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Although the OS-75 and the NB-150 have similar nominal beam
widths of about 4
(Darryl Symonds, personal
communication), the NB-150 appears to have much lower
sidelobes. Evidence is provided by the amplitude of the
return from the ocean bottom. The first return comes from
the downward-pointing sidelobe, followed by the peak
amplitude along the mainlobe, and then sidelobe energy from
larger angles from the vertical. In a plot of amplitude
versus depth, the return from the
ocean bottom shows up as a strong local maximum, roughly 40
db above the return from scatterers in the water
(Figure 8). Comparing the amplitude
profiles from NB and OS, we see in the latter a slower decline to the
noise floor below the maximum amplitude, and returns from
the nearly vertical sidelobe that are 12-15 db higher.
We suspect that the sidelobes of the OS-75 transducer on the
Endeavor are worse than on some other OS systems. Bottom
returns from an OS-75 mounted temporarily on the R/V Oceanus
and from an OS-38 mounted permanently on the JAMSTEC R/V
Kaiyo, although broader in depth range than those from
the Endeavor's NB, lack the sharp increase at 85% of the
water depth characteristic of the Endeavor's OS.
Figure 8:
Automatic gain control (AGC) counts as a function
of depth showing returns from
the bottom. All data are taken from the acoustic
interference test (test 3), with unsynchronized pinging
but with most of the resulting amplitude glitches filtered out.
Profiles were shifted slightly in depth to make the
maxima coincide; they were also shifted laterally to have
the same maximum AGC counts. Horizontal bars show
plus/minus one standard deviation. The grey line shows where
the first sidelobe returning from the bottom would be expected.
Just below this line, the OS amplitude increases sharply
to a plateau, then increases again to the maximum
AGC. The first sidelobe increase is weaker in the NB.
Below the AGC maximum, the NB amplitude falls to the
noise floor more steeply than the OS amplitude does.
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Next: Conclusions and recommendations
Up: Results
Previous: Accuracy
Jules Hummon
2001-10-24