Wheeler B. Howard

Acoustical properties of porous hose wind noise filters

Observation of infrasound signals in the atmosphere is often masked by wind noise. A common means of filtering out the wind noise is to connect a commercial porous (or soaker) hose to the manifold of a micro barometer. The filtering effect is attributed to the ability of the porous hose to average the pressure variations over its length. The pressure variations due to the turbulent wind field are incoherent on a scale equal to the hose length and, therefore, are reduced in the averaging process. Infrasound signals, with wavelengths much longer than the hose, are reduced little in the averaging process. There remains the question, ‘‘How does the porous hose respond to infrasound signals that have wavelengths comparable to the hose length?’’ To answer this question, measurements have been made of infrasound signals radiated from a jet engine during takeoff using an infrasound sensor with a porous hose connected to its port, and an array of infrasound sensors distributed along the length of the porous hose. ...

Advances in distributed arrays for detection of infrasonic events

Infrasound arrays normally consist of four to eight microbarometers spaced kilometers apart. Each of these microbarometers is connected to a pipe or porous hose array to reduce wind noise. This presentation describes a 100 sensor all weather distributed array and its use in continuously logging infrasound signals over an extended period of time. The array has been deployed next to a conventional porous hose array connected to a Chaparral 2.5 microbarometer. The effectiveness of the two arrays in canceling wind noise and detecting infrasound signals is compared. The hardware and software used by the distributed array to log the data and to process it so as to cancel wind noise, identify and separate the infrasound signals, and locate their sources will be discussed.

An investigation of the correlation of infrasound signals as a function of sensor separation and wind velocity

Wind noise is a common obstacle to the detection and analysis of infrasonic signals. The masking caused by the wind noise is often reduced by spatial averaging via pipe or porous hose arrays. An alternate method is to average the signals from multiple sensors in distributed arrays. In order to reduce the wind noise, the sensors in the distributed array must be far enough apart so that their wind noise signals are incoherent, but not so far apart that the sound loses its coherence. Few measurements have been reported of the infrasound correlation distance in the presence of wind. This paper reports results of measurements of this infrasound correlation when the infrasound sensors are separated by distances up to hundreds of meters. Techniques for combining infrasound signals from widely separated sensors and measuring their correlation in the presence of wind noise are discussed.

Locating infrasound events in wind with dense distributed arrays

Applications for infrasound suffer due to the presence of wind noise in the sensor data. Several techniques have been and are being used to mitigate the influence of wind noise in locating infrasonic signals of interest. One of these methods is the averaging of data from multiple sensors in a distributed array. A distributed array was employed to study infrasonic signals from airborne sources. Wind noise during testing masked the location of the infrasonic signals in the time domain. Postprocessing techniques using statistical measures for dense arrays were employed to recover the onset of the infrasonic events. Data from these tests and an explanation of the postprocessing techniques will be discussed.

Acoustic to seismic signatures of layered near‐surface soil

The near‐surface soil structure, which strongly influences agricultural productivity and soil erosion, is amenable to study by acoustic to seismic techniques. These techniques utilize the coupling between airborne sound and seismic vibrations of the soil to investigate the distribution of soil’s mechanical properties. Two field sites were chosen over which the acoustic to seismic coupling signature of the ground, was measured. A predictive model, assuming acoustic plane waves incident on a vertically stratified ground was developed to predict the acoustic to seismic response. In this presentation, the acoustic to seismic signatures from these sites are compared to model results using the ground truth from seismic refraction and cone penetrometer surveys as input to the model.

Detecting blast-induced infrasound in wind noise

Current efforts seek to monitor and investigate such naturally occurring events as volcanic eruptions, hurricanes, bolides entering the atmosphere, earthquakes, and tsunamis by the infrasound they generate. Often, detection of the infrasound signal is limited by the masking effect of wind noise. This paper describes the use of a distributed array to detect infrasound signals from four atmospheric detonations at White Sands Missile Range in New Mexico, USA in 2006. Three of the blasts occurred during times of low wind noise and were easily observed with array processing techniques. One blast was obscured by high wind conditions. The results of signal processing are presented that allowed localization of the blast-induced signals in the presence of wind noise in the array response.

Determination of soil background parameters via acoustic-seismic transfer function

Inversion methods for estimation of geoacoustic model parameters often use the scattered field data for obtaining the properties of viscoelastic layered media. This work presents a method to retrieve soil background parameters using the outdoor acoustic-seismic transfer function (admittance). Clutter in landmine detection is related to with spatial variations of soil parameters, so knowledge of soil parameters and their spatial variability are very important for landmine detection. The resonance method is extended and used for preliminary estimation of a set of parameters for a three-layered ground model. The least squares method is later used to choose the model with the best fit to experimental data. Results of the reconstruction show good agreement with the experimental data. A description of the resonant technique and the experimental setup are presented. The effect of a finite size of the sound sources often used in acoustic landmine detection on the acoustic-seismic transfer function is also discussed.

Modal analysis of broadband acoustic signals propagating in the top layer of the ground

Experimental measurements of the frequency dependence of sound propagation losses in the top layers of the ground were performed in the frequency range between 80 and 420 Hz. These measurements revealed that the distribution of energy on the frequency‐range plane has a regular structure. The features of this structure were modeled in the modal approach for a gradient model of the ground. It was shown that average sound speed in the ground and the approximate depth of the layers may be estimated through analysis of the experimentally measured frequency‐range distribution of energy. The technique explores the waveguide properties of the top layer of the ground. According to the mode theory, two asymptotes coming from the point (0,0) should exist on the wave number‐frequency plane. From the slope of these asymptotes, the minimum and maximal sound speeds in the waveguide were found to be 100 and 530 m/s. The approximate depth of the layers was estimated. Comparisons of obtained sound speeds and depth of layer...

Variation in acoustic to seismic signatures associated with natural depth variability of hardpans

Acoustic to seismic coupling techniques have been successfully used in the investigation of near‐surface soils. Spatial variation in the soil profile and associated near‐surface properties are manifested as differences in the acoustic to seismic transfer function. Two field sites were chosen which differ by the average depth of a naturally occurring hardpan. In this presentation, the measured acoustic to seismic response at multiple point locations within these two agricultural field sites is presented. Variability in attributes of the acoustic to seismic transfer function, namely the frequency, half‐width, and amplitude of the resonance, are compared to variability of the depths to the hardpan.

Geoacoustic inversion via acoustic‐seismic transfer function

Inversion methods for estimation of geoacoustic model parameters often use the scattered field data for obtaining the properties of viscoelastic layered media. This work presents a method to retrieve ground properties information using the outdoor acoustic–seismic transfer function (admittance). The resonance method developed in A. Nagle, H. Uberall, and K‐B. Yoo [Inverse Prob. 1, 99–110 (1985)] is extended and used for a preliminary estimation of a set of parameters for a three‐layered ground model. The least squares method is used afterward to choose the model with the best fit to experimental data. Results of the reconstruction show good agreement with the experimental data. A description of the resonant technique and the experimental setup are presented. The effect of a finite size of sound source on the acoustic–seismic transfer function is also discussed. [Work supported by ONR.]