Multi-channel underwater acoustic holographic recording system

We have constructed an experimental underwater acoustic holographic system that uses 100 piezoelectric transducers in a linear receiving array with the object area insonified by a single cylindrical acoustic radiator. The receiving array is swept through the water synchronously with a linear array of 100 light-emitting diodes positioned above the water. The holographic data is parallel-processed in 100 electronic channels, each driving its respective light-emitting diode during the sweep. A time-exposed photograph of the optical pattern thereby created is processed and used as an optical hologram. Many holograms have been made with the system in a laboratory tank. The reconstructed images demonstrate that the resolution attained is of the order of a few wavelengths. The principal practical problems were cross-talk between channels and tolerance of components. If the scanning of a larger system is carried out by the passage of a ship through the water, rapid surveying of the ocean floor becomes a possibility.