Sonic-sounding equipment consists of three essential parts: (1) a source that will emit a sound impulse, (2) an instrument for detecting or recording the outgoing and the returning signals, and (3) a means of measuring the time required for the sound to travel to the sea bottom and for the echo to return to the ship. Sound sources are of two general types: those that emit sound of audible frequency which is nondirectional in water, or those that emit high-frequency vibrations which are nonaudible and are classed as ultrasonic. The audible type is satisfactory for general use, but for sounding in shoal water or over a bottom that is very irregular, directional ultrasonic equipment must be used. Audibletype transmitters usually consist of a diaphragm that is vibrated by an electromagnet, although other devices are employed. The ultra- or supersonic transmitter depends upon the piezoelectric property of quartz crystals which, when subjected to a high voltage, vibrate at high frequency, and, as the process is reversible, the returning echo stimulates a current through the circuit, so that the same device is used as a transmitter and as a sound detector. In the audible-type sonic sounder the outgoing signal and the returning echo are picked up by a submerged microphone called a hydrophone.
Various devices are used to determine the time required for the sound impulse to travel to the bottom and return to the hydrophone. For sounding in deep water the simplest method is to measure the
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interval by means of a stopwatch. However, this method is not very accurate and is not practical in shallow water, where the time interval is small. Most instruments depend upon visual signals or a combination of visual and auditory signals to measure the time interval, and in some devices the interval is recorded automatically upon a moving tape. In practice, an instrument is set for a constant sounding velocity (p. 79), usually between 800 and 820 fathoms per second (1463 and 1500 m/sec), and hence the time interval is a direct measure of the depth obtained, using a constant sounding velocity. In accurate work the depths obtained in this way must be adjusted to allow for the vertical distribution of temperature and salinity. The time interval is commonly measured by means of a rotating disk revolving at a constant speed. This speed is determined by the graduations on the disk and the sounding velocity. For example, the disk may be graduated to read from zero to 1500 m, and, if rated for a sound velocity of 1500 m/sec, it will require 2 sec for one rotation of the disk; that is, 2 sec is the time required for the sound to travel to the bottom and back when the depth is 1500 m. The outgoing signal is activated automatically each time the disk is at zero. When earphones are used in deep-water instruments of this type, the position of the disk is noted at the instant the return echo is heard. The recorded depth will usually represent the average of several such measurements. For work in shallow water (less than 500 m) a flashing light signal activated by the outgoing sound impulse and the returning echo is commonly used. In such instruments the light is on a revolving arm that is mounted behind a graduated disk with a circular slit through which the light is visible. The outgoing sound impulse is emitted automatically each time the light passes zero on the depth scale, and the returning echo causes the light to flash, the depth being indicated by the graduations on the dial. In automatic recording devices the depths are marked upon a moving paper, and the plot obtained in this way represents an accurate profile of the bottom. Details concerning the construction and operation of sonic depth finders are given in the Hydrographic Review
, published periodically by the International Hydrographic Bureau. The instruments used by the U. S. Coast and Geodetic Survey are described by Rudé (1938)
and by Veatch and Smith (1939)