Curt A. L. Szuberla

Uncertainties associated with parameter estimation in atmospheric infrasound arrays

This study describes a method for determining the statistical confidence in estimates of direction-of-arrival and trace velocity stemming from signals present in atmospheric infrasound data. It is assumed that the signal source is far enough removed from the infrasound sensor array that a plane-wave approximation holds, and that multipath and multiple source effects are not present. Propagation path and medium inhomogeneities are assumed not to be known at the time of signal detection, but the ensemble of time delays of signal arrivals between array sensor pairs is estimable and corrupted by uncorrelated Gaussian noise. The method results in a set of practical uncertainties that lend themselves to a geometric interpretation. Although quite general, this method is intended for use by analysts interpreting data from atmospheric acoustic arrays, or those interested in designing and deploying them. The method is applied to infrasound arrays typical of those deployed as a part of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization.

A study of Pc 3 coherence at cusp latitudes

Previous studies have taken data from multi-instrument arrays, both ground-based and in situ, to determine the spatiotemporal characteristics of Pc 3 pulsations (22-100 mHz) at cusp latitudes. Correlations of Pc 3 frequency band magnetometer, photometer, and riometer data have been cited as evidence of a local (ionospheric), rather than a distant (magnetospheric), source for these cusp pulsations. Electron precipitation modulated at Pc 3 frequencies is thought to be the source mechanism for the unstructured portion of the Pc 3 spectrum; such precipitation can modify local ionospheric conductivities, allowing modulated currents to flow at like frequencies. In this paper we undertake a statistical analysis of induction coil magnetometer records taken at Cape Parry and Sachs Harbor, Canada, in May-June 1985. Comparing the distribution functions of interstation X-X and Y-Y polarization and coherence estimates, we arrive at a simple model to estimate an upper bound for the coherence length, 0(200 km), of Pc 3 source regions near the cusp. Such a determination is an important constraint in identifying the source of Pc 3 frequency band modulations of cusp latitude electron precipitation.

High-Altitude Infrasound Calibration Experiments

H. E Bass, E. T Herrin, P. Golden, R. Woodward, D. Drob, M. A H Hedlin, C. De Groot-Hedlin, K. Walker, M. Garces , C. Szuberla and R. Whitaker The University of Mississippi NCPA, 1 Coliseum Drive, University, MS 38677, USA Southern Methodist University, P. O. Box 750395, Dallas, TX 75275, USA Incorporated Research Institutions for Seismology, 1200 New York Avenue, NW, Suite 800, Washington, DC 20005, USA Naval Research Laboratory, Space Science Division, 4555 Overlook Avenue, Washington, DC 20375, USA University of San Diego California, Scripps Institute of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, USA Infrasound Laboratory, University of Hawaii, 73-4460 Queen Kaahumanu Highway #119, Kailua-Kona, HI 96740, USA University of Alaska, 903 Koyukuk Drive, Fairbanks, AK 99775, USA Los Alamos National Laboratory, EES-2 MS J577, Los Alamos, NM 87545, USA

Distribution of wave packet sizes in microbarom wave trains observed in Alaska

This work reports on a study of the distribution of wave packet sizes contained in intervals of continuous microbarom activity. Microbaroms are a class of atmospheric infrasound that is characterized by narrow-band, nearly sinusoidal, waveforms with periods near 5 s. They are known to be generated by marine storms, presumably through a nonlinear interaction of surface waves, however the detailed analysis of the process is still incomplete. The data analyzed were obtained using the University of Alaska infrasound array of four microphones located in central Alaska. Because of the narrow-band feature of the microbarom signals, the Hilbert transform is applicable as a method for finding phase breaks in the signal. The phase breaks are interpreted as the demarcation of the boundaries of wave packets. When applied to long sequences of microbaroms a broad distribution of packet lengths is found that diminishes monotonically with length and has a mean near 10 cycles and a variance nearly as large. The distribution function decreases exponentially with packet length. The distribution of packet sizes is influenced by the presence of multiple sources and multiple propagation paths between the sources and the sensor array. Identification of individual packets should open the way to a more detailed analysis of microbarom wave trains. After separating the wave train into individual wavelets the intermicrophone correlation is estimated as a function of microphone separation. As has been observed in earlier microbarom studies, a decrease in correlation was observed for microphone pairs orthogonal to the direction of propagation when compared to correlations between microphones spaced along the direction of wave propagation.

Interstation Pc3 coherence at cusp latitudes

Magnetic fluctuations in the 22–100 millihertz (Pc3) band are a consistent indicator of the presence of the cusp in the overhead ionosphere at high latitudes. Correlation of the signals from a variety of instruments have shown that the sources of these pulsations are local (ionospheric) rather than distant (magnetospheric) [Engebretson et al., 1990]. Modulated electron precipitation is presumed to be the source of the fluctuations through the modulations in ionospheric conductivity that they produce. Olson and Szuberla [1997] used data from a pair of cusp stations to deduce the scale size of the precipitating beams using a simple model in which the beams were assumed to have circular cross section. They obtained an upper bound for the coherence length of the order of 200 km. In this paper we extend the analysis of Olson and Szuberla by incorporating data from the Magnetometer Array for Cusp and Cleft Studies (MACCS) magnetometer array and the Australian ANARE antarctic sites to give a broader range of station separations. Using a statistical approach we computed the cumulative distribution function of the interstation coherence and from that distribution we established a measure of coherence, CL. The result of this analysis is a coherence that diminishes with inter-station distance as CL ≈ 1.4 exp(−S/250) where S is the station separation in km. When this result is interpreted in the context of the simple model mentioned above we find a coherence length of 140–180km.

Explosion localization via infrasound

Two acoustic source localization techniques were applied to infrasonic data and their relative performance was assessed. The standard approach for low-frequency localization uses an ensemble of small arrays to separately estimate far-field source bearings, resulting in a solution from the various back azimuths. This method was compared to one developed by the authors that treats the smaller subarrays as a single, meta-array. In numerical simulation and a field experiment, the latter technique was found to provide improved localization precision everywhere in the vicinity of a 3-km-aperture meta-array, often by an order of magnitude.

High‐altitude infrasound calibration experiments

At the 152nd Meeting of the Acoustical Society of America, Andre and Bass reported an infrasound experiment conducted at White Sands Missile Range during the 2005‐2006 time frame. The experiment consisted of exploding a 22.4 kg charge at altitudes from 31.3 km to 49.6 km then recording the waveforms at 30 infrasound arrays (not all at the same time) at distances up to 1200 km from the source. The analysis is not yet complete but some preliminary observations have been reported in the most recent issue of Acoustics Today. This talk will summarize the findings published in Acoustics Today and offer suggestions to others who might want to access and analyze the data.

High-latitude Observations of Infrasound from Alaska and Antarctica: Mountain Associated Waves and Geomagnetic/Auroral Infrasonic Signals

The Geophysical Institute at the University of Alaska Fairbanks has established and operated seven different infrasonic microphone arrays in Alaska, Canada, Sweden, and Antarctica from 1965 to the present in a continuing effort to study natural sources of infrasound, at high latitudes, in the pass band from 0.015 to 10 Hz. Recently, in association with the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization [ADD REF TO CHAPTER 2], modern 8-microphone infrasound arrays, with digital data-acquisition at 20 Hz, were installed as I55US in Windless Bight, Antarctica (2001) and as I53US in Fairbanks, Alaska (2002). Coherent infrasonic signals, observed over the period 2000–2008 at both stations, are studied here for both mountain associated waves (MAW) and unique high trace-velocity signals associated with geomagnetic and auroral activity at high latitude regions.

Seismic Envelope‐Based Detection and Location of Ground‐Coupled Airwaves from Volcanoes in Alaska

Abstract Volcanic explosions and other infrasonic sources frequently produce acoustic waves that are recorded by seismometers. Here we explore multiple techniques to detect, locate, and characterize ground‐coupled airwaves (GCA) on volcano seismic networks in Alaska. GCA waveforms are typically incoherent between stations, thus we use envelope‐based techniques in our analyses. For distant sources and planar waves, we use f ‐ k beamforming to estimate back azimuth and trace velocity parameters. For spherical waves originating within the network, we use two related time difference of arrival (TDOA) methods to detect and localize the source. We investigate a modified envelope function to enhance the signal‐to‐noise ratio and emphasize both high energies and energy contrasts within a spectrogram. We apply these methods to recent eruptions from Cleveland, Veniaminof, and Pavlof Volcanoes, Alaska. Array processing of GCA from Cleveland Volcano on 4 May 2013 produces robust detection and wave characterization. Our modified envelopes substantially improve the short‐term average/long‐term average ratios, enhancing explosion detection. We detect GCA within both the Veniaminof and Pavlof networks from the 2007 and 2013–2014 activity, indicating repeated volcanic explosions. Event clustering and forward modeling suggests that high‐resolution localization is possible for GCA on typical volcano seismic networks. These results indicate that GCA can be used to help detect, locate, characterize, and monitor volcanic eruptions, particularly in difficult‐to‐monitor regions. We have implemented these GCA detection algorithms into our operational volcano‐monitoring algorithms at the Alaska Volcano Observatory.

Discrimination of near-field infrasound sources based on time-difference of arrival information

A computationally efficient method for discriminating between near- and far-field infrasound sources using array time-difference of arrival (TDOA) information is described. Rather than assess wave-front curvature, the discriminant quantifies the statistical departure of TDOA information from that of a plane wave passing the array. Since the method constrains neither the functional form nor the amplitude characteristics of a signal it is suited for discrimination of signals across large-aperture infrasound arrays. Experimental results confirm theoretical predictions to a range of order ten array apertures. The discriminant is applied to data from an Antarctic infrasound array.