Daniel C. Bowman

The acoustic signatures of ground acceleration, gas expansion, and spall fallback in experimental volcanic explosions

Infrasound and high-speed imaging during a series of field-scale buried explosions suggest new details about the generation and radiation patterns of acoustic waves from volcanic eruptions. We recorded infrasound and high-speed video from a series of subsurface explosions with differing burial depths and charge sizes. Joint observations and modeling allow the extraction of acoustic energy related to the magnitude of initial ground deformation, the contribution of gas breakout, and the timing of the fallback of displaced material. The existence and relative acoustic amplitudes of these three phases depended on the size and depth of the explosion. The results motivate a conceptual model that relates successive contributions from ground acceleration, gas breakout, and spall fallback to the acoustic amplitude and waveform characteristics of buried explosions. We place the literature on infrasound signals at Santiaguito Volcano, Guatemala, and Sakurajima and Suwonosejima Volcanoes, Japan, in the context of this model.

Infrasound in the middle stratosphere measured with a free-flying acoustic array

Infrasound recorded in the middle stratosphere suggests that the acoustic wavefield above the Earths surface differs dramatically from the wavefield near the ground. In contrast to nearby surface stations, the balloon-borne infrasound array detected signals from turbulence, nonlinear ocean wave interactions, building ventilation systems, and other sources that have not been identified yet. Infrasound power spectra also bore little resemblance to spectra recorded on the ground at the same time. Thus, sensors on the Earths surface likely capture a fraction of the true diversity of acoustic waves in the atmosphere. Future studies building upon this experiment may quantify the acoustic energy flux from the surface to the upper atmosphere, extend the capability of the International Monitoring System to detect nuclear explosions, and lay the observational groundwork for a recently proposed mission to detect earthquakes on Venus using free-flying microphones.

A Comparison of the Ocean Microbarom Recorded on the Ground and in the Stratosphere

The ocean microbarom is an acoustic signal generated via nonlinear interaction of ocean surface waves. It can propagate for thousands of kilometers and represents a significant infrasonic noise source for ground infrasound stations across the globe. However, wind noise often compromises detections at ground stations. Furthermore, the microbarom may travel in elevated acoustic ducts that do not transmit enough energy for detections on ground stations. Here, the presence of the ocean microbarom on two high altitude balloon flights is investigated. A spectral peak consistent with the microbarom was observed on sensors in the stratosphere but not on those deployed on the ground near the flight path of the balloon. This is probably due to an elevated acoustic duct and/or a superior signal to noise ratio in the stratosphere. Thus, microbarom activity quantified solely with ground based sensors may underestimate the occurrence of the phenomenon. However, high levels of interference from flight system electronics and/other other payloads may have obscured other microbarom episodes during the balloon deployments.

Near real time weather and ocean model data access with rNOMADS

The National Oceanic and Atmospheric Administration Operational Model Archive and Distribution System (NOMADS) facilitates rapid delivery of real time and archived atmospheric and oceanic model outputs from multiple agencies. These data are free to the scientific community, industry, and the public. The rNOMADS package provides an interface between NOMADS and the R programming language. Like R itself, rNOMADS is open source and cross platform. It utilizes server-side functionality on the NOMADS system to subset model outputs for delivery to client R users. We discuss rNOMADS implementation and usage as well as provide two case studies. Users can download rNOMADS from within the R interpreter or from the Comprehensive R Archive Network (CRAN). HighlightsTimely access to weather and ocean model data is crucial in many areas in geoscience.We made an R interface to the NOAA Operational Model Archive and Distribution System.The package facilitates delivery of global and regional model outputs.Fifty-eight near real time and 10 archived weather and ocean model products are available.Models include the Global Forecast System and North American Mesoscale.

Direct measurement of the acoustic wave field in the stratosphere

Low frequency acoustic waves (infrasound) are generated by a variety of natural and anthropogenic phenomena. Although infrasound propagates throughout the atmosphere, the vast majority of acoustic studies utilize sensors on or near the Earths surface. This paper describes results from two infrasound arrays launched into the stratosphere, one in August 2014 and the other in September 2015. The observations presented here are the first stratospheric infrasound measurements reported in scientific literature in 50 years. Acoustic signals recorded on the balloon borne sensors were different than those recorded by nearby infrasound stations on the ground. The 0.2 Hz ocean microbarom was detected in the stratosphere, but was not observed on nearby ground stations. A series of narrow band signals were also observed in the stratosphere, some of which varied in frequency over tens of minutes. The source of these signals is unclear. Wind noise decreased with altitude during the ascent, becoming negligible above 20 km. It was absent when the balloon was neutrally buoyant, although it was pervasive on ground stations operating in the same region during the day. Spectral characteristics of stratospheric infrasound were similar between the two flights and also resembled the last experiment in the early 1960s, but spatiotemporal variations in signal strength and frequency were also observed. Future efforts should focus on characterizing infrasound sensor operation in extreme environments and increasing spatial and temporal frequency of acoustic measurements in the free atmosphere. Results from this study have implications for long range detection of events such as nuclear blasts, the quantification of acoustic energy that heats the upper atmosphere, and calibration of a proposed mission to place airborne acoustic sensors on Venus.

Geoacoustic Observations on Drifting Balloon-Borne Sensors

Infrasound microphones on free flying balloons experience very little wind noise, can cross regions that lack ground station coverage, and may capture signals that seldom reach the Earth’s surface. Despite the promise of this technique, until recently very few studies had been performed on balloon-borne acoustic sensors. We summarize the history of free flying infrasound stations from the late 1940s to 2014 and report on results from a series of studies spanning 2014–2016. These include the first efforts to record infrasound in the stratosphere in half a century, the presence of a persistent ocean microbarom peak that is not always visible on the ground, and the detection of distant ground explosions. We discuss the unique operational aspects of deploying infrasound sensors on free flying balloons, the types of signals detected at altitude, and the changes to sensor response with height. Finally, we outline the applications of free flying infrasound sensing systems, including treaty verification, bolide detection, upper atmosphere monitoring, and seismoacoustic exploration of the planet Venus.

Detection of Artificially Generated Seismic Signals Using Balloon‐Borne Infrasound Sensors

Abstract We conducted an experiment in Pahrump, Nevada, in June 2017, where artificial seismic signals were created using a seismic hammer, and the possibility of detecting them from their acoustic signature was examined. In this work, we analyze the pressure signals recorded by highly sensitive barometers deployed on the ground and on tethers suspended from balloons. Our signal processing results show that wind noise experienced by a barometer on a free-flying balloon is lower compared to one on a moored balloon. This has never been experimentally demonstrated in the lower troposphere. While seismoacoustic signals were not recorded on the hot air balloon platform owing to operational challenges, we demonstrate the detection of seismoacoustic signals on our moored balloon platform. Our results have important implications for performing seismology in harsh surface environments such as Venus through atmospheric remote sensing.

Mass, height of burst, and source–receiver distance constraints on the acoustic coda phase delay method

This research uses the acoustic coda phase delay method to estimate relative changes in air temperature between explosions with varying event masses and heights of burst. It also places a bound on source-receiver distance for the method. Previous studies used events with different shapes, height of bursts, and masses and recorded the acoustic codas at source-receiver distances less than 1 km. This research further explores the method using explosions that differ in mass (by up to an order of magnitude) and are placed at varying heights. Source-receiver distances also cover an area out to 7 km. Relative air temperature change estimates are compared to complementary meteorological observations. Results show that two explosions that differ by an order of magnitude cannot be used with this method because their propagation times in the near field and their fundamental frequencies are different. These differences are expressed as inaccuracies in the relative air temperature change estimates. An order of magnitude difference in mass is also shown to bias estimates higher. Small differences in height of burst do not affect the accuracy of the method. An upper bound of 1 km on source-receiver distance is provided based on the standard deviation characteristics of the estimates.

Detecting Lightning Infrasound Using a High‐Altitude Balloon

14 Acoustic waves with a wide range of frequencies are generated by lightning strokes dur15 ing thunderstorms, including infrasonic waves (0.1 to 20 Hz). The source mechanism for 16 these low frequency acoustic waves is still debated and studies have so far been limited 17 to ground-based instruments. Here we report the first confirmed detection of lightning 18 generated infrasound with acoustic instruments suspended at stratospheric altitudes us19 ing a free-flying balloon. We observe high-amplitude signals generated by lightning strokes 20 located within 100 km of the balloon as it flew over the Tasman Sea on 17 May 2016. 21 The signals share many characteristics with waveforms recorded previously by ground22 based instruments near thunderstorms. The ability to measure lightning activity with 23 high-altitude infrasound instruments has demonstrated the potential for using these plat24 forms to image the full acoustic wavefield in the atmosphere. Furthermore, it validates 25 the use of these platforms for recording and characterizing infrasonic sources located be26 yond the detection range of ground-based instruments. 27 Plain-language summary 28 Lightning generates sound waves across a wide range of frequencies, including be29 low the threshold for human hearing at 20 Hz. How these waves at less than 20 Hz, also 30 known as infrasound waves, are generated during a lightning stroke is currently an area 31 for debate. So far, measurements of lightning infrasound waves have been limited to mi32 crophones fixed to the ground and models have shown that only a small section of sound 33 waves actually reach the ground. Here we show lightning infrasound that has been de34 tected using microphones suspended over a thunderstorm using a balloon flying at 32 35 km height. This opens up the possibility of using balloons in future studies to make bet36 ter measurements of infrasound waves generated by lightning activity and in turn, give 37 a better idea of how they are generated. It also shows how balloons can be used to record 38 infrasound waves far away from land and therefore beyond the detection limit of ground39 based microphones. 40

Local-distance acoustic propagation from explosions

We study the effect of acoustic propagation from explosions on full waveforms using both empirical and numerical approaches. Empirically, we explore the effects of meteorology, terrain, etc., on explosion signatures by exploiting a rich dataset of explosion measurements in different regions to relate specific path effects to second-order effects in the waveforms. Numerically, we explore the effects using different full wave codes to understand observations from a unique experiment with both ground and air waveform and 3D wind field measurements. We discuss implications for explosion yield estimation for surface explosions and for underground events.