Rayleigh Flashcards
If an acoustic wave travels along the fibre with apparent velocity* VA and frequency fA, the
corresponding wavenumber is kA =_________. Therefore, for a fixed speed of sound, the wavenumber
filter applies a frequency ______to the data
what is meant by dynamic strain sensing
if the spatial response is converted to the spatial frequency (wavenumber) domain, this means that the gauge length introduces ____ in the wavenumber response and so it behaves like a wavenumber filter.
The finest scale is usually the (native) sample separation, determined by the sampling rate fd of the
acquisition system. The sample separation is often dictated by the need to estimate the _______
some designs of DVS systems provide the unwrapped phase at each location, whereas others provide the phase difference between successive probe pulses; the latter is of course the derivative of the former, but the geophysical meaning is subtly different in the two cases.
Alongside the development of acoustic sensor arrays, optical time-domain reflectometry (OTDR)
moved from multimode to single-mode fibre around 1980 and 1981 and this led to the discovery of
the _________ in these fibres.
In distributed fibre-optic vibration sensors, the displacement of the medium is converted to a dynamic
_____on the sensing fibre and that strain is read remotely by the interrogator
The multiplexing of these sensors was developed in the 1980s using the ____, ____ and ______ domains to distinguish the different sensor elements positioned along a fibre.
Rayleigh scattering is an elastic process caused by
localised inhomogeneities of the refractive index of the medium through which an electro-magnetic wave is travelling. These inhomogeneities cause re-radiation of a small fraction of the probe light with a _____pattern and have a fixed phase relation to the incident light.
DVS uses the time axis both to discriminate locations along the fibre (as is the case in other forms
of OTDR) and to ______
In the case of conventional (electrical) vibration sensors, the response is proportional to acceleration (for accelerometers)
what is Rayleigh scattering
Whereas the ability of other types of DOFS to distinguish closely spaced features is characterised by both
sampling resolution and spatial resolution (that in turn is defined by the pulse duration and acquisition
bandwidth), in DVS, a third parameter is involved, namely the ______.
However, it is possible in some systems to extract data for _________ gauge lengths; i.e. there can be some benefit in selecting an output trace separation that is smaller than the gauge length. This provides better clarity of images where the gauge length is similar to the distance between sensors in the conventional survey, but the higher output sampling trace for the optical data shows more detail, particularly for the slower waves
Although DVS/DAS systems
appear to show seismic images that are similar, for example, when viewed on a greyscale image, to those
recorded with conventional sensor arrays, the details of the wavelets recorded by DVS and by electrical
signals are ______in ways that can affect the interpretation and processing of the data
The main distinction between vibration and quasi-static measurements can be summarised by the fact
that, in the vibration measurement, the backscatter signature is analysed as a function of time at ____
source frequency, whereas for the static measurements, the backscatter signature is measured and processed
as a ____of probe frequency
Although a distributed vibration sensor (DVS) responds to vibration, the readings are not directly comparable to those of conventional vibration sensors, such as ____or ______.
In some cases, the signal is divided into frequency bands that
are selected to discriminate between different sources of vibration, a practice known as calculating the ________
the frequency of readings in fast time versus slow time is determined by
In distributed fibre-optic vibration sensors, the displacement of the medium is converted to a _______
strain on the sensing fibre and that strain is read remotely by the interrogator.
Although a distributed ibration sensor (DVS) responds to vibration, the readings are not directly comparable to those of conventional vibration sensors, such as accelerometers or geophones. The interpretation of DVS data must take these differences into account; they arise partly from the use of strain, rather than ________ directly and partly because the fibre sensor integrates the strain over a gauge length.
The connection between time and distance along the fibre is determined by the speed of light c. The tracking of vibration
signals at specific locations of the fibre occurs on a much _____ timescale, appropriate to the monitoring of mechanical vibration.
The finest scale is usually the (native) sample separation, determined by the_______ fd of the
acquisition system
what is fast time and slow time
the gauge length should be set to a larger value than
the fibre ______occupied by the probe pulse in order to ensure a monotonic response
what is the difference between fast and slow time visually
A distributed vibration sensor integrates the strain over a gauge
length and so behaves like a _____ filter in the spatial dimension along the fibre.
In some applications, particularly when a long observation time is required and an indication of the vibrational energy at each location is sufficient, the measurement can be summarised by calculating the _____ or ______ of the signal at each location over pre-defined timeframes (such as 1 s
or 30 s as appropriate to the application).
Quite separately, hydrophones or microphones measure fluctuations of the ambient _____; these sensors generally respond to isostatic pressure independently of the direction
from which the wave impinges on their surface
Locally, at an infinitesimal section of fibre dz at location z, the change in length dL due to a sinusoidal acoustic wave
travelling along the fibre will be (equation)
historically why have the frequency bands been segmented
how does frequency variation compare for Rayleigh dynamic versus slow strain sensing
what is meant by static strain sensing
the longer the gauge length is in relation to pulse width, the more ______the response with respect to strain will be.
In the case of conventional (electrical) vibration sensors, the response is proportional to particle
____(for geophones)
A sigle value is returned for each gauge and the response to a localised event anywhere within a gauge _____ will affect that value
A sigle value is returned for each gauge and the response to a localised event anywhere within a gauge length will affect that value. In that sense, the system is unable to distinguish the location of the event to better
than a gauge _____
dynamic variations of strain are detected as a function of position along the fibre, and this provides a distributed sensor of ______ vibration.
A mechanical vibration is a _____(or quasi-_____) displacement of the medium in which it takes
place
Acoustic signals or mechanical vibrations were amongst the first measurands to be investigated with optical fibre sensors, driven by interest in _______ applications.
why does Arthur call it DVS
The distance occupied by the probe pulse (a pulse is usually the probe waveform adopted in DVS) is
usually a few times _____than the sample separation.
Distributed vibration sensors convert the wave, whether a _____wave or a ______wave, into a strain
on the fibre
The signals generated by a coherent OTDR cannot be modelled analytically because they arise from ______ effects; nonetheless, statistical models can be built showing specific realisations that illustrate the behaviour of signals or the aggregated behaviour of a set of such signals
