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.
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Phone: (425) 643-9866
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Содержание SBE 45 MicroTSG
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