[S-VC33] Mechanism of volcanic eruptions

[SVC33-P13] Waveform and spectral characteristics of pulse-like acoustic waves reproduced by analogy experiment of bubble bursting

*Dan Muramatsu¹, Mie Ichihara¹, Osamu Kuwano² (1.Earthquake Reserch Institute, The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology)

Keywords:Volcano, Acoustic wave, Rheology

Pulse-like infrasound signals generated by bubble bursting have been reported in magmatic and hydrothermal eruptions. Their source process could be influenced by fluid rheology (Jolly et al., 2016; Muramatsu et al., 2022). We examine the relationships between fluid rheology and pulse-like acoustic waves by an analogy experiment of bubble bursting using 3wt% water dispersion of the chemically synthesized smectite (SUMECTON-SA, Kunimine Industries) as an analog material of volcanic mud. The sample has yield stress and shows thixotropy caused by microstructure recovery after applying shear. The aging test suggests that the recovery time scale is in the order of several dozens of seconds. Air bubbles were generated by injecting a constant airflow from the bottom of the sample with an injector. We measured bubble bursting at the surface with broadband microphones and cameras. To prompt the initial rising of bubbles, we pre-sheared samples from the tip of the injector to the surface by inserting a glass stick before air injection. We made some rest times during the experiment by stopping air injection to confirm the influence of structure recovery on the acoustic wave. The rest times were progressively extended, like 10 s, 30 s, and 100 s. Following this procedure, we conducted two tests using the same sample. The measured acoustic pulse has dominant peaks around 1000–2000 Hz, but this may contain a natural frequency of air column resonance inside the sample container. We deconvoluted each pulse spectrum by the reference event based on the method of Snieder and Safak (2006) and found that the pulse frequency tended to increase with time in the second test. Camera images confirmed a trapped bubble beneath the sample surface, shrunk with time. If the resonance within the trapped bubble caused the acoustic pulse, the increasing frequency could be explained by the shrinking of the bubble, but the bubble size and volume should be estimated for the discussion. The waveform did not show a significant difference before and after the rest in the first test but did show a slight difference in the second. However, we need additional tests to discuss relationships between the rest and the acoustic waves because water dispersion of smectite also shows time-dependent rheology on a long-time scale of several days (Laribi et al., 2005). As a next step, we solve the above problems by (i) removing air column resonance by improvement of the equipment, (ii) assessing longer-time scale rheology change on the acoustic wave, and (iii) estimating the size of the trapped bubble. Also, in the presentation, we discuss the resonance problem due to structure because this should be accounted for when we analyze infrasound from volcanic eruptions.