Field Studies - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 185

Sand traps are integral sensors, in that they measure the
amount of sediment that has been transported over a long
period. It is often desirable to have information on shorter
timescales. One approach is to use a sand trap with a bal-
ance that can read out the weight of sand at any given
moment, and thus the sand flux can be computed by dif-
ferentiating these data.
Another class of sensors measure saltation in real time. The
most common modern sensors in this category are the safire
('Saltation Flux Impact Responder', e.g., Baas 2004) and
SENSIT. The safire uses a metal tube attached to a piezo-
electric disk. When a sand particle strikes the cylinder, a
vibration is communicated to the piezoelectric disk, which
generates a small electrical pulse. Electronics in the sensor
filter amplify the signal to generate a digital pulse for each
impact above a threshold. A convenient feature of the elec-
tronics is that they also provide an analog voltage between 0
and 5 V which provides a count of the number of pulses in a
50 ms period. Since most field datalogging equipment records
analog voltages, this greatly simplifies data acquisition,
especially if a large array of sensors is to be monitored, since it
avoids the need to have separate pulse-counting systems.
The operation of the SENSIT (Stockton and Gillette
1990) is broadly similar, and it records information on an
internal data logger that can subsequently be downloaded to
a computer. Early versions of the sensor recorded both
impact kinetic energy and particle counts as a function of
time, but more recent sensors can store the information as
energy per individual impact event. Yet another device is
the Saltiphone (Spaan and Van den Abele 1991). These
various devices have been successfully used in field
investigations (Arens 1996; Gillette et al. 1997; Sterk et al.
1999; Helm and Breed 1999). A piezoelectric impact sensor
was carried to Mars on the Beagle 2 lander in order to detect
saltating particles. Even a simple microphone can detect
saltation, which often manifests as a recognizable hiss even
when particles do not impact the microphone itself.
Acoustic emissions from avalanching is an entire research
field of its own ('booming dunes')—see Chap. 10 .
Optical sensors can measure the passage of saltating or
suspended dust. An optical mass flux sensor (Butterfield
1999) has been employed in wind tunnel studies, and an
optical device has been proposed for Mars use (Merrison
et al. 2012).

Early dune studies relied either on manual recording of
measurements, or occasionally on photographic recording
(e.g., of the heights of the liquid columns in an array of
manometers). Chart recorders, or magnetic tape, were a
later solution for recording instrument data, but for the last
20 years or so, electronic datalogging has become by far a
more convenient solution.
Dataloggers take two principal forms: a standalone unit
(which can be as small as a box of cigarettes) or as a
peripheral to a personal computer. The latter arrangement is
usually cheaper for a given capability, and generally allows
larger capability in terms of the number of channels to be
recorded, how often they are sampled, and so on. However,
the need for a computer makes them more suitable for
laboratory installation, or (usually using a laptop computer)
for only brief, attended, operation. On the other hand, the
number of measurements is limited only by the disk

capacity of the computer, which can be huge.
Standalone dataloggers perform the same function, but
record the data in their own memory for subsequent
download via a serial cable to a computer (or, more
recently, record the data on removable storage like SD
memory cards which can be physically transferred to a
computer for analysis). They are generally smaller, more
robust, and with lower power requirements than a PC
solution. This means they can be left for long-term opera-
tion using battery or solar power, although the memory
capacity may be a factor in how long they might be left
unattended. A final solution is for data to be digitized
locally and transmitted via radio to a base station equipped
with a computer.

16.6 Aeolian Abrasion

One basic 'instrument' type that has been used successfully
in a variety of aeolian settings are diverse targets used to
document the amount of abrasion produced by wind-blown
particles. Wooden stakes and fence posts provided clear
visual evidence of the abrasive power of wind-blown sand
long before Bagnold initiated quantitative aeolian investi-
gations. Indeed, both natural and man-made materials still
provide graphic visualization of the erosive power of sand
and wind; for example, the distinctive 'butterfly' pattern
that typically forms on metal sheets attached to the base of
telephone poles in windy desert environments (e.g., Fig. 1.5
of Greeley and Iversen 1985), illustrates that the wind and
particle density profiles above a surface combine to produce
zones of maximized abrasion. Robert Sharp elaborated on
what could be learned from the abrasion of wooden stakes;
he placed carefully weighed rods and blocks of diverse
materials (such as Lucite, gypsum, and common building
bricks)

16.5 Data Acquisition

There is more to measuring something than just a sensor;
the data must be recorded, and when time-resolved mea-
surements are needed—especially at turbulent timescales
(see Chap. 3 ) with many sensors (Fig. 16.13 )—the data
management challenge can be formidable. in the sand-and-wind-rich environment of the