a brief emission of sound  

to send out (sound, light, etc.)   

the reflection of sound waves   

the use of acoustic techniques to acquire geological data 

a sound receiver used in marine seismic 

the resistance to the sound wave caused by the layers through which it passes 

a vehicle which creates a sound impulse with a single impact on the surface of the formation

the distance between the source and the receiver 

(height, length and depth); 4D is 3D plus time