Environmental Engineering Reference
In-Depth Information
7.2 Remote-sensing instruments and platforms
Remote sensing has the attribute of being able to supply information on a target
(a gas or cloud of particles) without touching it. This is especially useful when
making measurements at volcanoes where proximity to toxic gases or within range
of ejecta is hazardous. It is also useful for monitoring targets that are remote and
dif
cult to get to. Remote-sensing instruments are deployed on satellite platforms,
on aircraft, on unmanned aerial vehicles (see also Chapter 9 ) and, for ground-based
activities, they can be mounted on vehicles, on tripods or attached to moveable
mounts for scanning purposes. Here we are concerned with remote-sensing meas-
urements of gases, principally SO 2 and volcanic ash particles. Table 7.1 provides a
summary of the most widely used instruments deployed on satellite platforms and
on the ground, together with a few details of the wavelength ranges utilised, the
gases measured and the horizontal spatial resolution.
All of the instruments shown in Table 7.1 are passive; the main active instru-
ment useful for measuring volcanic species is the CALIOP (Cloud-Aerosol Lidar
with Orthogonal Polarization) lidar on board the Cloud-Aerosol Lidar and Infrared
Path
nder Satellite Observation (CALIPSO) satellite. CALIOP measures back-
scattered laser light along a narrow strip at nadir as CALIPSO travels along its
orbital path. The lidar is insensitive to SO 2 gas but can detect sulfate aerosols and
ash particles. The recent work by Winker et al .( 2012 ) gives a good overview of
the capability of CALIOP for measuring volcanic ash. There are some passive
microwave instruments (e.g. Special Sensing Microwave Instrument - SSMI - and
the Advanced Microwave Sounding Unit - AMSU/B) that might be able to
provide information on ash in the early stages of a volcanic eruption when par-
ticle sizes are large, radii
m. At the moment the spatial resolution of
passive microwave sensors in space is quite large (pixel sizes c .30
>
500
μ
100 km) and
more research is required before an assessment can be made of the utility of passive
microwave data. However, the Microwave Limb Sounder (MLS) has been used
to retrieve volcanic SO 2 from space (Read et al ., 1993 ; Prata et al ., 2007 ).
-
7.3 Remote sensing of volcanic ash
7.3.1 Volcanic ash
Volcanic ash is the term given to anything ejected from a volcano that is less than
2 mm in diameter. Volcanic ash encompasses a wide variety of compositional
types, size distributions and shapes. Particle size 1
and the size distribution are
1
For historical reasons, the volcanological and atmospheric science communities differ in the way they typically
describe particle size. Volcanologists de
ne particle size in terms of diameter whereas in atmospheric science,
particle size is described in terms of radius. The log-based Krumbein
ϕ
scale of Krumbein and Sloss ( 1963 )is
also commonly used in the geological sciences; it is de
ned as
ϕ ¼
log 2 D , where D is particle diameter.
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