YSI SonTek RiverSurveyor, System Manual

Page: 32 / 194

SonTek/YSI Inc
RiverSurveyor System Manual (May 1, 2007)
20
3.2. Doppler Instruments and Current Profiling
The following topics are covered in this section:
•
What is an Acoustic Doppler Profiler (ADP)?
•
What is current profiling?
•
What parameters are associated with current profiling?
•
How do I set up the profiling parameters in RiverSurveyor?
•
What data will I get from RiverSurveyor?
3.2.1. What is an Acoustic Doppler Profiler (ADP)?
An Acoustic Doppler Profiler (ADP) is an instrument that measures the velocity of water using a
physical principle called the Doppler shift. The ADP is the principle component of every River-
Surveyor system.
A SonTek ADP has three (or four) transducers mounted in the transducer head of the system.
Each of these transducers has a different orientation and generates a narrow beam of sound that
is projected through the water (the instrument cannot collect valid data “in air”). Reflections
from particles or “scatterers” (such as suspended sediment, biological matter, or bubbles) in the
water column are used to determine the water velocity.
The geometric orientation of each of the transducers allows the ADP to calculate the velocity of
the water in using a Cartesian (
XYZ ) coordinate system relative to the position and orientation of
the instrument. The internal compass and tilt sensor (roll/pitch) used with all RiverSurveyor sys-
tems is able to calculate the water velocities in Earth coordinates (East-North-Up or
ENU ) inde-
pendent of the system’s orientation. RiverSurveyor systems must always use ENU coordinates.
3.2.2. What is current profiling?
As mentioned above (§
3.2.1
), ADPs measure water current velocities along each of the trans-
ducer beams and transform these velocities into Cartesian ( XYZ ) or Earth ( ENU ) coordinates.
The beams are divided into discrete increments or cells (also known as range cells or depth cells )
of a specific length. The ADP will record the following data for each of these cells:
•
Three velocity values (one for each velocity component)
•
Three signal strength values (one for each transducer beam)
•
Three standard deviation values (one for each velocity component)
The velocity values are of foremost interest; signal strength and standard deviation are used to
determine the effective profiling range of the instrument and to estimate the quality of the veloc-
ity measurements.
Referring to
Figure 3-8
, current profiling can be thought of as dividing a river or stream into sev-
eral horizontal slices (rows) from top to bottom (columns). The “rows” represent individual
cells, and the “columns” represent vertical profiles. Each slice (row of cells) will contain water
flowing at a certain velocity. Slices/rows/cells closer to the bottom will tend to flow slower than
cells at mid-depth due to friction. The cells at the left and right edges of each row also tend to
flow slower than cells in the center of the row. The ADP measures the velocity of the water in
each of these cells and produces a velocity profile from the top of the column to the bottom of
the column. By moving the ADP from one side of a river to the other, we can then add all the ad-
jacent profiles together and know the velocity for all the water in the river.