Microbarograph Array Research Articles

What does infrasound signal duration tell us about signal propagation distance and waveguide structure?

A global network of microbarograph arrays is currently being constructed to support verification of the Comprehensive Nuclear-Test-Ban Treaty. The identification of explosively generated infrasound signals that have propagated thousands of kilometers to the arrays, and the subsequent association of signals across the sparse network, remains a challenge. One signal parameter that has not been extensively studied, but may assist in source identification procedures, is the signal duration. The durations of 42 high signal-to-noise ratio signals from 35 near-surface explosions, recorded at distances of between 25 and 6300 km, exhibit a weak relationship with source-to-receiver range; longer propagation paths result in longer signal durations. The variation in signal duration at a given source-to-receiver range depends upon the atmospheric waveguide structure. At propagation distances greater than 2000 km, long duration signals are generated within weak waveguides characterized by a small excess in stratospheric effective sound speed compared to that at the ground surface. These waveguides permit signal propagation with high celerity (up to 335 m/s). Shorter duration signals at these distances occur in stronger waveguides that exhibit greater along-path variability; these signals do not exhibit celerities greater than 315 m/s. The utility of signal duration within the context of explosion monitoring is explored.

Seasonal and diurnal variability of pressure fluctuation in the infrasound frequency range observed in the Czech microbarograph network

Infrasound environments in the Czech microbarograph network were studied. Reference Fourier amplitude spectra were calculated from data measured at three microbarograph sites of the network in May 2011-April 2012; directional analysis of infrasound arrivals in the frequency band 0.15-0.4 Hz was performed for the microbarograph array at Panska Ves in May 2014-April 2015. Diurnal, seasonal and site-to-site variability of the reference spectra was evaluated. Site-to-site variability is influenced by the location of the respective sensors in the open air and inside the observatory buildings and by local noise phenomena like wind turbines. Diurnal variability is well developed in summer with maximum ambient noise levels during the daytime and minima at night. Seasonal variability is observed at night with maxima in winter and minima in summer. Wind and wind eddies seem to be an important source of ambient noise in measurements in the Czech microbarograph network. A distinct spectral peak occurs near 0.2 Hz with amplitudes by about one order of magnitude higher in winter than in summer. Its seasonal variability is related to seasonal propagation of microbaroms from the source region in the Northern Atlantic.

Co-Phase Analysis of Atmospheric Wave Data

Summary Co-phase is a statistic designed for the detection and parameter estimation of signals by detector arrays. Ionospheric motions detected by an array of four phase-path sounders following a large earthquake are found by the co-phase technique to have a phase velocity equal to that of seismic Rayleigh waves of the same period, and to arrive from the direction of the epicentre. The calculation of co-phase for an 80-min sample of data from a.n 8-element array of microbarographs detects the presence of a signal from a high energy event despite a signal-to-noise ratio of less than unity. Co-phase analysis of acoustic signals generated by the Saturn-Apollo rocket launches indicates that these signals originate at ionospheric heights and propagate in a waveguide between a soundspeed maximum and a steep density gradient in the mesosphere.

Digital Computer Programs for Recording and Processing Infrasonic Array Data

We describe in detail the digital formats in which microbarograph array data are recorded at Alexandria Laboratories. Brief summaries are presented of 32 computer programs for altering the data formats, performing signal detection and analysis in both the time domain and the frequency-wave number domain, and for various theoretical calculations involving the acoustic radiation from explosion sources in a layered atmosphere. An example is included of the write-ups and flow charts of these programs.

Experimental Noise Reducers for an Active Microbarograph Array

Summary Brief outlines are given of the problem of identifying low frequency acoustic waves from atmospheric background noise and of the methods used by UKAEA to improve detection of these waves. An important aspect of the experimental work has been to develop devices to reduce atmospheric pressure noise, suitable for use at existing microbarograph recording sites, and to evaluate their performance in field tests. Some interim results from these experiments are given.

Analysis of the Spatial Coherence of Short-Period Acoustic-Gravity Waves in the Atmosphere

Summary The coherence of atmospheric acoustic-gravity waves has been measured in the period range 10–100 s at the Large Aperture Microbarograph Array in south-eastern Montana. The acoustic-gravity waves observed were signals generated by presumed nuclear explosions. The decrease of coherence with increasing distance between pairs of microbarographs is less rapid in the direction of wave propagation than transverse to it. Variation of direction of arrival over a small range of azimuth (±5°) explains the spatial behaviour of coherence in the direction normal to the wave propagation; variation of phase velocity of ±10 ms-1 explains the behaviour along the direction of wave propagation. Both effects may be due to inhomogeneities in the atmosphere; the velocity variation may be due to the presence in the signal of several normal modes of acoustic- gravity waves, each travelling at a slightly different phase velocity in the range 300–330 ms-1.

Automatic Processing of Multi-Array Long-Period Seismic Data

Summary Long-period seismic data from the three large arrays ALPA, LASA, and NORSAR are telemetered to the Seismic Array Analysis Center in Alexandria, Virginia, where they are processed in real time by employing a fast frequency-wavenumber (f-k) detection scheme. The processor incor- porates a novel f-k filtering technique which allows the separation of interfering signals and the extraction of small signals from noise. Surface wave magnitudes (Ms) are derived directly from the processor output. The Seismic Array Analysis Center (SAAC) in Alexandria, Virginia, receives the long-period seismic data transmitted continuously from the Large Aperture Seismic Array (LASA) in Montana and other large long-period arrays located in Alaska (ALPA) and Norway (NORSAR). As part of the evaluation of this network these data are processed to detect and measure Rayleigh waves associated with seismic events listed in the LASAISAAC Daily Event Summary. The choice of data processing methods was influenced by the fact that approxi- mately 100 events per day have to be processed in real time. Time domain beam- forming and plotting is a slow process which suffers from the disadvantage that the propagation azimuth and velocity must be assumed. Any disturbance visible on a particular beam may be associated with adjacent beams or with a side lobe. For these and other reasons, a frequency-wavenumber Cf-k) approach has been used for routine analysis of the LP data (Mack 1972). The heart of the processing scheme is the fast signal detection algorithm FKCOMB (Smart 1972), which makes used of frequency-wavenumber transforms. This refer- ence also describes the principles of thef-k analysis of transient waveforms. FKCOMB was originally used for the automatic processing of microbarograph array data (Smart & Flinn 1971). In this paper we merely summarize the method of analysis and outline how it is applied in the SAAC to detect Rayleigh waves and to obtain seismic surface wave magnitudes on a real-time basis. Frequency wavenumber analysis Frequency-wavenumber Cf-k) spectral estimation is a powerful technique for signal detection and waveform analysis of digitally recorded array data. The f-k spectrum of a given segment of array output is the squared modulus of the multi- dimensional Fourier transform of the data in time and space. Thef-k spatial represen- tation of a propagating wave is shown schematically in Fig. 1. Using discrete Fourier t Present address: Exploration Services Center, Mobd Oil Corporation, Dallas, Texas 75221.

The Summer Arctic Boundary Layer during the Arctic Ocean Experiment 2001 (AOE-2001)

Boundary-layer measurements made from the Swedish icebreaker Oden during the Arctic Ocean Experiment 2001 (AOE-2001) are analysed. They refer mainly to ice drift in the central Arctic during the period 2-21 August 2001. On board Oden a remote sensing array with a wind profiler, cloud radar and a scanning microwave radiometer, and a regular weather station operated continuously; soundings were also released during research stations. Tur- bulence and profile measurements on an 18-m mast were deployed on the ice, along with two sodar systems, a microbarograph array and a tethered sounding system. Surface flux and meteorological stations were also deployed on nearby ice floes. There is a clear diurnal cycle in radiation and also in wind speed, cloud base and visibility. It is absent in temperature and humidity, probably due to the very strong control by melting/ freezing ice and snow. In the advection of warm air, latent heat of melting maintains the surface temperature at 0 � C, while with a negative energy balance the latent heat of freezing of the salty ocean water acts to maintain the surface temperature >)2 � C. The constant presence of water at the surface maintains a relative humidity close to 100%, and this is also often facilitated by an increasing specific humidity through the capping inversion, making entrainment a moisture source. This ensures cloudy conditions, with low cloud and fog prevailing most of the time. Intrusions of warm and moist air from beyond the ice edge are frequent, but the local Arctic boundary layer remains at a relatively constant temperature, and is shallow and well mixed with strong capping inversions. Power spectra of surface-layer wind speed sometimes show large variance at low frequency. A scanning radiometer provides a monitoring of the vertical thermal structure with a spatial and temporal resolution not seen before in the Arctic. There are often two inversions, an elevated main inversion and a weak surface inversion, and occasionally additional inversions occur. Enhanced entrainment across the main inversion appears to occur during frontal passages. Variance of the scanning radiometer temperatures occurs in large pulses rather than varying smoothly, and the height to the maximum variance appears to be a reasonable proxy for the boundary-layer depth.

Observing Coherent Boundary Layer Motions Using Remote Sensing and Surface Pressure Measurement

Abstract In this paper, high-frequency pressure time series measured by microbarographs are used to extract information on the existence and characteristics of convective rolls in the convective boundary layer. Rolls are identified in radar and satellite data, and it is shown that the pressure signals associated with the rolls have been detected in an array of microbarographs. The methodology of obtaining further information on roll characteristics from the array, notably orientation and drift velocity, is discussed in some detail. It is shown that the pressure time series contain signals representing the roll motion, approximately normal to the mean wind, and signals representing turbulent structures that drift along the mean wind direction. As the along-wind signals may dominate the time series, care is needed to identify the roll motion. Filtering of the higher-frequency along-wind signals can isolate the roll motion details. Also, a new approach using “beam-steering diagrams” to discriminate rolls fro...

Acoustic propagation and atmosphere characteristics derived from infrasonic waves generated by the Concorde.

Infrasonic signals generated by daily supersonic Concorde flights between North America and Europe have been consistently recorded by an array of microbarographs in France. These signals are used to investigate the effects of atmospheric variability on long-range sound propagation. Statistical analysis of wave parameters shows seasonal and daily variations associated with changes in the wind structure of the atmosphere. The measurements are compared to the predictions obtained by tracing rays through realistic atmospheric models. Theoretical ray paths allow a consistent interpretation of the observed wave parameters. Variations in the reflection level, travel time, azimuth deviation and propagation range are explained by the source and propagation models. The angular deviation of a ray's azimuth direction, due to the seasonal and diurnal fluctuations of the transverse wind component, is found to be approximately 5 degrees from the initial launch direction. One application of the seasonal and diurnal variations of the observed phase parameters is the use of ground measurements to estimate fluctuations in the wind velocity at the reflection heights. The simulations point out that care must be taken when ascribing a phase velocity to a turning height. Ray path simulations which allow the correct computation of reflection heights are essential for accurate phase identifications.

A Ground-Based Network for Atmospheric Pressure Fluctuations

Abstract In 1992, a surface-based, mesoscale microbarograph array with four pressure sensors was installed near the Hohenpeisenberg, southern Germany, and has since been in continuous operation. In this paper, a description of the sensors, the network, and the data evaluation is given. The sensors are based on conventional microphone techniques, where the pressure difference between the ambient air and an internal reservoir is measured. The latter is connected with the former by an adjustable needle valve. Pressure fluctuations are resolved with an amplitude resolution of 3 µb and between periods of 2 s and 30 min. Sensors are calibrated by lifting over a given height. Time constants are determined with a pressure-pulse technique and are on the order of 300 s. Data are sampled at 1 Hz and are transmitted on-line to a central data processing unit. Each sensor is installed at the bottom of a 1.50-m-high container, which is mounted flush with the ground. The sensor is thermally insulated and protected such t...

Use of Ducting Theory in an Observed Case of Gravity Waves

Abstract This paper attempts to explain the properties of a gravity wave event observed on Mallorca (Balearic islands) using an array of microbarographs. The waves propagated coherently for a relatively long distance with a speed of about 29 m s−1 and were nondispersive. In the absence of a continuous forcing, long-lived atmospheric gravity waves can only exist if some mechanism prevents the vertical leakage of energy through wave propagation, trapping the gravity wave in a duct layer near the surface. Lindzen and Tung showed the necessary conditions for trapping and discussed the properties of neutral modes for constant stability and wind in the duct. The role of shear in the wind profile near the ground is examined here by first finding three distinct neutral mode types using a matrix eigenvalue method. Applying the wave theory given by Booker and Bretherton, those neutral modes in the sheared duct that have critical levels within the stable duct will be mostly absorbed at their critical level. Therefor...

Gravity‐Wave Observations Using an Array of Microbarographs In the Alearic Islands

AbstractTime series of surface pressure from an array of microbarographs in the Balearic Islands (western Mediterranean) are used to obtain wavelength, phase speed and direction of propagation of gravity waves on the Balearic Islands. These pressure waves are commonly found during the summer and they force the large seal‐level oscillations observed in the inlet of Ciutadella; a phenomenon locally known as ‘rissaga’. Four events are described here, each of which has a duration of between 30 and 48 hours. Radiosonde data from Palma de Mallorca are used to relate the waves to the vertical structure of the atmosphere. the various source mechanisms of these waves and also the possibility of the existence of a duct layer trapping the wave energy near the surface are discussed. the waves are found to have no significant variation of phase speed with frequency and so they are basically non‐dispersive. This is an indicator pointing either to a local generation of gravity waves by wind‐shear instability or to the existence of neutral trapped waves, in the lower troposphere.

Meteorological Tower, Microbarograph Array, and Sodar Observations of Solitary-like Waves in the Nocturnal Boundary Layer

Five short-duration disturbances of the stable nocturnal boundary layer, ranging in depth from about 15 m to over 500 m, were observed using a 300 m meteorological tower, 2 sodars, and a 4-station microbarograph array. Four of the events showed characteristic pulse-like sodar signatures; the fifth occurred within the minimum range of the sodars. We show that the events have the characteristics expected of strongly nonlinear deep-fluid internal solitary waves that are transporting and recirculating cold air at a supercritical velocity. The unique sodar signatures, tracing out the form of the waves, are attributed to small-scale turbulent temperature inhomogeneities marking the boundary of the recirculating air as it is transported through the ambient nocturnal boundary layer by the wave. All the events are shown to deviate from the Benjamin-Davis-Ono (BDO) theory, because of their amplitude/wavelength and amplitude/stable layer thickness ratios are considerably larger than normally thought appropriate for the weakly nonlinear BDO theory. These events exhibit characteristics that are consistent with those predicted by the nonlinear theory of Tung et al.

A Climatology of Gravity Waves and Other Coherent Disturbances at the Boulder Atmospheric Observatory during March–April 1984

Gravity waves and other coherent disturbances observed at the Boulder Atmospheric Observatory (BAO) during the period between mid-March and mid-April 1984 were studied using data collected by an array of microbarographs located around the 300-m meteorological tower, together with data from the tower. Five passbands were considered in the period range 1-20 min; it was found that, for most of the time, the atmospheric state at these passbands displayed highly coherent structure. For disturbances of up to 5-min periods, a relationship was found between the turbulent kinetic energy measured on the tower and the amplitude of the rms pressure field at the ground, but, for longer periods, no such relationship was found.

A Diagnostic Study of the Influence of a Gravity Wave upon Regional Weather

Abstract On 20 May 1977 a periodic pressure disturbance over the upper Midwest was detected by synoptic and mesoscale microbarograph arrays. Analysis indicates that a pressure wave with an amplitude of approximately 1.0 mb, a phase speed of approximately 14 m s−1, a period of between 1.5 and 3 h, and a horizontal wavelength of approximately 100 km moved through Illinois, Iowa and Wisconsin from the south within the warm sector of an occluded cyclone. We interpret them pressure data as the signature of an internal atmospheric gravity wave. At several locations convective precipitation occurred coincident with the advancing ridges of the pressure wave, suggesting that the gravity wave influence convection. Openings in the cloud cover correlated with the location of the gravity wave. Simple modeling confirms the destabilization potential of the gravity wave.

The Design and Operation of a Microbarograph Array to Measure Pressure Drag on the Mesoscale

During the ALPEX field program, pressure was measured along three Alpine cross sections. In order to attain high accuracy, pressure sensors had to be protected against the influence of wind-induced dynamic pressure effects and against gravity wave signals. Corrections based on pre- and postexperiment laboratory and on biweekly field calibrations enabled the compilation of a pressure dataset with an estimated absolute accuracy of 0.3 hPa and a relative accuracy of about 0.1 hPa. Time series plots and time-space plots of the data were generated for quality control purposes.

High Temporal-Resolution Infrared-Radiometric Observations of the Atmospheric Boundary Layer

An experiment designed to study the relationship of radiometric fluctuations to those of temperature and associated variables in the atmospheric boundary layer is described. Simultaneous and colocated observations were conducted by a ground-based infrared radiometer, fast response temperature, humidity, and wind sensors at 8 levels on the 300-m meteorological tower of the NOAA Boulder Atmospheric Observatory, an acoustic sounder, and a microbarograph array. The observed events included the arrival and development of a drainage flow, atmospheric waves, and convective plume activity. The analysis of the time series of the radiometer output compared with the directly measured meteorological variables and with the acoustic sounder record points out the potential of the ground-based radiometric technique for remote sensing of atmospheric fluctuation phenomena.

Traveling atmospheric pressure waves measured during a solar eclipse

An array of microbarographs near Saskatoon, Saskatchewan, detected traveling pressure waves at a time and with a direction of motion that indicates an association with the solar eclipse of February 26, 1979. The velocity was 10 m/s moving toward azimuth 65°; the wave period and amplitude were approximately 120 s and 12.0 Pa, respectively; the duration of the wave train was approximately 3 hours. The probability that it was a free gravity wave is examined and rejected. A plausible source mechanism similar to the low level nocturnal jet is suggested.

Utilization of seismically recorded infrasonic‐acoustic signals to monitor volcanic explosions: The El Chichon Sequence 1982—A case study

Infrasonic‐acoustic signals from five explosive eruptions of El Chichon volcano during March 29 through April 4, 1982, were recorded by a microbarograph array and Seismic Research Observatory (SRO) seismograph collocated near McKinney, Texas. Analyses of these signals for frequencies from 0.0033 to 0.025 H have demonstrated functional relationships of amplitude to frequency consistent with ω−1 for microbarometric data, ω−2 for vertical mode seismic data, and ω−3 for radial mode seismic data. These observed slopes are in agreement with those theoretically predicted by Sorrells (1971). Estimations of kinetic energy releases by the explosions of El Chichon are presented and range from 4.9×1022 ergs to 2.0×1023 for the largest eruption on April 4, 1982, to 4.2×1021 to 6.1×1021 ergs for the smallest eruption on April 3, 1982. The method of Posey and Pierce (1971) to estimate the explosivity from long‐period infrasonic signals is not likely to be applicable, strictly speaking, to the shorter‐period data. Thus it is believed that the energy release for the smallest eruption is overestimated by possibly an order of magnitude. Comparable estimations of volcanic explosivity from SRO radial seismic data and microbarometric array data strongly recommend using SRO and International Deployment Accelerometers network data for far‐field analyses of volcanic explosions and construction of a rigorous volcanic explosivity scale.