lobes of the beam (not shown in the simplified Figure 1) allows the towfish to 

 "see" a continuous 180-deg zone, up to the surface and down to the bottom 

 directly below the towfish. However, geometric distortions in the printed 

 record compress the area just below the fish, and lack of energy in the side 

 lobes does not produce an accurate image of the surface. 



7. Electrical energy, supplied through the electromechanical tow 

 cable, is applied to the piezoelectric transducers in the towfish. This 

 energy causes them to vibrate, creating sound waves which travel through the 

 water. Sound is reflected from the seabed or structure, received by the same 

 transducers, transmitted back up the tow cable to the recorder, and printed on 

 continuous chart paper (Figure 2). The recorder provides controls to adjust 

 the range, paper speed, printing intensity, and signal gain. 



8. Side-scan sonar transducers typically vibrate at preselected fre- 

 quencies from 50 to 500 kHz. Most of the popular commercial units operate at 

 frequencies between 100 and 500 kHz. The 100-kHz frequency provides greater 

 range, up to 400 m on each channel and is most often used for sea bottom 

 mapping and locating objects. A frequency of 500 kHz gives a shorter range, 

 up to 100 m per beam (also known as a channel), but provides greater resolu- 

 tion and is recommended for detailed inspection of coastal structures under 

 most circumstances. 



Sonograph Characteristics 



9. Properly interpreting the continuous side-scan sonar image record, 

 commonly known as a sonograph, is often difficult without an understanding of 

 the principles of operation, conditions at the survey site, and the area being 

 viewed. The examples presented in this report will provide a basic under- 

 standing of the techniques of interpreting side-scan sonar image records. 

 However, the only way to become proficient is through experience. Klein Asso- 

 ciates, Inc. (1985), and Flemming et al. (1982) provide additional information 

 on interpreting side-scan sonar images. 



10. The continuous paper image of the bottom or structure produced by 

 the recorder is remotely similar to low level oblique aerial photographs. 

 However, the physics of sound transmission in water is sufficiently different 

 from light transmission in air to produce different image characteristics. 

 Training in the physics of side-scan sonar and in perceiving the distorting 



