Geoscience Reference
In-Depth Information
sand transport in the field, in the Libyan desert in 1938) are
worth recalling, since they underscore some of the general
challenges of field data acquisition:
A rather neat sensor has been developed, notionally for
Mars applications, by Merrison et al. (2006) using light
structured by a holographic optical element and a photodi-
ode to detect the sequence of pulses of reflected light as a
dust particle crosses the light pattern. This yields the speed
of the particle—of course, if the particle is a saltating sand
grain rather than a dust particle, its speed may not be the
same as that of the wind.
The design of the manometer, which was home-made, had to
embody several special features. It had to register both velocity
and static pressures at least six pitots simultaneously, to be
capable of carriage by car across rough country without
requiring readjustment, to be set up, leveled and zero-ed rap-
idly, and to be read easily under the unfavorable conditions of
an open dune during a violent sand storm.
16.3.6
Large-Scale Tracer Methods
16.3.4
Ultrasound
In order to measure the wind flow pattern over a dune or
dunefield, a variety of approaches can be applied. One
method is to install an array of wind sensors as described
above. Other approaches include release of tracers such as
smoke or bubbles, or the use of kites or streamers (e.g.,
Fig.
5.1
) to visualize the flow. A promising modern tech-
nique in boundary layer meteorology is to use a small
Unmanned Aerial Vehicle (UAV) to measure the wind field;
comparison of its air speed and its ground speed allows
recovery of the wind.
The transit time of a pulse of sound through a flowing
medium will depend on the intrinsic propagation speed of
sound through it, and the speed and direction of the flow.
The sound speed of air is temperature-dependent, but this
can usually be corrected in real time, e.g., by measuring the
pulse transit time in both directions. This measurement can
be performed with audible sound, but more usually is per-
formed with ultrasound, in particular to facilitate measure-
ment of short pulses. The wider the physical separation, the
more accurately can the transit time be measured, but the
weaker the pulse and the larger the volume of air over
which the measurement is averaged: 10-20 cm is typical,
crossed in about 500 microseconds.
Most frequently, a set of transducers is mounted in a
tetrahedral or other arrangement such that the transit time in
several different directions and the three components of the
wind direction can be calculated (see the discussion on
are capable of high sample rates and thus for characterizing
turbulent flows.
Because of the more complex signal processing required
compared with other sensors, ultrasonic anemometers are
typically rather expensive. Such anemometers have been
proposed for planetary measurements (an application for
which their lack of moving parts makes them ideal), but
have not yet flown. A challenge for Mars application is that
coupling transducers to the thin atmosphere is poor, so
adequate signal levels are difficult to obtain.
16.4
Measuring Saltation
The simplest way of measuring saltation, or specifically the
sand flux, is a sand trap. At its crudest this might be a bucket
set flush into the ground, so that moving sand accumulates
in it. A rather higher-fidelity approach is a rectangular box,
usually with a slightly flared opening (see Fig.
21.1
). For
unattended operation over long periods, this might be
equipped with a door that closes after a finite period, or
closes and stops a clock after a given amount of sand has
accumulated. Another design (Jackson 1996) operates like a
rain gauge, emptying a bucket after 1 g of sand has accu-
mulated. Often several traps are used together, e.g., at dif-
ferent heights. When the mass of sand can be measured
before any one of the traps becomes full, then both the
vertical and the temporal distribution of the moving sand
can be determined. Many diverse designs have been used
for the sand traps, but each has its own set of issues with
regard to influencing the wind conditions present at the
entrance to the trap (e.g., all traps produce some amount of
'back pressure' opposed to the wind at their entrances,
hence box traps tend to be somewhat trapezoidal in cross
section, with a narrow opening upwind, flaring wider
downwind). Sand traps have proved most useful when they
can be calibrated in the controlled conditions found inside a
wind tunnel prior to their deployment in the field. Thus far,
sand trap results have proved to be generally consistent with
predictions of sand flux based on both wind tunnel experi-
ence and on saltation theory.
16.3.5
Optical Tracer Methods
The flow of a medium can be estimated if there are visible
tracer particles in it. Even small amounts of dust or smoke
are enough to permit speed measurements via laser Doppler
anemometry, although this is a fairly expensive apparatus.
Similar systems have been used at a distance in the field
(Doppler lidar) to measure the dust loading and wind speed
in dust devils.
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