Field Studies - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 173

16

Field Studies

Sand dunes are an outdoor phenomenon, and one of the joys
of dune research is to go out into the field, tramp around and
over dunes, and study them closely. Here we review some
of the field research techniques that have been and are
employed. Crudely speaking, these techniques may be
gathered in four categories: the study of sand, the study of
shape and structure, observation of wind, and the study of
aeolian processes.
We close with some discussion about the special chal-
lenges of making field observations with robot proxies on
other worlds. As discussed by Clancey (2012), this practice
challenges our somewhat arbitrary distinction in Chaps. 16
and 17 of field and laboratory work. While Mars rovers have
acted as surrogate field geologists, moving around,
observing, and occasionally manipulating the environment,
they also carry sophisticated instrumentation that is more
traditionally retained in a laboratory on Earth. Nonetheless,
in the spirit of exploration, we discuss them in this section.

Even where sand is to be merely sampled in the field, in
order to permit closer study in the laboratory, elaborate
protocols may be required. For example, samples for OSL
dating require that samples from depth be acquired without
exposure to sunlight, so stainless steel tubes which can be
sealed in a light-tight manner are driven into the dune.
A variety of field instruments exist for measuring the
cohesion of soils. A common one is the cone penetrometer,
a device driven or hammered into the ground and the force
needed to do so is measured (or, equivalently, the number of
calibrated hammer blows to drive a cone a given distance is
recorded). The Apollo astronauts used such an instrument
on the moon. Another measurement is the shear vane, a
cruciform set of plates which are turned in the ground.
A property of interest is the saltation threshold—the
windspeed or shear stress at which material can be lofted
from the ground (this is perhaps studied more in the context
of dust than sand). This can be measured in the field with a
portable wind tunnel, an approach pioneered by Bagnold
(see Chap. 17 ) .
A more convenient apparatus is the PI-SWERL (Portable
In Situ Wind Erosion Lab) developed at the Desert
Research Institute (e.g., Etyemezian et al. 2007). Essen-
tially, this is a chamber placed over the ground, with a
spinning blade to develop shear (not unlike a lawnmower).
The wind shear applied to the surface can be related to the
blade speed, which is controllable, and air is sucked out of
the chamber through an optical dust counter, so the dust
emission as a function of shear stress can be quantified. The
chamber is easily moved from spot to spot to assess the
heterogeneity of the dust emission threshold.
Some interestingly ad-hoc methods of estimating soil
strength or saltation threshold have been tried. Li et al.
(2010) report a correlation of friction velocity with mea-
surements of the size of crater produced when shooting
pellets into the ground with an airgun.

16.1 Sand Measurements in the Field

Although the more elaborate investigations of sand compo-
sition require a laboratory (see Chap. 17 ), some basic prop-
erties are easy to evaluate in the field. The use of a set of sieves
can determine the size distribution, which can also be crudely
gauged by close examination with a microscope or even a
hand lens. Particle shape, and the heterogeneity of sand (how
much quartz vs dark minerals vs shell fragments) can be
similarly estimated by eye, or more quantitatively by spec-
troscopy or color imaging (see Chap. 18 ) . As noted in Fig. 5.
10 , the presence of magnetic minerals is readily indicated by
using a permanent magnet. Some laboratory techniques have
become sufficiently miniaturized to employ in the field—-
hand-held X-ray fluorescence instruments, much like those
used on spacecraft, are now available, for example.