It often is assumed that an echo sounding trace is a continuous and 

 true profile of the ocean bottom, but it is not. First, the trace is a 

 series of individual samples taken at short, but distinct, intervals 

 depending on the repetition rate built into the transmitter. Secondly, 

 the leading edge of the trace is the slant-distance to the nearest 

 reflecting surface within the cone of sound emitted by the transducer. 

 The shape of the trace will coincide with that of the bottom directly 

 under the vessel only when the bottom is flat. 



The directivity of a transducer is a function of the transducer dia- 

 meter and the frequency of transmission used. As the diameter and 

 the frequency are increased, the sound beam becomes more directional. 

 However, narrow beams must be stabilized, so that they will not be 

 affected by the motion of the ship. This stabilization presents a 

 serious problem when a large transducer is used*, Also, as the frequency 

 is increased, the attenuation of the signal is greater and the available 

 depth range becomes less. 



B. EXISTING INSTRUMENTS 



The echo sounding equipment now available can be divided into three 

 general classes of instruments: general purpose, shallow water, and 

 deep water instruments. 



1. General purpose instruments 



These instruments are designed to give the navigator the 

 necessary information for entering and leaving harbors. They usually 

 record to an intermediate depth of approximately 200 fathoms, and 

 many models are available from commerical sources. 



2. Shallow water instruments 



Shallow water instruments usually are portable and are normally 

 used in the range of to 50 fathoms. They are reasonably accurate, if 

 the power source is constant. The Edo 255 is considered typical of this 

 type. 



3. Deep water instruments 



These instruments have recorders that can be used for the full 

 range of depths and are quite accurate if provided with a constant 

 power source. They are hull-mounted and require that the ship be 



XI- 2 



