Environmental Engineering Reference
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the acoustic beam, which typically characterize the
sedimentary content of multiple orders of magnitude
more water than point samplers. Like bulk-optic
techniques, empirical calibrations are required to
convert the ABS measurements to SSC. Complex
post-processing requires compensations for physical
properties of ambient water such as temperature,
salinity, and pressure, and, in some cases, suspended
materials. Additional compensations are needed for
instrument characteristics such as frequency, power,
and transducer design.
The purchase price of a commercially available
single-frequency Doppler in situ instrument is about
two to four times that of a fully equipped turbidim-
eter. Because biofouling has little if any effect on the
performance of the sensor, fi eld-maintenance costs
are probably less than that for a turbidimeter. The
instrument-measurement realm is multiple conic
beams. Instrument calibrations can be performed
using physical samples collected within the volume
of the beam; however, they are often supplanted by
cross-section calibrations.
The development and application of the ABS tech-
nology can be broadly grouped into two approaches,
based primarily on the instrumentation type and
target application (the underlying theory is equiva-
lent for the two approaches). The fi rst approach uses
specially designed acoustic instrumentation often
using multiple frequencies to compute SSCs and
grain sizes over relatively short ranges (1-2 m). This
approach has primarily been applied using fi xed
deployments to study near-bed sediment transport
processes in the marine environment. There are
ample publications describing the development and
application of this approach (see, for example, Hanes
et al. 1988; Sheng & Hay 1988; Hay 1991; Thorne
et al . 1991, 1993, 1995, 1996; Hay & Sheng 1992;
Thorne & Campbell 1992; Crawford & Hay 1993;
Richards et al . 1996; Schaafsma & Hay 1997;
Thorne & Hardcastle 1997; Thorne & Buckingham
2004; Thorne & Meral 2008). A review paper by
Thorne & Hanes (2002) provides a good overview
of the technique. This approach requires calibration
of a “system constant” for each instrument, which
is typically accomplished in the laboratory (Thorne
& Hanes 2002). At least one commercially available
instrument that uses this technique but lacks Doppler
capability is available (Aquatec Group 2008).
The second approach uses commercially available
in situ acoustic Doppler current profi les (ADCPs; the
term ADCP is used generically and does not imply a
particular manufacturer unless specifi ed.) This
approach is particularly suited to monitoring sus-
pended-sediment fl ux because ADCPs provide three-
dimensional velocity profi les as well as acoustic
backscatter information. As stated above, the under-
lying theory is the same, though for the ADCP
approach the sonar equations are typically formu-
lated in logarithmic form (i.e. in decibels (dB); see
next section) whereas for the fi rst approach the linear
form of the equations are used (i.e. in terms of
pressure or voltage). The increasing popularity of
ADCPs for characterizing hydrodynamics in fl uvial,
estuarine, and coastal environments has facilitated
the concurrent estimation of suspended-sediment
properties in these environments as well.
Theoretical aspects of the ADCP approach have
been well documented (see, for example, Thevenot
et al. 1992; Reichel & Nachtnebel 1994; Deines
1999; Gartner 2004). Applications have been docu-
mented for a wide range of environments (see, for
example, Schott & Johns 1987; Thevenot et al .
1992; Thevenot & Kraus 1993; Jay et al . 1999; Klein
2003; Gartner 2004; Topping et al . 2004, 2006,
2007; Hoitink & Hoestra 2005; Hortness 2006;
Wall et al. 2006; Tessier et al . 2008; among many
others). At least one commercial software product is
available to convert backscatter to SSC (Land &
Jones 2001). Comparisons of SSC computed from
acoustic backscatter with SSC values determined
from water samples have been found to agree within
about 10-20% (Thevenot et al . 1992; Thorne et al.
1991; Hay & Sheng 1992).
The theoretical development presented below is
constructed in terms of the logarithmic form of the
sonar equations, which is the typical form used for
the ADCP approach. This form is particularly suited
to this approach because commercially available
ADCPs typically provide the conversion factor from
raw backscatter counts to decibels (see below), which
facilitates accounting for transmission losses and
empirical calibration of backscatter to SSC. The
logarithmic form of the sonar equations can be
inverted to obtain an expression for SSC:
SSC computed =
+( ( )
10 ABRB
(2)
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