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developed for deep water (with shallow water being largely ignored), and the “simple model” approach to looking at scattering from the above list of ocean and seabed objects didn’t have enough knowledge of the ocean and seabed available as input to make it feasible. The Navy wanted operational numbers for array performance as quickly as possible, and so only one real option was available – direct mea- surement. As money for at-sea work was more plentiful then than today, that was in fact an attractive option. Thus Bill Carey took to sea and made his direct measurements.
In doing at-sea measurements of the horizontal array coherence, there are a number of issues which have to be dealt with carefully. The first concern is geometry. A geometry where the array is broad- side to the source is preferred, as this obviates the problem of the ambiguity of the horizontal beam steering angle versus the vertical multipath angle. In a broadside geometry, all the multi-paths arrive on the same zero degree steered beam. The next concern is signal
to noise ratio (SNR). Ambient noise from continuous sources in shallow water “Kuperman-Ingenito noise,”(Kuperman and In- genito, 1980) produces very short (fraction of a 400 Hz wavelength) horizontal correlation scales, and so one needs to be well above
this noise level to see longer scales experimentally. Noise from
large, discrete sources (e.g. ships) mimics the multi-paths from the experimental source, and can give spurious results (larger correlation lengths, if the noise source is on the experimental source-to-receiver line and smaller lengths if off it). Source or receiver motion (or both) is yet another concern. This motion quickly produces a large number of realizations of the environment, and even can affect the statistics of the measurement if the environment one passes through is non-stationary. Using a fixed source and receiver geometry cures this, in that one has a bathymetrically stationary system and an oceanographically more slowly varying environment to deal with. Another experimental issue is broadband versus narrowband signals. Generally, the longer integration time needed to get good SNR with narrowband signals is a disadvantage, but given a fixed geometry, longer times (up to several minutes) are often available. A final experimental consideration is adequate measurement of the ocean and seabed condition, so that one can correlate the coherence length
measured to the regional and seasonal ocean condition. Carey’s mea- surements provided this “general context” of where, when, and in what ocean condition the data were taken, but they did not have the detail to comment on all the individual acoustic scattering mecha- nisms that were discussed above. That level of experimental detail of the environment would have to wait for the “Shallow Water 2006” (SW06) experiment performed two decades after Carey’s experi- ments (Tang et al, 2007).
Measurements of Lcoh
At this point, it is appropriate to show both Carey’s measurements and the later SW06 measurements, to see the experimental story. In Figure 2, we show a synopsis of his data, along with some calculated points from our “simple theory”, which we will discuss soon. Carey’s 400 Hz data, at first glance, shows a 20-40λ spread of Lcoh, with
an average of about 30λ. One also might suspect that the coherence length is increasing with source-to-receiver range, which could be due to higher mode stripping by differential attenuation of (seabed- interacting sound) or other effects.
Figure 2 : A synopsis of Bill Carey’s coherence measurements, along with two theory points based on “simple” calculations of shelf-break front effects. Four of the experiments had multiple source/receiver ranges.
12 | Acoustics Today | Winter 2014