SonTek, a Xylem brand
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RiverSurveyor S5/M9 System Manual (February 2013)
Appendix I. CastAway CTD Integration
Background - Sound Speed and Acoustic Measurements
Underwater acoustic measurements, particularly acoustic depth range measurements, are de-
pendent on information about the speed of sound in the water to collect accurate data. Sound
can travel through the water at different speeds based on changes in temperature, salinity and
density (Table 5). For example, sound will travel faster in hot salty water and travel slower in
cold fresh water.
Change
Effect on
Sound
Speed
Temperature
1° C
4.0 m/s
Salinity
1 PSU
1.4 m/s
Depth
100 m
1.7 m/s
Table 5. Effect of temperature, salinity and
depth changes on the speed of sound in wa-
ter
The water in almost every lake, river, estuary and ocean is stratified into different layers. Tem-
perature conditions in one layer may be notably different from conditions in another layer (Fig.
137). As sound transitions from one layer to the next it will also refract and change direction.
These changes in sound speed and direction throughout the water column cause errors in
acoustic depth measurements. Depth measurement errors are generally largest in deeper riv-
ers or in estuaries where water temperature and salinity at the surface can vary significantly
from the water beneath it. For more detailed information about how temperature and salinity af-
fect depth calculations, refer to SonTek Technical Note:
Range, Velocity, Sound Speed and
Snell’s Law
.
Similar to acoustic current measurement systems, hydrographic surveying instruments (i.e.
multibeam echosounders, side scan sonar) measure the time it takes a sound pulse to travel
from the instrument to the seafloor to calculate distance. Hydrographic surveyors apply sound
speed corrections to their data to reduce depth measurement errors induced by sound speed
variability. To measure sound speed surveyors use a CTD (Conductivity, Temperature, Depth)
sensor to collect a vertical sound speed profile during a hydrographic survey. The sound speed
correction is then applied to the acoustic sounding data yielding more accurate depth measure-
ments. These sound speed corrections can also be applied to ADP measurements to improve
their accuracy.
Application to ADP measurements
While it has been common practice for hydrographic surveyors to apply sound speed correc-
tions to bathymetric data, corrections have not commonly been applied to ADP data for two pri-
mary reasons. First, the advantage of higher accuracy measurements has been overshadowed
by the complexity of applying corrections and velocity measurements are impacted less by
sound speed than depth measurements. In many environments sound speed corrections will
only slightly improve ADP measurement accuracy; however there are environments such as es-
tuaries where corrections will significantly increase accuracy. With growing requirements for
Figure 137. A stratified Water Column
