YSI SonTek ADVField, Техническая документация инструкция

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SonTek/YSI
Acoustic Doppler Velocimeter Principles of Operation (September 1, 2001) 9
7.2. Sound Speed
The ADV uses sound speed to compute velocity from the measured Doppler shift. This section
discusses how to correct ADV velocity data for errors in the sound speed value used for data col-
lection. Any errors in sound speed are typically very small, so postprocessing corrections are
rarely required.
The speed of sound in water is primarily a function of temperature and salinity. Figure 5 shows
sound speed as a function of temperature (at several salinity levels). Figure 6 shows sound speed
as a function of salinity (at several temperature levels). Generally, a temperature change of 5
°
C,
or a salinity change of 12 ppt, results in a change in sound speed of 1%. The full range of typical
temperature and salinity levels, from -5 to 50
°
C and 0 to 60 ppt, gives a sound speed range of
1375 to 1600 m/s (total change of 14%).
Changes in sound speed affect ADV velocity measurements in two ways: conversion of the
Doppler shift to velocity, and measurement geometry as determined by the location of the sam-
pling volume. The geometry change occurs since the ADV sampling time is determined by the
specified sound speed. If sound speed is incorrect, the true location of the sampling volume will
differ from the location assumed in the probe calibration file. The change in geometry causes
sound speed errors to have an increased effect on the accuracy of velocity data.
Correcting ADV velocity for sound speed errors can be done two ways. The first method uses a
best-fit approximation of the geometry change. This correction, shown below, is accurate to
about 0.5% for sound speed errors up to about 5%.
Horizontal velocity correction (Vx and Vy for 3D probes; Vx for 2D probes):
V
true
= V
orig
* (1 + 1.93 * ((C
true
/ C
orig
) - 1))
Vertical velocity correction (Vz for 3D probes; Vy for 2D probes):
V
true
= V
orig
* (1 + 0.94 * ((C
true
/ C
orig
) - 1))
Where:
V
true
= Corrected velocity value
V
orig
= Un-corrected (original) velocity value
C
true
= True speed of sound
C
orig
= Speed of sound used in original calculations
Figure 5 – Sound Speed vs. Temperature Figure 6 – Sound Speed vs. Salinity
0 10 20 30 40
1400
1450
1500
1550
1600
Temperature (degrees C)
S
o
u
n
d
spe
ed (
m
/s
)
Sal. 0 ppt
Sal. 35 ppt
0 10 20 30 40
1400
1420
1440
1460
1480
1500
1520
1540
1560
Salinity (ppt)
S
o
u
n
d
spee
d (
m
/s
)
Temp. 0 C
Temp. 15 C
Temp. 30 C