An example of one ping of data from a sidescan sonar is shown in Figure 7.

Figure 7 - One ping of data collected by a sidescan sonar system.  The strength of the echoed sound wave is plotted along the y-axis and time in seconds is plotted along the x-axis.  The blue waveform is a component of the actual values (voltages) that are measured by the transducers arrays.  The components are combined to produce the discrete magnitudes (red crosses).  These data were collected by a shallow-towed 12 kHz sonar system located almost 5000 meters above the seafloor.  Because it takes sound more than six seconds to travel from the sonar and back again at a speed of ~1500 m/sec, there is no amplitude recorded in the early part (left-hand side) of the figure.  After ~6.5 seconds, the first returned echo, called the bottom detect, is finally received by the system.  The strength of the measured echo generally decreases for an additional eight seconds until it is time to stop recording data and transmit another ping.

The terms “sidescan,” “backscatter” and “reflectivity” are often used interchangeably, and incorrectly, when describing data collected by sidescan sonar. For the record, these are the correct definitions of these terms:

Sidescan is the correct term for the instrument that is used to collect the data.
Backscatter is the measure of the amplitude of a reflected acoustic signal bouncing off the seafloor (or riverbed, etc.).  Backscatter results from the small (centimeter scale) reflective surfaces on the seafloor.
Reflectivity refers to the broader scale reflective properties of the seabed (i.e., the side of a hill, the flat region directly under a sonar system).

The strength of any given “target” depends on both its reflective properties and the extent to which it contributes to the backscattered signal. It is assumed that the surface reflects incoming sound waves equally in all directions: