b'Acoustic methods in geophysicsFeaturealong the track of the flow, in the process frequently scouring stream channels to bedrock. Debris dams, fences, and traps are widely deployed to protect infrastructure downslope from debris flows, but these can be overtopped and early warning of imminent flow arrival could be critical where settlements lie in the probable path of debris flows. Liu etal. (2015) reported that detection based on infrasound monitoring, involving signal processing and the elimination of extraneous infrasound from sources such as lightning or vehicles, could result in the issuing of an early warning to downslope areas very rapidly - possibly with 10-30 seconds of debris flow initiation. For many typical debris flows, such a warning could provide up to five minutes within which key evacuations could occur. Related work has trialled infrasound detection for providing similar early warnings of landslides and mudflows, which frequently prove to be lethal and which are very damaging to infrastructure.UltrasoundHigh frequencies sounds are generated by a number of processes in rock materials. Ultrasound in nature is also less familiar than the audible frequencies, but can result from the incremental fracturing of rocks, such as that produced in alpine conditions by freezing-related processes. The brief release of acoustic energy linked to fracture growth generates high frequencies. The development of rock cracking in alpine environments progressively reduces the strength of rock faces, and is likely to be both a precursor of, and a trigger for, alpine rockfalls (Draebing and Krautblatter 2019). Girard etal. (2013) recorded frost cracking using sensors to collect the emitted ultrasound signals at a field site in the Swiss Alps. This involved fractures over 1-10 cm length scales, having displacements of 0.5 - 1 m. By detecting Figure 5.DAS data showing the detection of the 10 January 2018 (M = 7.5,the acoustic emissions in the frequency range 20-100 kHz, depth 19 km) Honduras earthquake on a broadband seismometer (Goldstone,Girard etal. (2013) were able to record ~ 6.5 x 105 events during GSC) in southern California, USA, and on part of a co-located DAS fibre optica one-year monitoring period. These events occurred during cable (modified after Yu etal. 2019). both freezing and fully-thawed conditions. Such methods are shedding light on the actual mechanisms behind rock weathering for stations 600-900, together with the GSC seismometer datain alpine environments, and the seasonal conditions under which resolved into radial, vertical, and tangential components. Thefracturing occurs. Weber etal. (2018) provide a further case study, lower panel shows the time-frequency plot of the stacked DASinvolving in-situ field monitoring of ultrasonic acoustic emissions stations. on the flanks of the Matterhorn (Swiss Alps).The DAS method can be extended to rely on sub-sea cables also. Further, acoustic optical fibre methods applied toSonification and audificationdown-borehole monitoring have been able to detect rock displacements as small as 1 nm, and can be used to interpretFinally, it is appropriate to note that the analogy drawn earlier aspects of groundwater flow conditions at depth, thebetween volcanoes as infrasound sources and giant musical groundwater movement leading to small deflections of the fibreinstruments can be extended to a further area of development optic cable (Becker etal. 2020). in the application of acoustic methods. Given the very large sets of data that can result from long-term acoustic monitoring (or Early-warning of damaging environmental processes indeed, from other sources of geophysical data), appropriate tools for data reduction, analysis, and mining are increasingly In relation to a number of hazardous processes, infrasoundneeded. As a result, exploitation of the ability of the human ear appears to offer a basis for developing early-warning signals.to detect patterns and changes in time-series data converted to Debris flows are among the processes in steep topography thataudible frequencies (via sonification and audification) is being can pose a severe hazard to settlement and infrastructure. Theysuggested in a wide range of geophysical applications, including can rapidly move tens of thousands of m3 of rock and weatheredheliophysics (Alexander etal. 2014), seismic facies analysis material, including very large boulders, downslope at speeds(Amendola etal. 2017), and others. Further developments, of 5-10 m s1. Such flows are common after heavy rainfall inincluding the use of combined visual and acoustic analysis of mountainous areas, and are also common in the aftermath ofgeophysical data, are being proposed (DellAversana etal. 2017). wildfires when heavy thunderstorm rainfall is received. ManyAn example of the sonification of the eruptive sequence of the such debris flows have been mapped in bushfire-affected areasLone Star geyser (Yellowstone National Park, USA) is provided by of south-eastern Australia, and have resulted in significantthe Volcano Listening Project (https://volcanolisteningproject.damage to infrastructure. They often begin in areas upslope,org/volcanomusic/lonestargeyser/). This clearly demonstrates and may grow in volume as additional debris is incorporatedthe utility of sonification for the identification of patterns in time 45 PREVIEW JUNE 2022'