The MPL sound velocimeter: An instrument for in situ sound velocity measurements in the deep ocean

A sound velocimeter designed to measure sound speed to a precision of 1 part in 10⁵ has recently been tested at sea. The device was used in conjunction with a CTD to check agreement with sound speeds computed from popular empirical equations [C.‐T. Chen and F. J. Millero, J. Acoust. Soc. Am. 62, 1129–1135 (1977); V. A. DelGrosso, J. Acoust. Soc. Am. 56, 1084–1091 (1974)]. Disagreements will be discussed. High‐precision travel‐time measurement is achieved through use of a phase‐comparison technique. For this purpose, the device determines the number of cycles of the acoustic signal between the transmitted pulse and received echo, adjusting the frequency until the echo and transmitted pulse are in phase. The travel time is the product of the signal period and the number of cycles between the outgoing and incoming pulses. The velocimeter’s capability of measuring travel time at several different frequencies near 4 MHz serves as a check on the timing consistency, since the travel time should be independent of frequency over this narrow range. The total distance traveled by the pulse is about 8.6 cm. With proper calibration, this pathlength can be known to a precision of 1 part in 10⁵.