Seismic sonars are used to survey below the seafloor.  Sound pulses travel through the water, enter the seabed, and are then reflected back to the surface by components in the seafloor.  The reflections are recorded by the seismic receiver array at or near the surface.  The sub-bottom acoustic returns are analysed to determine the thickness of each layer of material in the seafloor. To some degree, returns can be classified to establish material type by layer, ie. clay, silt, gravel, rock, etc.  Seismic surveys of this type are often used to hunt for natural resources.

Very little acoustic energy makes it back to the seismic array from the the sea floor.  To detect these small acoustic pulses amidst all the background noise requires enormous receiving arrays.  These hydrophone arrays are arranged in a several, long (typically 6-8 km) streamers towed behind the ship.  This is illustrated in the diagram below.

In order for the hundreds of hydrophones to work effectively as an acoustic array, two difficulties must be overcome:

1. the sound speed at the hydrophones must be accurately known for beam steering, and
2. each hydrophone's location must be accurately known in three dimensions at all times.

The seismic arrays are also very expensive to purchase and maintain.  Therefore, wherever possible, technology is used to protect or insulate the array and thus increase array longevity. 

For any acoustic array, including seismic arrays and multibeam systems, tuning the array to listen in one specific direction requires knowledge of sound speed at the hydrophones.  The listening angle is a function of the distance between the hydrophones and the speed of sound. 

For illustration purposes, we can simplify the calculation to two hydrophones - instead of many more - and work in two dimensions - instead of three.  In the diagram below, an acoustic plane wave approaching the array at the listening angle desired will encounter hydrophone A first and then it must travel the additional distance e before encountering hydrohone B.  There will therefore be a time delay t between the detection of the signal between hydrophones A and B. 

To maximize the response in a specific direction  we want the signal at both hydrophones to constructively interfere when the hydrophone outputs are added.  This can be accomplished by adding a time delay of t to hydrophone A and then summing the two hydrophone outputs.

As shown in the figure the sine of the listening angle is angle directly proportional to c, the sound speed.  This is the reason that sound speed measurements at the array are critical to an array's directional accuracy.

Since the seismic array is moving, the sound speed can change as the array is towed through the water.  In addition, since the array is large the sound speed at one end can be different from the other.  For this reason sound speed sensors should be distributed throughout the array.

As you can see in the diagram above, in addition to sound speed it is critical to know the exact locations of each hydrophone to accurately calculate the listening angle.  Unfortunately, streamers undulate behind the ship in response to heading changes, speed changes, currents, water temperature changes, and salinity changes.   Since GPS does not work underwater, elaborate systems are implemented to determine the exact hydrophone positions behind the ship.  System components include:

When making turns, the array cannot be used, and changing direction with such a long array takes a long time.  Since time is money turns are executed as infrequently as possible and as quickly as possible.  However, when the ship changes direction the outer most streamers see an increase in speed.  This places additional strain on the streamer.  Buy incorporating speed sensors on the outer streamers turns can be executed as fast as possible without exceeding the limitations of the outside streamers.