passive acoustic monitoring, inbased on seismometer data. The aftershocks could be shown to which pre-existing signals are detected. This has been extendedhave arisen on multiple faults around the epicentre.to detecting and locating other events, including bolide impacts. Hupe etal. (2022) have reviewed some additional areasAn example of DAS data is shown in Figure 5, which shows where global IMS data may prove useful. Finally, it is appropriatethe detection of the 10 January 2018 (M = 7.5, depth 19 km) to mention the 15 January 2022 eruption of the Hunga volcanoHonduras earthquake on a broadband seismometer (Goldstone, (Tonga). This event resulted in co-eruptive infrasound that wasGSC) in southern California, USA, and on part of a co-located detected at multiple IMS stations, and even audible sound wasDAS fibre optic cable. This cable had previously been established reported as far as 10 000 km from Hunga, taking the form offor telecommunications, and ran for ~ 50 km around the several boom sounds. This event yielded important globalGoldstone experimental site. In total, ~5000 DAS stations, at seismo-acoustic data, especially on the nature and transmissiona spacing of 4 m, were analysed along a 20 km section of the of atmospheric waves (Matoza etal. 2022). DAS cable. The figure (upper panel) shows the DAS data stacked JUNE 2022 PREVIEW 44'