Atmospheric Acoustic Remote Sensing - Chapter 4 ppt

4 Sound Transmission and Reception The essence of an acoustic remote-sensing system is in generating sound into a wellformed beam which interacts with the atmosphere in a known manner and then detecting that interaction In Chapter we learned about the nature of the atmosphere into which the sound is projected, and in Chapter the way in which sound travels In this chapter we describe how to form a beam of sound, how scattered sound is detected, and how systems are designed to optimize retrieval of various atmospheric parameters The main emphasis of Chapter is on geometry and timing, but details on some of these aspects are left to Chapter 4.1 GEOMETRIC OBJECTIVE OF SODAR DESIGN The boundary layer atmosphere is often strongly varying in the vertical, but horizontally much more homogeneous The geometric design objective for vertically profiling instruments is therefore to localize the acoustic power sufficiently in space so that atmospheric properties are obtained from well-defined height intervals at a particular time This means that the vertical resolution has to be defined, typically by using a pulsed transmission But since sound will spread spherically from the source, height resolution also depends on angular width of the beam transmitted Here we concentrate on SODAR (SOund Detection And Ranging) systems, for which the acoustic beams are often non-vertical, as shown in Figure 4.1 Here the pulse duration is and the angular width of the acoustic beam is ±∆ in azimuth angle and ±∆ in zenith angle From Figure 4.1, the vertical extent of the pulse volume is ≈ c cos + 2z∆sin ∆ , which has a term increasing with height z Taking c =20 m, and = 20°, the vertical extent of the pulse volume near the ground is c cos = 18.8 m but, for a beam half-width of ∆ = 5°, this increases to 50 m at z = 500 m This emphasizes the need to keep the product sin ∆ small Also, if ∆ is too large then the pulse volume will include a wide range of radial velocity, the Doppler spectrum will be wider, and the ability to detect the peak position of the Doppler spectrum, in the presence of noise, will be compromised But we will see later that the wind velocity component estimates of u and v have errors which depend on 1/sin , so it is important that not be too small On the other hand, must also not be so large that the volumes sampled by the various SODAR beams which point in different directions, are so spatially separated that their wind components become uncorrelated The resulting design must therefore be a delicate balance between modest values and a narrow beam width ∆ Typical designs have 15°<