A  Towfish depth below the surface

B  Towfish altitude above the bottom

C  Slant Range to the target

D  Acoustic shadow length

The key to understanding and interpreting side scan sonar records is the geometry of use (refer to figure image above and the sample records below ).

The body of the sonar towfish containing the transducers is towed at some depth below the surface. The start time for the display is the transmission of the sonar pulse. The sonar is measuring and displaying the ranges of reflectors from the tow body, and every thing must be referred to this position.
 
Since the same transducer both produces the sound pulse and listens to the echoes, the transmitted puree is also treated as a signal, in fact a very strong one. This will produce a very dark mark at the start of the display for each channel.
 
Next there will be a period of time during which the sonar pulse is moving through the water column and not returning any echoes. This will appear as a blank area on the record unless some thing such as a school of fish is in this region.
 
Depending on where the sonar is in the water column the first echo to arrive may be from the water surface or from the sea floor beneath the sonar. Although the sonar beam is narrow in the horizontal plane, it is broad in the vertical plane and will reach both the sea surface and the bottom directly beneath the sonar. In the image above the sonar is closer to the surface than the bottom and the first echo to be printed will be the surface return, appearing as a discrete line.
 
Next the bottom echo will arrive, followed by returns from the sea floor at succesively greater distances from the sonar. These will be displayed at correspondingly increasing distances from the dark line produced by the initial output pulse. Note that these are slant ranges and do not represent the actual horizontal distance of the points on the bottom from the vessel track. The true horizontal offset can be found easily by calculation, and will be discussed in detail in a later section.
 
Objects protruding above the bottom will prevent sound from reaching the sea floor for some distance beyond the object. This will produce an acoustic shadow which appears on the final record as a light area.
 
The image below shows the sonar towfish closer to the bottom than the surface

Whether looking at one or two channels of data, no matter how they are oriented, keeping in mind the geometry behind the image formation process will be a great aid in interpreting the record.