Geotechnical News December 2010
29
Geotechnical Instrumentation News
Advanced Geotechnical Applications of
Distributed Fiber-Optic Sensing
Alexander M. Puzrin
Michael Iten
Dominik Hauswirth
Introduction
Distributed fiber-optic (FO) strain
sensors are offering new possibilities
in the field of geotechnical monitoring.
By integrating a single FO cable into
soil or structure, an unprecedented
amount of accurate, spatially resolved
data can be obtained. Current
commercially available technology
allows for strain measurements in the
microstrain (με) range (0.0001%) with
a spatial resolution of 1m along a 30km
long fiber.
In this article we describe recent
novel geotechnical FO technology ap-
plications in the laboratory and field.
The emphasis is to sketch the FO cable
layout, integration and the monitoring
results, with details of the projects giv-
en elsewhere (Iten et al., 2009a; Haus-
wirth et al., 2010; Iten & Puzrin, 2010).
For locating landslide boundaries, a
soil-embedded sensor system, a road-
embedded sensor and the reactivation
of an old inclinometer are described.
In addition, a new monitoring ground
anchor is presented. Finally, laboratory
testing of a novel sensor technology
offering spatial resolution below 5cm
indicates the direction where FO sen-
sor technology is heading: substitution
of hundreds of individual local strain
gauges with one single FO cable.
Distributed Fiber-Optic Sensing
Measurement Technology
Continuous strain can be measured
along optical fibers by several
techniques based on the Brillouin
scattering effect: spontaneous Brillouin
Optical Time Domain Reflectometry
(BOTDR) occurs when a light pulse
guided through a silica fiber is
backscattered by a nonlinear interaction
with thermally excited acoustic waves.
In the more refined Brillouin Optical
Time Domain Analysis (BOTDA), two
counter-propagating light waves (pump
and probe) at different frequencies
interact via stimulated acoustic waves.
The scattered light undergoes a
frequency shift, which is directly re-
lated to the strain and temperature in
the medium. Thus, in addition to the
strained FO cable, a loose fiber must
be placed for temperature compensa-
tion. The backscatter is recorded in the
time domain to obtain information of
the scattering location along the fiber
and the frequency shift of the signal is
analyzed and converted into strain and
temperature data. The strain measured
is the average value over the spatial
resolution (typically >1m), which cor-
responds directly to the length of the
light pulse sent down the fiber. Remote
control and automatic measurement
mode is possible.
Recently, a significant breakthrough
was achieved in narrowing the spatial
resolution down to 5cm with extremely
short pulse durations in the Brillouin
Echo Distributed Sensing (BEDS)
setup. The BEDS concept is based on
observing a “negative” gain created by
a very short-time phase shift applied on
the pump that interferes destructively
with the reflected light. BEDS is not
commercially available yet, but first
testing in soil has shown its potential
for future applications.
Table 1 gives an overview of the
listed technologies (see also Thévenaz,
2010).
Fiber-Optic Cables
FO cables used for integration into
different environments have to
comply with several requirements,
such as being strong enough to
withstand harsh installation conditions,
transmitting strain applied on the
jacket without loss to the fiber core,
allowing unproblematic handling
and offering flexible adjustment to
project modifications. The quality of
the FO cable and its fixations strongly
influences the overall measurement
accuracy of the sensing system.
Increasingly, specialty FO cables
for strain sensing are available from
cable manufacturers. Most important
for the user is to focus on the quality
Table 1. Comparison of distributed FO strain sensing technologies, according to manufacturer information
BOTDR
BOTDA
BEDS
Brillouin Optical Time
Domain Reflectometry
Brillouin Optical Time
Domain Analysis
Brillouin Echo Distributed
Sensing
Measurement accuracy
20με to 40με
2με to 10με
10με to 20με
Spatial resolution
1m
1m
0.05m
Max. distance
30km
30km
More than 5km
Availability
Commercially
Commercially
Lab prototype
Comment
Single fiber
Loop required
Loop required
