b'Acoustic methods in geophysicsFeatureAcoustic methods in geophysics - from the 19 thcentury to contemporary developmentssuggest that infrasound can be sensed up to 400 km from a major waterfall acoustic source (Bedard 2021). The capacity to detect infrasound has also been suggested to underlie the ability of African elephant to detect and migrate toward thunderstorms (and the resulting vegetation growth), detected over distances of more than 100 km from the location where rain fell (Garstang etal. 2014). Anthropogenic sound sources are generally regarded as undesirable owing to possible effects on organisms, especially in the oceans. There remains uncertainty about the possible effects of infrasound resulting from wind farm operations on human health, though research on this topic is ongoing.David Dunkerley david.dunkerley@monash.edu In terms of geophysical acoustic signals arising in the environment, we can list among the principal energy sources Many environmental processes generate sound, including theearthquake and volcanic activity, landslides, rockfalls, debris audible frequencies associated with some earthquake activityflows, river flow, intense rainfall, wind, meteorite impacts, surf and with explosive volcanic eruptions. Volcanoes possessingwaves, tsunami, singing sand dunes, willy-willies, tree fall, steep-walled craters have been likened to giant musicalice and glacier movement, and the physical breakdown or instruments, and analysis of their changing acoustic emissions,shattering of rocks, including rock weathering processes in the especially as the dimensions of the crater change, may providecryosphere. Thermal-hydrational rock splitting is likely to have aids to the prediction of eruptions (Johnson etal. 2018). Indeed,been the source of sounds said to resemble gunfire, whose acoustic methods have proven useful in making inferencesoccurrence on a number of mornings puzzled the explorer about the mechanics of volcano behaviour more generally,Charles Sturt during his explorations of inland Australia in including that of Whakaare / White Island, New Zealand (Walshthe 1840s. Such sounds were also reported by explorer David etal. 2019), which erupted on 9 December 2019 with the loss ofLivingstone from his travels in North Africa. Contemporary 22 lives. The goal of this paper is to provide a brief history of thereports of acoustic emissions from rock splitting events geophysical application of acoustic methods, from beginningsare rare, though it is informative to see (and hear) the very in the 19th century, to contemporary developments. energetic rock exfoliation event fortuitously recorded on a rock Seismic P waves in solid earth materials behave in similar waysdome in California (see the supplementary Video 1 attached to sound waves in air and water. Mechanical properties andto the paper by Collins etal. 2018) (https://static-content.ambient conditions within each conducting medium exert keyspringer.com/esm/art%3A10.1038%2Fs41467-017-02728-1/influences on aspects such as the velocity of transmission. This isMediaObjects/41467_2017_2728_MOESM3_ESM.mov). During around 6 km s1 in rock, but only about 25% of this (1.5 km s1)this event, fragments of rock were thrown quite violently some in water, and just 20% (1.22 km s1) in air. P waves (hereafterdistance into the air, and evidently significant stored energy was sound) from many sources in the environment either originatereleased during the exfoliation process.in, or are transmitted by, the atmosphere, oceans, and solid earth materials. Some energy sources originate in just oneTwo early historical instances of acoustic analysis - the realm, such as earthquake waves in the solid earth, whilst othersspeed of sound in waterarise from interactions among lithosphere, atmosphere and hydrosphere, exemplified by the acoustic signal of surf wavesMuch contemporary work involving environmental acoustics striking a rocky coast. Anthropogenic activities including therelies on the detection and analysis of pre-existing sound, movement of road and rail vehicles, motorised vessels, militaryboth natural and anthropogenic, extending from infrasound or mining operations and wind turbine operations contributeto ultrasound frequencies, and can be classified as involving additional sources of acoustic energy. passive acoustic methods. In contrast, many early experiments on environmental acoustics employed active acoustic methods, Sound is customarily classified as being audible if fallingwith the purposeful generation of artificial sound mechanically within range of human hearing (nominally 20 Hz to 20 kHz).or using explosives; the sound that was analysed consequently Lower frequencies - infrasound - may be partially heard or felt.usually lay in the audible frequency range.These include geophysical signals extending into the mHz range, and include much of the acoustic energy associatedThe speed of sound in water, determined through the with volcanic activity. Frequencies above the audible rangepioneering experiments of Colladon and Sturm in the 19th - ultrasound - can again arise from solid earth processes, andcentury, provides an important example. Beginning in 1826, can extend to 100 kHz and beyond. It is interesting to note inthey carried out multiple experiments on Lake Geneva. In passing that whilst acoustic signals from many sources, andearly trials, Colladons father, in one of two boats, generated spanning the wide frequency range just mentioned, can beunderwater sound using a submerged bell weighing 65 kg. employed to generate geophysical and other insights, they canThis could be struck by a hammer, via a lever mechanism that also have significant effects in the biosphere. These include ansimultaneously ignited some gunpowder to set off a flash of hypothesised role of infrasound in avian navigation (Patrick etal.light. The flash was detected by Colladon himself in second boat 2021), including the use of infrasound emitted by waterfallsmoored 14 km away (and in later experiments with a larger, as navigation beacons by pigeons, whose routes of travel500 kg bell, at a distance of 50 km). Colladon listened for the JUNE 2022 PREVIEW 42'