Define Black & White Fathometer - Non-Destructive Testing?

Most commonly used sounding device is black & white fathometer. The units are compact and easy to use. They consist of a transducer, which is suspended in the water, a sending/receiving device, and a recording chart, which displays the depth on paper. High frequency sound waves, generally in the range of 200 kHz, emitted from the transducer travel through the water until they strike the channel bottom and are reflected back to the transducer. The fathometer measures the time it takes the sound waves to return to the transducer, and converts that time to the depths of water, which are displayed on a graphic recorder in the form of a continuous plot of the channel bottom.