Info

distant storms and local winds. The ships are treated as discrete sources, and the shipping noise is propagated over great-circle routes to the receiver elements using a wide-angle, finite-element parabolic equation (FEPE) model. The arriving contributions from all ships are added coherently at the receiver, and the array response is determined by beamforming with various shading schemes. Environmental and shipping information is automatically extracted from US Navy standard databases: HITS for determination of shipping densities; DBDB for bathymetry; GDEM for sound-speed profiles; and LFBL for bottom-interaction parameters (refer to Chapter 10, specifically Table 10.5).

7.5 The noise notch

The so-called noise notch is often manifested in the vertical directionality of low-frequency noise fields generated by those noise models utilizing range-independent TL inputs. Figure 7.6 presents a vertical noise directionality diagram generated by the fast ambient-noise model (FANM). This particular example is for a low-frequency (50 Hz) case under the assumption of a range-independent ocean environment. Consistent with the conceptual picture discussed in Chapter 6 and earlier in this chapter, most of the low-frequency noise arrives near the horizontal. Furthermore, slightly higher

Vertical received angle (°)

Figure 7.6 Vertical noise directionality [dBre |xPa2(Hzstr-1)] computed by the FANM noise model at a frequency of 50 Hz. Rays with positive angles arrive at the receiver from below the horizontal plane.

Vertical received angle (°)

Figure 7.6 Vertical noise directionality [dBre |xPa2(Hzstr-1)] computed by the FANM noise model at a frequency of 50 Hz. Rays with positive angles arrive at the receiver from below the horizontal plane.

levels are observed in the direction of the sea surface than in the direction of the sea floor. This result is also expected in that the bottom-reflected rays are attenuated more than the corresponding surface-interacting rays are. Figures 7.5 and 7.6 both show this effect. The feature of real interest, however, is the horizontal notch (or null) that is predicted by range-independent ray theory. Since the notch feature is not always observed in measurements at sea, this situation presents a paradox. The generation of notch features is not limited to range-independent ray-tracing techniques. Carey et al. (1987), for example, were able to produce similar notch features using a PE model under similar range-independent environmental assumptions.

Anderson (1979) made measurements of the vertical directionality of noise in the North Pacific Ocean in September 1973. He did not observe the horizontal null (notch) in the directional spectrum of the noise that is predicted by ray theory for a horizontally homogeneous (range-independent) ocean.

This aspect can be explored further by examining basic ray-theoretical considerations in typical deep-ocean, range-independent environments. Specifically, this notch (or null) is the result of ray arrivals being excluded from a region that is centered on the horizontal axis of the receiver and limited in angular width to ±0R, where 0R defines the limiting ray as:

cs where 0R is the limiting ray angle at the receiver, cR the sound speed at the receiver position and cs the sound speed at the source position.

This corresponds to those rays leaving the source horizontally (0s = 0°). Equation (7.8) then follows directly from Snell's law. Figure 7.7 illustrates the geometry for defining the limiting ray. The sound-speed profile is based on measured data presented by Anderson (1979). Assuming that the noise source plane is near the surface, and that the receiver is located at 1,200 m (just below the deep-sound channel axis), then Equation (7.8) yields

Was this article helpful?

0 0

Post a comment