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Appendix C: How the LISST-ABS works
Basics
The LISST-Acoustic Backscatter Sensor, LISST-ABS operates in a
manner similar to radar. A short pulse of high-frequency sound is
transmitted by a ceramic transducer (see Figure).
The pulse travels outward, and its location at any time is at a distance
equal to the product of speed of sound in water
c
, and time
t
after
transmission,
ct
. Particles in the pulse (called 'cell', 'range cell', ' range
bin', or 'sample volume') scatter sound in all directions. Some of this
sound travels backward toward the transducer. This scattered sound
takes the same amount of time
t
to reach back to the transmitter. This
is the backscatter signal. The total time for this signal out and back is
2
t
. Thus the signal sensed at time
2t
after pulse transmission
corresponds to scattering from a range cell a distance
ct
from the
transmitter. This is the essence of acoustic backscatter. The strength of
the backscatter pressure P at the transducer from a range R =
ct
is
related to the outgoing pressure pulse P
o
, the geometric spreading
G(R), and attenuation by the combination of water and sediments. It is
helpful now to consider the signal via the sonar equation:
log
10
[P/P
o
] = -G(R) - 2[
w
+
s
]R + log
10
(
) + N(R) (1)
This equation shows the weakening of the signal due to geometric
spreading G(R), attenuation by absorption by water (
w
) and sediment
(
s
), and a noise floor N(R). The signal of interest is the scattering by
particles,
. The LISST-ABS actually measures the backscatter signal
from two range cells located at c
t
1
and c
t
2
from the transducer, so that it
can compensate for attenuation by particles in water. To understand,
consider the difference of two signals such as above, but with
assumption of equal concentration. By difference, we have:
log
10
[P
1
] - log
10
[P
2
] = [-
G(R)
2
w
R] -
s
R (2)
This shows that the difference in signals from two range bins is equal to
the geometric spreading factor, and attenuation. The attenuation term
is usually very small for LISST-ABS until sediment concentration
becomes high, >1g/L. [we ignore noise floor, it is small]. Thus the two-
point backscatter determines the quantity in square bracket. Any
increase in the difference determines water attenuation term.
Содержание LISST-ABS
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