Beam sidelobes

Although the OS-75 and the NB-150 have similar nominal beam widths of about 4$^{\circ}$ (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.