<s>
Rayleigh	O
scattering	O
based	O
distributed	B-Algorithm
acoustic	I-Algorithm
sensing	I-Algorithm
(	O
DAS	O
)	O
systems	O
use	O
fiber	O
optic	O
cables	O
to	O
provide	O
distributed	O
strain	O
sensing	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
Coherent	O
Rayleigh	O
Optical	B-Protocol
Time	I-Protocol
Domain	I-Protocol
Reflectometry	I-Protocol
(	O
COTDR	O
)	O
.	O
</s>
<s>
Unlike	O
other	O
distributed	O
fiber	O
techniques	O
such	O
as	O
those	O
based	O
on	O
Brillouin	B-Algorithm
or	O
Raman	B-Architecture
scatter	O
,	O
distributed	B-Algorithm
acoustic	I-Algorithm
sensing	I-Algorithm
is	O
only	O
able	O
to	O
detect	O
changes	O
in	O
temperature	O
rather	O
than	O
its	O
absolute	O
value	O
.	O
</s>
<s>
Distributed	B-Algorithm
acoustic	I-Algorithm
sensing	I-Algorithm
relies	O
on	O
light	O
which	O
is	O
Rayleigh	O
backscattered	O
from	O
small	O
variations	O
in	O
the	O
refractive	O
index	O
of	O
the	O
fiber	O
.	O
</s>
<s>
Brillouin	B-Algorithm
scatter	O
occurs	O
due	O
to	O
the	O
interaction	O
between	O
the	O
light	O
and	O
acoustic	O
phonons	B-Language
travelling	O
in	O
the	O
fiber	O
.	O
</s>
<s>
As	O
the	O
light	O
is	O
scattered	O
by	O
a	O
moving	O
phonon	B-Language
,	O
its	O
frequency	O
is	O
shifted	O
by	O
the	O
Doppler	O
effect	O
by	O
around	O
10GHz	O
.	O
</s>
<s>
Light	O
is	O
generated	O
at	O
both	O
above	O
(	O
anti-Stokes	O
shift	O
)	O
and	O
below	O
(	O
Stokes	B-Algorithm
shift	I-Algorithm
)	O
the	O
original	O
optical	O
frequency	O
.	O
</s>
<s>
Brillouin	B-Algorithm
scatter	O
is	O
much	O
weaker	O
than	O
Rayleigh	O
scatter	O
and	O
so	O
the	O
reflections	O
from	O
a	O
number	O
of	O
pulses	O
must	O
be	O
summed	O
together	O
to	O
enable	O
the	O
measurements	O
to	O
be	O
made	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
maximum	O
frequency	O
at	O
which	O
changes	O
can	O
be	O
measured	O
using	O
Brillouin	B-Algorithm
scatter	O
is	O
typically	O
a	O
few	O
tens	O
of	O
Hz	O
.	O
</s>
<s>
Raman	B-Architecture
scatter	O
occurs	O
when	O
light	O
is	O
scattered	O
in	O
interaction	O
with	O
molecular	O
vibrations	O
in	O
the	O
fiber	O
.	O
</s>
<s>
As	O
with	O
Brillouin	B-Algorithm
scattering	I-Algorithm
,	O
both	O
Stokes	O
and	O
anti-Stokes	O
components	O
are	O
produced	O
and	O
these	O
are	O
shifted	O
from	O
the	O
wavelength	O
of	O
the	O
incident	O
light	O
by	O
several	O
tens	O
of	O
nanometers	O
.	O
</s>
<s>
By	O
measuring	O
the	O
ratio	O
in	O
intensity	O
between	O
the	O
Stokes	O
and	O
anti-Stokes	O
components	O
,	O
an	O
absolute	O
value	O
of	O
temperature	O
can	O
be	O
measured	O
by	O
a	O
distributed	B-Algorithm
temperature	I-Algorithm
sensing	I-Algorithm
(	O
DTS	O
)	O
system	O
.	O
</s>
<s>
The	O
larger	O
wavelength	O
shifts	O
compared	O
to	O
Brillouin	B-Algorithm
scatter	O
mean	O
that	O
it	O
is	O
easier	O
to	O
separate	O
the	O
scattered	O
Raman	B-Architecture
light	O
from	O
the	O
un-shifted	O
Rayleigh	O
scattered	O
component	O
.	O
</s>
<s>
However	O
the	O
intensity	O
of	O
the	O
Raman	B-Architecture
scatter	O
is	O
lower	O
than	O
the	O
Brillouin	B-Algorithm
scatter	O
and	O
so	O
it	O
is	O
normally	O
necessary	O
to	O
average	O
for	O
many	O
seconds	O
or	O
minutes	O
in	O
order	O
to	O
get	O
reasonable	O
results	O
.	O
</s>
<s>
Therefore	O
,	O
Raman-based	O
systems	O
are	O
only	O
suitable	O
for	O
measuring	O
slowly	O
varying	O
temperatures	O
.	O
</s>
<s>
Phase-sensitive	O
coherent	O
optical	B-Protocol
time-domain	I-Protocol
reflectometry	I-Protocol
(ϕ-	O
OTDR	B-Protocol
)	O
is	O
a	O
technique	O
that	O
can	O
provide	O
sufficient	O
sensitivity	O
and	O
resolution	O
for	O
these	O
distributed	B-Algorithm
acoustic	I-Algorithm
sensing	I-Algorithm
systems	O
.	O
</s>
<s>
Standard	O
optical	B-Protocol
time-domain	I-Protocol
reflectometry	I-Protocol
techniques	O
use	O
light	O
sources	O
with	O
coherence	O
lengths	O
,	O
which	O
are	O
shorter	O
than	O
pulse	O
lengths	O
.	O
</s>
<s>
On	O
the	O
contrary	O
,	O
in	O
ϕ-OTDR-based	O
sensors	O
,	O
the	O
coherence	O
length	O
of	O
lasers	O
is	O
longer	O
than	O
their	O
pulse	O
length	O
.	O
</s>
<s>
The	O
use	O
of	O
distributed	B-Algorithm
acoustic	I-Algorithm
sensing	I-Algorithm
has	O
also	O
been	O
demonstrated	O
to	O
monitor	O
hydraulic	O
stimulation	O
in	O
enhanced	O
geothermal	O
systems	O
(	O
EGS	O
)	O
as	O
well	O
as	O
the	O
injection	O
of	O
carbon	O
dioxide	O
in	O
several	O
carbon	O
capture	O
and	O
storage	O
(	O
CCS	O
)	O
projects	O
.	O
</s>
