Detection Of Infrasonic Signals Research Articles

On one approach to the detection of infrasonic signals of irregular natural phenomena in the instrumental observations time series at the student interdisciplinary testing ground

The work is devoted to the analysis of time series and the problem of processing signals obtained as a result of the design approach implementation during the organization of instrumental observations of irregular natural phenomena at the student interdisciplinary testing ground. The objective of the work is to study the methods of processing noisy signals obtained as a result of monitoring the infrasonic environment, which make it possible to automate the search for fragments of the time series generated by irregular natural phenomena. At the beginning of the work, a brief explanation of the essence of the measuring scientific experiment carried out within the framework of the project approach used in the additional education of students and schoolchildren shall be given. The following is a review of publications describing various approaches to the analysis of nonstationary time series obtained in the process of instrumental observations. As the main method of time series analysis, it is proposed to use the algorithm for calculating the fractal dimension of the time series, proposed by T. Higuchi [1]. During studying of the time series of infrasonic signals, a number of regularities were discovered that contribute to the development of an original procedure for processing and transforming the signal under study, which makes it possible to determine the time intervals of fragments of the time series corresponding to the signals of the desired natural phenomena. The essence of the proposed approach lies in the preliminary preparation of the time series by processing the data with a simple normalized difference filter, previously smoothed by performing the coenvolution (convolution) operation with a Gaussian kernel; determining the step of segmenting the normalized time series, calculating fractal dimensions and averaged amplitudes for each of the segments of the time series and obtaining on their basis vectors of changes in dimensions and amplitudes with their subsequent element-wise multiplication. It is shown that the maximum values of the components of the resulting vector are indicators of timestamps for the location of the desired signals.

Evaluating factors influencing infrasonic signal detection and automatic processing performance utilizing a regional network.

Physical and deployment factors that influence infrasound signal detection and assess automatic detection performance for a regional infrasound network of arrays in the Western U.S. are explored using signatures of ground truth (GT) explosions (yields). Despite these repeated known sources, published infrasound event bulletins contain few GT events. Arrays are primarily distributed toward the south-southeast and south-southwest at distances between 84 and 458 km of the source with one array offering azimuthal resolution toward the northeast. Events occurred throughout the spring, summer, and fall of 2012 with the majority occurring during the summer months. Depending upon the array, automatic detection, which utilizes the adaptive F-detector successfully, identifies between 14% and 80% of the GT events, whereas a subsequent analyst review increases successful detection to 24%-90%. Combined background noise quantification, atmospheric propagation analyses, and comparison of spectral amplitudes determine the mechanisms that contribute to missed detections across the network. This analysis provides an estimate of detector performance across the network, as well as a qualitative assessment of conditions that impact infrasound monitoring capabilities. The mechanisms that lead to missed detections at individual arrays contribute to network-level estimates of detection capabilities and provide a basis for deployment decisions for regional infrasound arrays in areas of interest.

Infrasound Signal Detection and Back Azimuth Estimation Using Ground‐Coupled Airwaves on a Seismo‐Acoustic Sensor Pair

AbstractWe present a new infrasonic signal detection and back azimuth determination technique that requires just one microphone and one three‐component seismometer. Ground‐coupled airwaves (GCAs) occur when an incident atmospheric acoustic wave impinges on the ground surface and is partially transmitted as a seismic wave. GCAs are commonly detected hundreds of kilometers away on seismic networks and are observed to have retrograde particle motion. Horizontally propagating acoustic waves and GCAs have previously been observed on collocated infrasound and seismic sensor pairs as coherent with a 90° phase difference. If the sensors are spatially separated, an additional propagation‐induced phase shift is present. The additional phase shift depends on the direction from which the acoustic wave arrives, as each back azimuth has a different apparent distance between the sensors. We use the additional phase shift, the coherence, and the characteristic particle motion on the three‐component seismometer to determine GCA arrivals and their unique back azimuth. We test this technique with synthetic seismo‐acoustic data generated by a coupled Earth‐atmosphere 3‐D finite difference code, as well as three seismo‐acoustic data sets from Mount St. Helens, Mount Cleveland, and Mount Pagan volcanoes. Results from our technique compare favorably with traditional infrasound array processing and provide robust GCA detection and back azimuth determination. Assuming adequate station spacing and sampling, our technique provides a new and robust method to detect infrasonic signals and determine their back azimuth, and may be of practical benefit where resources are limited and large sensor networks or arrays are not feasible.

Infrasound phenomenology, propagation, and detection

Infrasound is generally understood to refer to acoustic disturbances in the atmosphere in frequency bands below the threshold of human hearing, but above the frequencies at which internal gravity waves propagate. The infrasonic band is nominally taken to be 0.05 Hz up to about 20 Hz. Infrasonic signals tend to be generated by large, violent events and propagate efficiently in sound ducts formed by wind jets and temperature gradients in the middle and upper atmosphere. Infrasonic signals can be detected at very large distances from the source, sometimes even globally. This tutorial will present an overview of the generation of infrasound by both natural and anthropomorphic sources and of the subsequent propagation and detection of infrasonic signals. Two particular sources will be discussed in detail: the signal generated by a large explosion and the so-called microbarom signal generated by colliding ocean waves. Signal propagation through the atmosphere will then be discussed. Available open source infrasound propagation packages and their use will be introduced, the significant atmospheric sound ducts will be identified, and the difficulties inherent in modeling propagation through a dynamic atmosphere will be emphasized. The current state and availability of atmospheric specification will be touched upon. Finally, the use of array processing for the extraction of infrasonic signals from the pressure fluctuations inherent to a turbulent atmosphere will be discussed.

Statistical analysis of infrasound signatures in airglow observations: Indications for acoustic resonance

The detection of infrasonic signals in temperature time series of the mesopause altitude region (at about 80–100km) is performed at the German Remote Sensing Data Center of the German Aerospace Center (DLR-DFD) using GRIPS instrumentation (GRound-based Infrared P-branch Spectrometers). Mesopause temperature values with a temporal resolution of up to 10s are derived from the observation of nocturnal airglow emissions and permit the identification of signals within the long-period infrasound range.Spectral intensities of wave signatures with periods between 2.5 and 10min are estimated applying the wavelet analysis technique to one minute mean temperature values. Selected events as well as the statistical distribution of 40 months of observation are presented and discussed with respect to resonant modes of the atmosphere. The mechanism of acoustic resonance generated by strong infrasonic sources is a potential explanation of distinct features with periods between 3 and 5min observed in the dataset.

Mesopause temperature perturbations caused by infrasonic waves as a potential indicator for the detection of tsunamis and other geo-hazards

Abstract. Many geo-hazards such as earthquakes, tsunamis, volcanic eruptions, severe weather, etc., produce acoustic waves with sub-audible frequency, so called infrasound. This sound propagates from the surface to the middle and upper atmosphere causing pressure and temperature perturbations. Temperature fluctuations connected with the above mentioned events usually are very weak at the surface, but the amplitude increases with height because of the exponential decrease of atmospheric pressure with increasing altitude. At the mesopause region (80–100 km height) signal amplitudes are about two to three orders of magnitude larger than on the ground. The GRIPS (GRound-based Infrared P-branch Spectrometer) measurement system operated by the German Remote Sensing Data Center of the German Aerospace Center (DLR-DFD) derives temperatures of the mesopause region by observing hydroxyl (OH) airglow emissions in the near infrared atmospheric emission spectrum originating from a thin layer at approximately 87 km height. The GRIPS instrument is in principle suited for the detection of infrasonic signals generated by e.g. tsunamis and other geo-hazards. This is due to the fact that the infrasound caused by such events should induce observable short-period fluctuations in the OH airglow temperatures. First results obtained during a field campaign performed at the Environmental Research Station "Schneefernerhaus", Zugspitze (47.4° N, 11.0° E) from October to December 2008 are presented regarding potential sources of meteorological and orographical origin. An adequate distinction of the overlapping infrasonic signatures caused by different infrasound sources in the OH temperature record is needed for the ascription to the proper source. The approach presented here could form a contribution to a hazard monitoring and early warning system.

Global Detection of Infrasonic Signals from Three Large Bolides

We present the infrasonic observations of three large bolides that were observed at numerous International Monitoring System (IMS) infrasound arrays on a global scale. First, a simple procedure for the global association of infrasound detections from large infrasound events is outlined. Infrasound signals are associated with large events based on arrival time, backazimuth and uniqueness at a given IMS array. Next, we apply the algorithm to three bolides and investigate some of the factors affecting the detectability of infrasound from large events. Our findings suggest that site-noise effects significantly degrade the capability of the IMS infrasound network, suggesting that more effort is required to reduce ambient site noise. These results have implications for the use of infrasound measurements (in particular those from IMS stations) as a tool for evaluating the global flux of near-Earth objects.

Infrasonic Signal Detection and Source Location at the Prototype International Data Centre

This paper describes an automatic and interactive data processing system designed to locate impulsive atmospheric sources with a yield of at least one kiloton by detecting and characterizing the airborne infrasound radiated by the source. The infrasonic processing subsystem forms part of a larger system currently under development at the Prototype International Data Center (PIDC) in Arlington, Virginia where seismic, hydroacoustic, radionuclide and infrasonic methods are used to detect and locate impulsive sources in any terrestrial environment. Infrasonic signal detection is achieved via a coincidence detector which requires both the normalized cross correlation and the short-term-average/ long-term-average ratio of a beam in the direction of maximum correlation to exceed predetermined threshold values simultaneously before a detection is declared.The infrasound propagation model currently used to infer travel-time information assumes the horizontal sound speed across the ground to be 320.0 m/s. This crude model is currently being replaced by a model which predicts travel-time information through a ray-tracing algorithm for acoustic waves in an atmosphere with seasonal representations for temperature and wind. A novel feature of the source location process is the fusion of all available arrival information, whether it be seismic, hydroacoustic or infrasonic to locate a single source where it is reasonable to hypothesize a common source. In its final configuration the infrasonic subsystem will routinely process data from the global 60-station International Monitoring System (IMS) infrasonic network currently under development.

An inexpensive, portable, and rugged system for recording the low-frequency sounds of cetaceans

Many studies have been done on the low-frequency sounds of cetaceans. However, few of these studies have involved coincident photoidentification, biopsy, and behavioral observation. In order to make acoustical recordings in conjunction with the regular visual fieldwork done from inflatable boats on the baleen whales of the St. Lawrence, a recording system was designed to meet several requirements. It had to be very inexpensive; light enough to be carried onto the boat each day and deployed by hand; and rugged enough to survive being pounded and soaked when stored on the open boat. The resulting system consisted of two main parts: a recording system made up of DAT recorder, amplifiers, filters, batteries and voltage regulators; and a surface-motion isolation system made up of a spar buoy, omnidirectional hydrophone, cable and damping plate. A mixing circuit allowed for real-time detection of infrasonic signals through a pair of headphones. In addition to the description of the system and its calibration, samples of blue, fin, minke and humpback whale recordings will be presented, and a brief comparison of the system with and without the spar buoy will be shown. [Work supported by a graduate fellowship from NSF.]