it is important to maintain a constant vessel speed throughout the survey; the 

 maximum speed should be about 5 knots; about 2 knots is best if very high reso- 

 lution is important. Tow speeds greater than 2 to 5 knots decrease the number 

 of pulses hitting "targets" in the survey area. Consequently, resolution will 

 be reduced and smaller objects on the bottom may be missed altogether. Also, 

 with higher vessel speeds the distortion of objects on the record becomes a 

 problem. Objects are compressed parallel to the path of travel at vessel speeds 

 greater than about 2 knots. At 5 knots compression is about 50 percent of the 

 real size in the ship-path dimension. 



Until recently, all side-scan sonar records gave a somewhat distorted view 

 of features on the sea floor because differences in the tow speed and recorder 

 speed yielded records with different scales in both horizontal directions. 

 However, microprocessor mapping units from several manufacturers are available 

 which give scale-corrected or isometric records with constant scales in both 

 directions. These new mapping systems are important for surveys where very 

 accurate target dimensions and geographic positions are required. For most 

 surveys the conventional side-scan sonar provides sufficient accuracy. 



Successful and accurate interpretation of side-scan sonar records is an 

 acquired skill that is based on experience and judgment as much as on training. 

 Training is required in the beginning to understand the physics of how the 

 systems function, but the interpretation of the sonographs depends mostly on 

 distinguishing between tonal intensities and relating target shapes to scaled- 

 down geological features. This expertise is developed through experience in 

 examining records ideally from well-known sea floor areas where sediment samples, 

 bottom photography, or diver observations are available. 



When the sonar signal is transmitted it is attenuated and scattered somewhat 

 as it travels through the water, and further energy losses occur when the pulses 

 strike the sea floor. The intensity of the returning signal will determine the 

 tonal shading on the sonogram. Intensity is largely dependent on the composi- 

 tion and porosity of sediment on the seabed, as well as the bottom topography. 

 Relationships between tonal intensity and sediment characteristics are not fully 

 understood, but in general, the denser and coarser the sediment, the greater the 

 reflectivity, and consequently the darker the tone on the sonograph. Therefore, 

 outcrops of dense bedrock on the seabed will have very high reflectivity and be 

 the darkest; gravel patches will be darker than sand, and sand will be darker 

 than fine-grained muddy sediments. It is important to bear in mind that there 

 is an inverse relationship of acoustical Impedance to sediment porosity. High 

 porosity sediment (e.g., clays ~75 percent) has a relatively low impedance and 

 low reflectivity and appears light on sonographs; whereas low porosity material 

 (e.g., medium sands ~40 percent) has higher impedance, higher reflectivity, and 

 appears as darker tones. However, porosity does not depend solely on the grain 

 size; other factors such as grain shape, degree of sorting, and sediment compac- 

 tion can cause sediments of different sizes to be similar in porosity and there- 

 fore have very similar tonal shades. 



The topography of the sea floor also affects reflectivity and may cause 

 effects on the records similar to changes in sediment composition and porosity. 

 Slopes on sand waves and ridges, and projections such as boulders and outcrops 

 that face the outgoing acoustical signal will give a strong reflection on the 

 near side of the record, but they will also produce a pronounced acoustical 

 shadow on the fsr side, where no signal is reflected and a white area is produced 



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