Catherine de Groot-Hedlin

Synthesis of earthquake-generated T-waves

T-phases excited by earthquakes propagate with low transmission loss along the ocean sound channel, and are characterized by long duration wavetrains, often having multiple peaks associated with bathymetric highs in the source region. Although T-waves were first identified almost fifty years ago, their activation has not been adequately explained. In this paper we show that, for continental margin earthquakes, realistic T-phase coda can be computed by expressing the acoustic energy in terms of several low order modes excited by point sources distributed uniformly over the seafloor. The conversion of seismic energy to acoustic energy is thus consistent with seafloor scattering.

Inversion of magnetotelluric data for 2D structure with sharp resistivity contrasts

We have developed a linearized algorithm to invert noisy 2-D magnetotelluric data for subsurface conductivity structures represented by smooth boundaries defining sharp resistivity contrasts. We solve for both a fixed number of subsurface resistivities and for the boundary locations between adjacent units. The boundary depths are forced to be discrete values defined by the mesh used in the forward modeling code. The algorithm employs a Lagrange multiplier approach in a manner similar to the widely used Occam method. The main difference is that we penalize variations in the boundary depths, rather than in resistivity contrasts between a large number of adjacent blocks. To reduce instabilities resulting from the breakdown of the linear approximation, we allow an option to penalize contrasts in the resistivities of adjacent units. We compare this boundary inversion method to the smooth Occam inversion for two synthetic models, one that includes a conductive wedge between two resistors and another that includes a resistive wedge between two conductors. The two methods give good agreement for the conductive wedge, but the solutions differ for the more poorly resolved resistive wedge, with the boundary inversion method giving a more geologically realistic result. Application of the boundary inversion method to the resistive Gemini subsalt petroleum prospect in the Gulf of Mexico indicates that the shape of this salt feature is accurately imaged by this method, and that the method remains stable when applied to real data.

Excitation of T-phases by seafloor scattering

T-phases excited by suboceanic earthquakes are classified into two types: abyssal phases which are excited near the earthquake epicenter at seafloor depths far below the SOFAR velocity channel, and slope T-phases which are excited at continental, or ocean island slopes and ridges at distances up to several hundreds of kilometers from the epicenter. In this article, it is demonstrated that approximate time-frequency characteristics of both classes of T-phase can be synthesized under the assumption that T-phases are excited by scattering from a rough seafloor. Seafloor scattering at shallow depths preferentially excites low order acoustic modes that propagate efficiently within the ocean sound channel minimum. At greater depths, scattering excites higher order modes which interact weakly with the seafloor along much of the propagation path. Using known variations in near-source bathymetry, T-phase envelopes are synthesized at several frequencies for several events south of the Fox Islands that excited both types of T-phase. The synthesized T-phases reproduce the main time vs frequency features of each type of arrival; a higher frequency, nearly symmetric arrival excited near the epicenter and a longer duration, lower frequency arrival excited near the continental shelf, with a peak amplitude at about 5 Hz.

Western U.S. Infrasonic Catalog: Illuminating infrasonic hot spots with the USArray

[1] In this study reverse time migration is applied to signals recorded by the 2007–08 USArray, presumably due to acoustic-to-seismic coupling, to detect and locate in two-dimensional space and time 901 sources of atmospheric infrasound, defining the Western United States Infrasonic Catalog (WUSIC). The detections are visually inspected and ranked. Uncertainties are estimated using a bootstrap technique. The method correctly locates most rocket motor detonations in Utah and a bolide explosion in Oregon with an average spatial accuracy of 50 km and 25 km, respectively. The origin time statistics for 2007 and 2008 events are nearly identical and suggest a predominant human origin. The event locations illuminate repeating sources of infrasound, or “infrasonic hot spots,” in Nevada, Utah, and Idaho that are spatially associated with active military areas. The infrasonic arrivals comprise several branches that are observed to a range between 200 and 1500 km to the east and west of the epicenter in the winter and summer, respectively. The optimum group velocities are Gaussian distributed and centered at 295 m/s. A seasonal variation in optimum group velocities exhibits good correlation with atmospheric temperature. The results show that relatively dense seismic networks fill in the gaps between sparsely located infrasound arrays and provide valuable information for regional infrasonic source location and propagation studies. Specifically, the catalogs presented here can be used to statistically validate and improve propagation models, especially above the middle stratosphere where winds are not directly measured by ground-based weather stations or meteorological satellites.

Atmospheric Variability and Infrasound Monitoring

The propagation of infrasound through the troposphere, stratosphere, and thermosphere is primarily controlled by the thermal structure of the atmosphere, winds, atmospheric attenuation, and reflections from the ground terrain. It is well known that the temperature and circulation of the atmosphere vary continuously, both temporally and spatially, and that solar heating drives this change. Studies show that atmospheric absorption, and thus signal attenuation, is most severe in the thermosphere. A detailed knowledge of the variations in the atmosphere - with time, altitude, and geographic location - is needed to correctly interpret infrasound waveforms to extract useful information about infrasound sources. This will provide information that could be used to correctly identify the source, to estimate its location and time, and to predict if along a particular path a signal should be detected above noise. Much progress has been made in our understanding of the atmosphere; however, methods continue to be developed with the goal of defining atmospheric structure at a fine enough scale to accurately synthesize waveforms. A key component of this development and validation of our atmospheric modeling capability is the study of ground-truthed atmospheric events. We present results from selected case studies that shed light on the temporal and spatial variations in sound propagation characteristics to provide an overview of sound transmission through our ever-changing atmosphere and our quest for accurate atmospheric models.

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

Testing low/very low frequency acoustic sources for basin-wide propagation in the Indian Ocean

Low/very low frequency acoustic signals were transmitted to distant receivers in the Indian Ocean. The aim was to test methods for characterizing the hydroacoustic capability of the International Monitoring System (IMS) that discriminates for nuclear tests in the region. Several acoustic sources were deployed between Seychelles and Fremantle, Australia, and the IMS receivers comprised a network of hydrophones off Diego Garcia and Australia. Two of the three acoustic sources tested produced basin-scale propagation of impulsive signals. Single glass spheres imploded within the sound channel produced a clear signal at frequencies above ∼40 Hz, at ranges of hundreds to a thousand kilometers. Five-sphere glass implosions were recorded at ranges up to 4400 km. Near-sea surface shots from a large airgun array were recorded in several cases at ranges of hundreds to thousands of kilometers, the frequency of the highest signal-to-noise ratio arrivals varied within the 5–100 Hz band. High background noise level was ...

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.

A Case Study on the Far-Field Properties of Propagating Tropospheric Gravity Waves

AbstractMesoscale gravity waves were observed by barometers deployed as part of the USArray Transportable Array on 29 June 2011 near two mesoscale convective systems in the Great Plains region of the United States. Simultaneously, AIRS satellite data indicated stratospheric gravity waves propagating away from the location of active convection. Peak perturbation pressure values associated with waves propagating outside of regions where there was precipitation reached amplitudes close to 400 Pa at the surface. Here the origins of the waves and their relationship to observed precipitation are investigated with a specialized model study. Simulations with a 4-km resolution dry numerical model reproduce the propagation characteristics and amplitudes of the observed waves with a high degree of quantitative similarity despite the absence of any boundary layer processes, surface topography, or moist physics in the model. The model is forced with a three-dimensional, time-dependent latent heating/cooling field that...

Criteria for discretization of seafloor bathymetry when using a stairstep approximation: Application to computation of T-phase seismograms

Acoustic solutions for numerical models in which an overly coarse discretization of a stairstep boundary is employed to simulate smoothly varying bathymetry are degraded in a way that simulates scattering. Geometrical optics approximations are used to derive discretization criteria for simulating a smoothly sloping interface for the case of a source embedded in either an acoustic or an elastic seafloor, and applied to modeling T-phases. A finite difference time-domain modeling approach is used to synthesize T-phases for both smoothly sloping and rough seafloor boundaries. It is shown that scattering at a rough seafloor boundary yields ocean-borne acoustic phases with velocities near those of observed T-phase, while smooth seafloor models yield T-phases with slower horizontal velocities. The long duration of the computed T-phases for both the rough acoustic and elastic models is consistent with energy being scattered into the sound channel both as it transits the ocean/crust boundary, as well as at several...