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Geotechnical News • December 2012
25
GEOTECHNICAL INSTRUMENTATION NEWS
Contact versus non-contact
(remote) monitoring
Traditional geotechnical monitoring
is based on a “contact” approach. In
other words, the sensors are installed
directly in contact with the ground/
structure, both on the surface (e.g.
crackmeters) or inside (e.g. incli-
nometers). In contrast, most remote
methods are based on a “non-contact”
approach, i.e. the data collection is
based on sensors that are installed far
away from the monitoring site. How-
ever, based on the degree of interac-
tion with the ground/structure, remote
monitoring methods can be divided in
two main subcategories:
• Partially remote
. Defined as those
methods that, even if based on a
remote sensor, require the installa-
tion of some additional sensors or
targets at the monitoring site (e.g.
antennas for D-GPS, prisms for to-
tal stations).
• Fully remote
. Defined as those
methods that do not require any in-
stallation at the monitoring site.
When moving from contact monitor-
ing to fully remote monitoring, the
following changes must be considered:
• A progressive reduction of interac-
tion with the ground/structure.
• An increasing size of the investi-
gated area.
• Aprogressive reduction of the local-
ization precision of the monitoring
point (spatial resolution).
• An increasing of the spatial infor-
mation density.
Furthermore, for remote methods,
noise related to wave propagation
through the atmosphere must be
accounted for. Hence, when moving
from contact to non-contact monitor-
ing an increased complexity in data
processing and care in the data analy-
sis and interpretation is required.
sification based on the following main
features:
• Type of platform. The type of plat-
form will be divided on the basis of
the sensor location:
à
à
“ground based” when the
sensor is installed on the
ground surface;
à
à
“aerial based” when the sen-
sor is installed on an airplane;
à
à
“satellite based” when the
sensor is installed on a satel-
lite.
• Type of wave. The type of wave that
the sensor collects will be divided
on the basis of the following cat-
egories:
à
à
visible (wavelength range:
400nm – 700nm);
à
à
infrared (wavelength range:
700nm – 1mm);
à
à
microwaves (wavelength
range: 1mm – 1m);
• Type of sensor. Sensors will be di-
vided between active and passive:
à
à
“active sensors”, emit a wave
and receive the reflection of
the emitted wave from the
ground/structure;
à
à
“passive sensors” receive the
wave naturally emitted by the
ground/structure following a
“natural” emission (e.g. the
sun).
Terrestrial laser scanning (TLS)
TLS is a ground based fully remote
technique that uses a visible and near
infrared wave active sensor. TLS
collects the coordinates of several
points, thus achieving 3D models of
the ground/structure. By comparison
of point clouds collected at different
times, ground/structure deformation is
detected. The main fields of applica-
tion are slope instabilities, dams and
mines.
Terrestrial interferometric synthetic
aperture radar (TInSAR)
TInSAR is a ground based fully
remote technique that uses a micro-
wave active sensor. TInSAR collects
2D images of large areas (few km
2
)
with a high sampling rate. By com-
parison of SAR images collected
at different times, ground/structure
deformation is detected. The main
fields of application are slope instabili-
ties, dams, mines, heritage structures
and civil buildings.
Robotic total station (RTS)
RTS is a ground based partially remote
technique that uses a visible or near
infrared active sensor. RTS collects
the precise position of several prisms
installed on the ground/structure. By
comparison of the prism positions
at different times, ground/structure
deformation is detected. The main
fields of application are slope instabili-
ties, dams, mines, civil buildings and
heritages structures.
Reflectorless robotic total station
(RRTS)
RRTS is a ground based fully remote
technique that uses a visible or near
infrared active sensor. RRTS collects
the precise position of several natu-
ral targets on the ground/structure.
By comparison of the natural target
position at different times, ground/
structure deformation is detected. The
main fields of application are tunnel-
ing in urban areas, civil buildings and
heritages structures.
Satellite interferometric synthetic
aperture radar (SInSAR)
SInSAR is a satellite based fully
remote technique that uses a micro-
wave active sensor. It is based on the
collection (since 1992) of 2D images
of large areas (several km
2
) with a
low sampling rate. By comparison of
images collected at different times,
ground/structure deformation is
detected. The main fields of applica-
tion are fluid extraction/pumping, tun-
neling in urban areas, civil buildings
and slope instabilities.
Remote methods: a quick
overview
A brief description of the basic operat-
ing principle of the seven methods is
presented below, together with a clas