The Images are Computed from Echoes
A dolphinoid click, the graph of which in time looks like this
echoes off a "fish" that looks like this
making echoes at sensors (hydrophones) that look like these three samples:
The slight differences among such received echoes is the basis of echoscopic imaging.
The number of dots in that simulated fish is 328. Fish with more than 4000 have been used, but a smaller number results in less time-in-computation and in more experiments. Each dot contributes, at each sensor, to the echo at that sensor.
The colored vertical lines mark the toa, the times-of-arrival, of the features used in in Feature-Base Passive (FBP) sonar (and, in the future) radar.