YSI SonTek RiverSurveyor, System Manual

Page: 49 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
37
3.5. Echo Sounders
The following topics are covered in this section:
•
What is an Echo Sounder?
•
How do Echo Sounders work?
•
How do Echo Sounders work with RiverSurveyor?
•
How do I set up an Echo Sounder in RiverSurveyor?
3.5.1. What is an Echo Sounder?
An echo sounder is a device that measures water depth. Echo sounders typically consist of a sin-
gle transducer, a control box, power cable, and a serial interface. The control box usually has a
display to show the depth readings. Some echo sounders have advanced features such as topog-
raphic logging and vessel speed measurement.
Although they can be integrated into the RiverSurveyor system, echo sounders are not included
as a standard part of any RiverSurveyor or RiverCat system.
3.5.2. How do Echo Sounders work?
Echo sounders measure the time it takes for an ultrasonic sound wave to travel from the trans-
ducer to the riverbed (or water surface), which reflects the wave, and then back to the transducer.
Since the velocity of the sound wave is known, the depth of water from the transducer face can
be calculated.
The speed of sound in water is dependent on temperature, salinity, and depth. However, echo
sounders used with RiverSurveyor systems are typically operated in relatively shallow depths
(less than 200 m), so depth will have negligible effects.
3.5.3. How do Echo Sounders work with RiverSurveyor?
Echo sounders provide depth information to RiverSurveyor. However, since RiverSurveyor sys-
tems already receive the depth of water below the ADP using bottom track, why would you need
an echo sounder? Because the depth information provided by an echo sounder has some benefits.
•
Certain environmental conditions can limit or prohibit the use of bottom-track for depth data
(§
3.3.5
). These typically include moving bed, a poorly defined river bottom (e.g., very high
suspended sediment concentrations on the bed), or a water depth that exceeds the maximum
bottom-track depth (
Table 3-1
).
•
The echo sounder uses one transducer for depth measurement while bottom-track uses an
average of all the ADP’s transducers. A single beam is more representative of the actual wa-
ter depth directly below the ADP. The multiple beams of an ADP cover a wider area and do
not point at the area directly below the ADP because of their slant angle. The difference in
individual ADP beam depths is especially prevalent in areas where there is significant
change in the bathymetric gradient.
•
An additional depth measurement provides a contingency and a second check for bottom-
track depth results.