310 Lecture 16 
The side lobes in the vertical plane are important (Fig. 16.9) and give useful 
information. Figure 16.10 shows a record taken with this equipment. Starting 
from the left-hand side of the record and referring also to Fig. 16.9, there is 
first the transmission mark (somewhat ragged because the blade has been eaten 
away at this point). Then comes a hard line which is the bottom echo from the 
vertical side lobe, followed closely by the echo from the third side lobe, then by 
those from the second and first side lobes, and finally the echoes from the main 
beam extending across the rest of the record. 
Figure 16.10 shows an area where the bottom has been largely swept clean 
of sediment and the bedrock is exposed. Figure 16.11 shows a sediment-covered 
area where sand waves have been formed. It will be seen that the side lobes 
give a quantitative measure ofthe bottom relief and aid in the interpretation of 
the acoustic picture. 
}¢—___________- ‘g00 yps_. ———______ >} 
|_<—____________ AppROx. 2 MILES 
Fig. 16.11. Asdic record showing sand waves in the Irish Sea. 
