Image Processing Reference
of sonic energy. Originally these pulses were generated mechanically; modern
systems use an electronic device called a transducer. The transducer also serves
as a sound detector; it is like a speaker and microphone in one. Underwater objects
reflect some of the energy in the sonic pulses back to the transducer, yielding
information about them. A simple case of sonar imaging is a depth finder. A narrow
beam of sound pulses is directed straight down from a transducer. The pulses reflect
off the bottom (or any object with a density that differs from water) and travel back
to the transducer. An electronic circuit measures the time difference between the
emission of the outgoing pulse and the reception of the reflected pulse and converts
this time difference into a distance. That distance is twice the depth of the water.
Imagine a wreck lying on the bottom of the ocean, 200 m below the surface,
as shown in Fig. 5.1. The height of the wreck is 20 m. If depth measurements are
made in the vicinity of the ship, they find that the sonar distance will vary between
180 m and 200 m. The 180 m measurement is the 200 m ocean depth minus the
height of the ship above the ocean floor. If the boat were driven around and depth
measurements taken at many different points on the ocean surface, the readings
will generate a crude sonar image of the submerged ship. It will appear as a 20-m
high feature on the ocean floor.
High-resolution sonar imaging is an extension of the depth finder, with the
addition of sophisticated data processing and a data collection system that
measures the shape and strength of the return pulses in addition to their round-
trip times. The shapes of the return pulses are modified by the geometry of
Figure5.1 Schematic of ship's depth finder.