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APPLICATION NOTE NO. 6 Revised August 2004

DETERMINATION OF SOUND VELOCITY FROM CTD DATA

Use of CTD measurement for determination of sound velocity is appealing because these instruments are

simpler and more rugged, and because their resolution, accuracy, and stability lead to far better precision

than can be obtained with direct SV measuring devices. For example, specifications of 0.01 mS/cm

conductivity, 0.01 degrees C temperature, and 1 meter in depth are readily achieved with good quality CTD

equipment. Assuming that the relationship between C, T, and D and SV is exactly known (see below), the

resulting uncertainty in SV would be as follows:

Error Type Sound Velocity Error

temperature error of 0.01 deg C 0.021 meters/second

conductivity error of 0.01 mS/cm 0.011 meters/second

salinity error of 0.01 psu 0.012 meters/second

depth error of 1 meter 0.017 meters/second

The equivalent SV errors (considered at 15 degrees C, 42.9 mS/cm, 35 psu, and 0 pressure, i.e., typical open-

ocean surface conditions) are much smaller than those usually claimed for direct-measurement instruments.

The question about the absolute accuracy of the inference of SV from CTD data is more difficult to answer.

The main reason for this is apparently the result of differences in the instrumentation used by various

researchers and is compounded by the difficulty of performing direct measurements of sound velocity under

controlled conditions of temperature, salinity, and (especially) pressure. For example, three widely used

equations (Wilson, 1959; Del Grosso, 1972; Millero and Chen, 1977) show differences in absolute sound

speed on the order of 0.5 meters/second for various combinations of water temperature, salinity, and pressure,

despite being based on careful measurements made under laboratory conditions.

The work of Millero and Chen is, however, the most modern, and it builds upon and attempts to incorporate

the work of earlier investigators. Accordingly, the SV/CTD relationship described by these researchers in

their paper of 1977 was used as a major component in the derivation of the Equation of State (Unesco

technical papers in marine science no. 44). Millero and Chen’s 1977 equation is also the one endorsed by the

Unesco/SCOR/ICES/IASPO Joint Panel on Oceanographic Tables and Standards, which comprises the

internationally recognized authority for measurements of ocean parameters (in Sea-Bird’s SEASOFT

software, users may select any of the 3 equations mentioned above).

Pike and Beiboer, 1993, made a careful comparison of algorithms used to calculate sound velocity. They

concluded that use of the Wilson equation should be discontinued, and that the Chen and Millero algorithm

should be used on the continental shelf while the Del Grosso formula is more appropriate for deep ocean

waters and long path lengths. Their paper includes tables showing valid temperature and salinity ranges for

each of the algorithms.

Sea-Bird Electronics, Inc.

13431 NE 20

Street

Bellevue, WA 98005

Phone: (425) 643-9866

Fax: (425) 643-9954

Web: www.seabird.com

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Sea-Bird Electronics SBE 38 Manual do Utilizador Página 75