[1] The effects of variability of the ground on land mine detection and false alarm rates are analyzed within the framework of a viscoelastic-layered model of the ground. A matrix technique was used to describe sound interaction with layered viscoelastic ground. The resonance method in combination with a global search method is used to estimate a set of parameters for a three-layered viscoelastic ground model. Results of the estimation show good agreement between computed and experimental data. The effect of a finite size of sound source on the acoustic-to-seismic transfer function is discussed. The effect of variability of ground properties on the acoustic-to-seismic transfer function (admittance function) is analyzed. Analysis is performed on the plane of parameters of the layered ground in a wide frequency range for all angles of incidence. It is revealed that small variations in the shear speed in the top layer of ground will not cause variation in the acoustic-to-seismic transfer function at low frequencies but may cause strong variation at high frequencies. Results of outdoor measurements of the acoustic-to-seismic transfer function are presented, and a correlation between the high magnitude of the acoustic-toseismic transfer function in certain frequency ranges and moisture content on the surface is revealed. A simple model explaining the correlation between moisture content in the upper layer, the acoustic-to-seismic transfer function, and ground properties is suggested.
Agricultural, hydrological and civil engineering applications have realized a need for information of the near subsurface over large areas. In order to obtain this spatially distributed data over such scales, the measurement technique must be highly mobile with a short acquisition time. Therefore, some type of remote sensing or geophysical technique must be utilized. Geophysical measurements are sensitive to the distribution of physical properties, such as the electrical conductivity, dielectric constant, mass density, mechanical properties, etc., of the ground. In most instances, the geophysical properties must be reconciled with physical properties used by the soil scientist. This paper presents a preliminary investigation of the use of acoustic to seismic (A/S) coupling measurements for measuring the depth to the top of the fragipan horizon. Fragipans influence the hydrology and ecohydrology of the soil on a field scale. A suite of traditional geophysical measurements was taken to characterize the soils at two sites with different depths to the fragipan horizon. Data at these two field sites indicate that this A/S technique is sensitive to the spatial variability of the depth to the fragipan horizon. At present, inversion of the A/S data for the fragipan depth requires use of data from a separate geophysical measurement or soil cores to provide a field calibration.
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. Measurements are reported for the hose oriented both in the direction and perpendicular to the direction of the sound source. [Work supported by the Space and Missile Defense Command is gratefully acknowledged.]
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.
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