resolution of the imagery as well as decrease the accuracy of the various 

 automatic corrections. Also, the captain should remember that the towfish 

 will tend to fly deeper for a given cable length at low speeds. When turning, 

 care must be taken so that the fish will not hit bottom and the cable will not 

 become fouled in the propellers. Although slower tow speeds are desirable in 

 that they allow more time to look at a point target, low speed in choppy con- 

 ditions may cause excessive boat motions. In addition, time required to com- 

 plete the survey must be considered, since periods of low currents are 

 generally short in the vicinity of coastal structures. 



25. Currents are undesirable because the towfish will tend to orient 

 itself in line with the current, possibly not desirable from a scanning angle 

 standpoint. If a speed log is being used, there may be an error in the speed 

 correction since the log may not give the true over-the-bottom speed. How- 

 ever, it should be pointed out that in zones where currents are unavoidable, 

 at least some qualitatively useful data can be obtained, whereas the same con- 

 ditions might be unsafe for divers. 



26. In calm conditions it may be possible to get into close range and 

 use the higher resolution 500-kHz frequency for sharp, detailed images, par- 

 ticularly of features selected for closer scrutiny from a screening run at 

 100 -kHz. Keep in mind that, generally, as frequency increases, usable range 

 decreases, and as range increases, resolution decreases. The transmission of 

 acoustic energy in water is also affected by conditions of turbidity, salin- 

 ity, velocity, and thermal nonuniformities . Speed of acoustic energy travel 

 can differ from that assumed by the manufacturer in calibrating the unit, and 

 discontinuities due to such conditions can cause refraction and/or reflection 

 of pulses. Range of operation can be limited by turbid conditions due to a 

 reduced backscatter limit (the range at which the system is unable to discrim- 

 inate between targets and background noise) . None of the errors introduced by 

 nonuniformities can be removed from the data with current equipment; there- 

 fore, it is best to schedule around conditions where there is strong stratifi- 

 cation, if possible. Keeping the towfish out of the wake is important for an 

 additional reason: air entrained in the water column will interfere with the 

 transmission of the acoustic pulses. The same interference occurs in wave- 

 breaking zones on the structure. For this reason, surveying at high tide on 

 calm days is recommended. 



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