U.S. patent application number 16/746617 was filed with the patent office on 2020-07-16 for remote sensing of natural gas leaks.
This patent application is currently assigned to New Era Technology, Inc.. The applicant listed for this patent is New Era Technology, Inc.. Invention is credited to T. Boyd Tolton.
Application Number | 20200225111 16/746617 |
Document ID | / |
Family ID | 59239806 |
Filed Date | 2020-07-16 |
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United States Patent
Application |
20200225111 |
Kind Code |
A1 |
Tolton; T. Boyd |
July 16, 2020 |
REMOTE SENSING OF NATURAL GAS LEAKS
Abstract
A method of detecting natural gas releases that includes the
step of traversing a target area with a gas-filter correlation
radiometer having a field of view oriented towards the target area.
The gas-filter correlation radiometer receives reflected radiation
in a passband from the target area and produces gas-filter
correlation radiometer signals from the received reflected
radiation. A surface reflectivity spectral profile of the target
area is determined. The presence of methane in the target area is
then determined based upon the received reflected radiation and the
surface reflectivity spectral profile of the target area.
Inventors: |
Tolton; T. Boyd; (Edmonton,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
New Era Technology, Inc. |
Boardman |
OH |
US |
|
|
Assignee: |
New Era Technology, Inc.
Boardman
OH
|
Family ID: |
59239806 |
Appl. No.: |
16/746617 |
Filed: |
January 17, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15630882 |
Jun 22, 2017 |
10578514 |
|
|
16746617 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2201/0214 20130101;
G01N 2021/3531 20130101; G01N 21/3518 20130101; G01N 2021/1793
20130101; G01N 2201/0216 20130101; G01N 2201/068 20130101; G01M
3/38 20130101; G01N 2021/3509 20130101 |
International
Class: |
G01M 3/38 20060101
G01M003/38; G01N 21/3518 20060101 G01N021/3518 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 27, 2016 |
CA |
2934093 |
Claims
1. A method of detecting a leak of a hydrocarbon, the method
comprising the steps of: traversing a target area with a gas filter
correlation radiometer having a field of view oriented towards the
target area, the gas filter correlation radiometer comprising a
bandpass filter and a gas correlation cell, the bandpass filter
having a bandpass including at least part of the range of 4150
cm.sup.-1 to 4450 cm.sup.-1 and being arranged to filter radiation
passing through the gas correlation cell in a first path and
through an evacuated cell in a second path, the gas correlation
cell containing a gas having a spectral band within the bandpass of
the bandpass filter overlapping a spectral band of the hydrocarbon;
detecting radiation that has passed through the gas correlation
cell to generate a first signal; detecting radiation that has
passed through the evacuated cell to generate a second signal; and
comparing the first signal and the second signal to determine if
the hydrocarbon is present in the target area; and in which the
first signal and the second signal are accumulated and compared
over an integration period of 0.01 seconds or less.
2. The method of claim 1 in which the bandpass includes at least
part of the range 4175 cm.sup.-1 to 4275 cm.sup.-1.
3. The method of claim 2 in which the bandpass includes a methane
peak at 4350 cm.sup.-1.
4. The method of claim 1 in which the bandpass is included in the
range of 4150 cm.sup.-1 to 4450 cm.sup.-1.
5. The method of claim 1 in which the gas correlation radiometer
further comprises an InGaAs infrared detector.
6. A gas filter correlation radiometer, comprising: a bandpass
filter, a beam splitter following the bandpass filter providing a
first path through the gas filter correlation radiometer and a
second path through the gas filter correlation radiometer; a gas
correlation cell on the first path, the bandpass filter having a
bandpass including at least part of the range of 4150 cm.sup.-1 to
4450 cm.sup.-1 and being arranged to filter radiation passing
through the gas correlation cell, the gas correlation cell
containing a gas having a spectral band within the bandpass of the
bandpass filter overlapping a spectral band of the hydrocarbon; an
evacuated cell on the second path; a first detector arranged to
receive radiation that has passed along the first path and produce
output comprising a first signal; a second detector arranged to
receive radiation that has passed along the second path and produce
a second signal; and detector electronics having the first signal
and the second signal as input, the detector electronics being
configured to compare the first signal and the second signal; and
in which the gas filter correlation radiometer is configured to
accumulate and compare the first signal and the second signal over
an integration period of 0.01 seconds or less.
7. The gas filter correlation radiometer of claim 6 in which the
bandpass includes at least part of the range 4175 cm.sup.-1 to 4275
cm.sup.-1.
8. The gas filter correlation radiometer of claim 6 in which the
bandpass includes a methane peak at 4350 cm.sup.-1.
9. The gas filter correlation radiometer of claim 6 in which the
bandpass is included in the range of 4150 cm.sup.-1 to 4450
cm.sup.-1.
10. The gas filter correlation radiometer of claim 6 in which the
gas correlation radiometer further comprises an InGaAs infrared
detector.
11. The method of claim 1 in which the gas filter correlation
radiometer traverses the target area at an altitude of 300 meters
or less.
12. The gas filter correlation radiometer of claim 6 configured to
be mounted on a vehicle for traversing a target area at an altitude
of 300 meters or less.
13. A vehicle configured to traverse a target area at an altitude
of 300 meters or less, the vehicle having mounted a gas filter
correlation radiometer as claimed in claim 6.
Description
FIELD
[0001] Remote sensing of natural gas leaks.
BACKGROUND
[0002] This invention relates to remote sensing techniques to
detect gas leaks. In particular, mounting a remote sensing
instrument on a ground or aerial vehicle that can survey a target
area, such as a pipeline, and measuring absorption of upwelling
electromagnetic radiation that has passed through gas-filter
correlation radiometer (GFCR).
[0003] A GFCR is a remote sensing radiometer that uses a sample of
the gas as a spectral filter, providing enhanced sensitivity and
selectivity to that gas. Incoming radiation is passed through a
correlation cell, which is undergoing a gas-density modulation
along its optical path. The radiation is then passed through a
bandpass filter, which passes only a specific spectral (passband)
range selected to cover an absorption band of the gas of interest.
The radiation is then measured by an infrared detector. GFCRs have
been used in different configurations for over three decades in
remote sensing instrumentation.
[0004] Methane (CH.sub.4) comprises approximately 95% of the
composition of natural gas. However, CH.sub.4 exists in fairly
large quantities in the atmosphere (it is well mixed in the
atmosphere with a concentration of approximately 1.7 ppm).
Therefore, detecting a gas leak required detection of a small
increase on a large background. Events such as passing near a
source region of CH.sub.4 (such as a farm) or an increase in the
altitude of the airplane (an increase in the atmospheric path
length) might result in the false signature of a leak.
[0005] To reduce the influence of the background, some past
attempts have tried to detect the excess CH.sub.4 of a natural gas
leak by detecting the absorption of CH.sub.4 in the infrared
wavelength regions where the absorption bands are greatest--for
example, at 7.8 .mu.m (2180 cm.sup.-1) or 3.3 .mu.m (3000
cm.sup.-1). This provides the advantage that the upwelling
radiation is primarily emitted from the earth's surface. This
minimises the background CH.sub.4, as only the CH.sub.4 located
between the remote sensing instrument and the earth's surface is
detected. However, for underground pipe--since the temperature of
the surface and the leaked CH.sub.4 are nearly the same--the
radiative contrast between the surface and the leaked methane is
very small, greatly reducing the detectivity/detectability of the
leak. Also, the thermal noise introduced within the instrument
itself becomes a serious design constraint.
[0006] As the background of CH.sub.4 becomes very large, the solar
radiation reaching the instrument would have passed through entire
atmosphere. The best known satellite instrument to attempt to
measure lower atmospheric trace gases using GFCRs was the MOPITT
(Measurements Of Pollution In The Troposphere) instrument launched
on NASA's Terra satellite. MOPITT was a satellite instrument
launched in December 1999. MOPITT was designed to measure the
concentrations CH.sub.4 in the lower atmosphere utilising the 2.3
.mu.m wavelength. The 2.3 .mu.m CH.sub.4 channels of MOPITT failed
as the signal-to-noise ratio (SNR) of the measurements did not
provide enough resolution to measure the concentration of CH.sub.4
to a resolution .ltoreq.1%, which was required for global
atmospheric chemistry models. As a consequence of this failure,
attempts to measure CH.sub.4 in lower atmosphere using the 2.3
.mu.m wavelength have been discounted.
SUMMARY
[0007] In an embodiment, there is provided a gas filter correlation
radiometer, comprising a bandpass filter, a beam splitter following
the bandpass filter providing a first path through the gas filter
correlation radiometer and a second path through the gas filter
correlation radiometer; a gas correlation cell on the first path,
the bandpass filter having a bandpass including at least part of
the range of 4150 cm-1 to 4450 cm-1 and being arranged to filter
radiation passing through the gas correlation cell, the gas
correlation cell containing a gas having a spectral band within the
bandpass of the bandpass filter overlapping a spectral band of the
hydrocarbon; an evacuated cell on the second path; a first detector
arranged to receive radiation that has passed along the first path
and produce output comprising a first signal; a second detector
arranged to receive radiation that has passed along the second path
and produce a second signal; and detector electronics having the
first signal and the second signal as input, the detector
electronics being configured to compare the first signal and the
second signal.
[0008] In an embodiment, there is provided a method of detecting a
leak of a hydrocarbon, the method comprising traversing a target
area with a gas filter correlation radiometer having a field of
view oriented towards the target area, the gas filter correlation
radiometer comprising a bandpass filter and a gas correlation cell,
the bandpass filter having a bandpass including at least part of
the range of 4150 cm-1 to 4450 cm-1 and being arranged to filter
radiation passing through the gas correlation cell in a first path
and through an evacuated cell in a second path, the gas correlation
cell containing a gas having a spectral band within the bandpass of
the bandpass filter overlapping a spectral band of the hydrocarbon;
detecting radiation that has passed through the gas correlation
cell to generate a first signal; detecting radiation that has
passed through the evacuated cell to generate a second signal; and
comparing the first signal and the second signal to determine if
the hydrocarbon is present in the target area.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] There will now be described preferred embodiments of the
invention, with reference to the drawings, by way of illustration
only and not with the intention of limiting what is defined by the
claims, in which like numerals denote like elements and in
which:
[0010] FIG. 1 is a schematic of the gas filter correlation
radiometer;
[0011] FIG. 2 is a schematic of an alternative embodiment of the
gas filter correlation radiometer;
[0012] FIG. 3 depicts a helicopter using the gas filter correlation
radiometer to detect a leak in a pipeline;
[0013] FIG. 4 depicts an overhead view of a helicopter traversing a
pipeline and shows successive fields of view, including an exploded
view of a portion of a field of view being sampled;
[0014] FIG. 5 is graph of the spectral absorbance of methane;
[0015] FIG. 6 is a graph of the spectral absorbance of methane in
the 3.3 .mu.m wavelength range;
[0016] FIG. 7 is a graph of the spectral absorbance of methane in
the 7.7 .mu.m wavelength range;
[0017] FIG. 8 is graph of the spectral absorbance of ethane;
[0018] FIG. 9 is a graph of the spectral absorbance of ethane in
the 3.3 .mu.m wavelength range;
[0019] FIG. 10 is a graph of the spectral absorbance of ethane in
the 6.7 .mu.m and 12 .mu.m wavelength ranges;
[0020] FIG. 11 is a graph of the transmission of a 1 cm long gas
cell filled with 1 atm of CH.sub.4 at 20.degree. C. overlaid with a
wide bandpass filter (4150 to 4450 cm.sup.-1) and a narrow bandpass
filter (4175 to 4275 cm.sup.-1);
[0021] FIG. 12 is a graph of the sensitivity (.DELTA.D2A) to leak
methane for a wide bandpass filter, a narrow bandpass filter, a 3.3
.mu.m realSens.TM. radiometer and a 3.3 .mu.m realSens.TM.
radiometer with no terrestrial emission included (a realSens.TM.
radiometer is a radiometer made by Synodon of Edmonton, Alberta,
Canada, and is designed in accordance with the general description
of the exemplary disclosed radiometer, other than use of the 2.3
.mu.m peak and band;
[0022] FIG. 13 is a graph of various surface types across the wide
and narrow 2.3 .mu.m bandpasses;
[0023] FIG. 14 is a graph of various surface types across the wide
and narrow 3.3 .mu.m realSens.TM. bandpass;
[0024] FIG. 15 is a graph of D2A signals as a function of effective
Rsurf, for different surface types, for the wide bandpass 2.3 .mu.m
realSens.TM. radiometer;
[0025] FIG. 16 is a graph of D2A signals as a function of effective
Rsurf, for different surface types, for the narrow bandpass 2.3
.mu.m realSens.TM. radiometer;
[0026] FIG. 17 is a graph of D2A signals as a function of effective
Rsurf, for different surface types, for the 2.3 .mu.m realSens.TM.
radiometer;
[0027] FIG. 18 is a graph of the change in D2A signal as a function
of Rsurf, for modified calculation parameters using results from a
wide bandpass 2.3 .mu.m realSens.TM. radiometer
[0028] FIG. 19 is a graph of the change in D2A signal as a function
of Rsurf, for modified calculation parameters using results from a
2.3 narrow bandpass .mu.m realSens.TM. radiometer;
[0029] FIG. 20 is a graph of the change in D2A signal as a function
of Rsurf, for modified calculation parameters using results from a
3.3 .mu.m realSens.TM. radiometer;
[0030] FIG. 21 is a graph of the transmission of a 100 ppm-m gas
cell (at 20.degree. C.) of ethane and methane;
[0031] FIG. 22 is a schematic of an alternative embodiment of the
optical configuration of realSens.TM. radiometer using a two focal
plane array (FPA) configuration;
[0032] FIG. 23 is a schematic of the optical configuration of
realSens.TM. radiometer;
[0033] FIG. 24 is an illustration of the configuration of a
truck-mounted embodiment of a 2.3 .mu.m realSens.TM.
radiometer;
[0034] FIG. 25 is a schematic of a 320.times.256 pixel FPA with the
proposed 32.times.1 binned-pixel arrays mapped; and
[0035] FIG. 26 is a schematic of a 320.times.256 pixel FPA with the
proposed 32.times.8 binned-pixel arrays mapped.
DETAILED DESCRIPTION
[0036] In this document, the word "comprising" is used in its
non-limiting sense to mean that items following the word in the
sentence are included and that items not specifically mentioned are
not excluded. The use of the indefinite article "a" in the claims
before an element means that one of the elements is specified, but
does not specifically exclude others of the elements being present,
unless the context clearly requires that there be one and only one
of the elements.
[0037] Leaks may be detected using a short wavelength absorption
band of CH.sub.4--for example, at 2.3 .mu.m (4350 cm.sup.-1)--to
measure CH.sub.4 leaks, and detecting this short wavelength
absorption band may have a number of benefits over measurements at
longer wavelengths--for example, at 3.3 .mu.m or 7.8 .mu.m. For
example, the absorption lines of CH.sub.4 at 2.3 .mu.m are denser,
the amount of solar energy is significantly higher, the
reflectivities of most surface types are higher, and the upwelling
thermal energy emitted by the surface is significantly less.
[0038] The instrument used for detection of leaks is a type of
gas-filter correlation radiometer (GFCR). GFCRs have been used in
different configurations for over 3 decades in remote sensing
instrumentation.
[0039] The following terminology is used concerning signals in the
realSens.TM. detector instrument: the COR signal is a signal
measuring the light passing through a correlation cell (containing
a gas, such as C.sub.2H.sub.6 or CH.sub.4), the REF signal is a
signal measuring the light that passed through a reference cell not
containing any optically active gas, the DIFF signal represents the
difference between the COR and REF signals, the AVG signal
represents the average of the COR and REF signals, and the D2A
signal represents the ratio of the DIFF and AVG signals.
[0040] The basic concept of the instrument is a standard GFCR
configuration, consisting of two radiometers viewing the same FOV
(field of view). For example, this may be achieved using a single
optical chain by splitting the optical chain in the middle and
using wedges to separate the focused image of each side onto a
single Focal Plane Array (FPA). FIG. 22 shows a schematic of this
configuration. In a realSens.TM. detector, the FPA may consist of
two 32.times.1 linear array separated by 10 mm. An alternate
configuration which could be considered is to completely separate
the REF and COR channels into two separate optical chains, as shown
in FIG. 23. This may make the optical design easier but would
require two separate detector systems (FPAs) and alignment of the
FOVs would be more difficult to ensure. FIGS. 1 and 2 show in more
detail possible configurations of a realSens.TM. detector
instrument and FIGS. 3 and 4 show exemplary use of these
configurations.
[0041] Referring to FIG. 1, there is shown a GFCR 101 incorporated
within a housing 100, with a detector section, such as a pair of
photodiode arrays 102A, 102B mounted in the housing. Radiation from
source 126 passes through a window 103 in the housing 100 is
collected by collector optic 124 and filtered by bandpass filter
116 and then directed by collimating lens 122 onto beam splitter
106. The bandpass filter 116 has a bandpass including at least part
of the range of 4150 cm-1 to 4450 cm-1 and is arranged to filter
light passing through the gas correlation cell, the gas correlation
cell containing a gas having a spectral band within the bandpass of
the bandpass filter overlapping a spectral band of the hydrocarbon.
The gas may for example be methane or ethane. The bandpass may
include at least part of the range 4175 cm-1 to 4275 cm-1 or a
range of the range of 4150 cm-1 to 4450 cm-1. In an exemplary
embodiment, a 40 cm.sup.-1 wide band-pass filter 116 centred at
4350 cm.sup.-1 is specified. The filter width is 1.3% of the
central wavenumber. The passband of filter 116 may be selected to
include an ethane or methane absorption peak and exclude radiation
falling outside of the peak. Beam splitter 106 formed by a
partially reflective mirror splits the radiation from the radiation
source 126 along paths 110 and 112. On the first radiation path
110, the radiation passes through gas correlation cell 114 and is
focused by detector lens 104A onto a detector, here a photodiode
102A. On the second radiation path 112, the radiation is directed
by mirror 120 through an evacuated gas cell 118 and is focused by
lens 104B onto a second detector, here photodiode 102B. The gas
correlation cell 114, also called a gas filter or absorption cell,
contains a gas, such as methane or ethane, to be detected.
[0042] The gas correlation cell 114 may for example be a 1 cm cell
with for example a concentration of ethane provided by one
atmosphere of pure C.sub.2H.sub.6. The second path 112 has a
different path length of C.sub.2H.sub.6, such as may be obtained by
providing the cell 118 with for example no C.sub.2H.sub.6, as for
example an evacuated gas cell or a cell containing a gas that is
optically neutral in relation to the ethane spectra of interest.
The output of the photodiodes 102A, 102B is provided to suitable
electronics, such as a computer 108, for processing according to
gas filter correlation radiometer techniques. The GFCR 101 may use
a beam splitter, for example, in the form of a partially reflective
mirror as shown in FIG. 1, or in the form of a bi-prism, as shown
in FIG. 2, or may selectively direct the incoming radiation through
separate paths, in a time division manner, using for example a
chopper. The use of a beam splitter versus a chopper is a trade-off
between simultaneity of the two received signals and loss of signal
intensity. A beam splitter, such as a partially reflective mirror
or a bi-prism, is preferred for gas leak detection because it
provides simultaneous measurement of both detector signals. This
can be important because the signals are fast varying due to the
forward motion of the helicopter and the variation in the
reflective surface.
[0043] A different optical configuration is shown in an alternative
embodiment in FIG. 2. Radiation from source 226 passes through a
window 203 in housing 200, is collected on collector optic 224 and
focused to a field stop 216. The field stop 216 is used to limit
the field of view. The radiation from source 226 is then directed
by collimating lens 222 onto prisms 206 and 207 which form the
front of a compound gas cell 215 formed by gas cell walls 228, gas
cell separator 230, and a plane parallel gas cell window 232. The
prisms 206 and 207 split the radiation from the radiation source
226 along paths 210 and 212 by causing the radiation to diverge
while passing through gas cells 214 and 218. On the first radiation
path 210, the radiation is directed by prism 206 through gas
correlation cell 214 and is focused by detector lens 204 onto the
photodiode 202A. On the second radiation path 212, the radiation is
directed by the prism 207 through an evacuated gas cell 218 and is
focused by detector lens 204 onto photodiode 202B.
[0044] The compound gas cell 215 with prisms 206 and 207 may also
be located between the field stop 216 and the collimating lens 222,
or between the detector lens 204 and the photodiodes 202A and 202B.
Likewise, the prisms 206 and 207 may be located at either the front
of the compound gas cell 215 or at the back of the compound gas
cell 215.
[0045] The gas correlation cell 214, also called a gas filter or
absorption cell, contains a gas, such as ethane, to be detected.
The gas correlation cell 214 may for example be a 1 cm cell with
for example a concentration of ethane provided by one atmosphere of
pure C.sub.2H.sub.6. The second path 212 has a different path
length of C.sub.2H.sub.6, such as may be obtained by providing the
cell 218 with for example no C.sub.2H.sub.6, as for example an
evacuated gas cell or a cell containing a gas that is optically
neutral in relation to the ethane spectra of interest. The output
of the photodiodes 202A, 202B is provided to suitable electronics,
such as computer 208, for processing.
[0046] The detector signal on the path 112 is:
S 1 = G .intg. .lamda. 1 .lamda. 2 I .lamda. .tau. filter d .lamda.
##EQU00001##
[0047] where I.sub..lamda. is the intensity of the radiation from
the radiation source 126, .tau..sub.filter is the transmissivity of
the filter 116, .lamda..sub.1 is the low pass of the filter 116,
.lamda..sub.2 is the high pass of the filter 116 and G is the gain
of the photodiode 102B.
[0048] The detector signal on the path 110 is:
S 2 = G .intg. .lamda. 1 .lamda. 2 I .lamda. .tau. filter .tau.
corr . cell d .lamda. ##EQU00002##
[0049] where .tau..sub.corr cell is the transmissivity of the
correlation cell 114.
[0050] If
S avg = S 1 + S 2 2 ##EQU00003##
and S.sub.diff=S.sub.1-S.sub.2 then the calculation made by the
computer is:
S inst = S diff S avg , ##EQU00004##
which yields a signal that is dependent on the presence of the
target gas in the radiation path from the source 126 to the
photodetector 102B. The calculation of the difference in the
received signals for both optical paths is made for each pixel of
the photodetectors 102A, 102B to yield an image of the field of
view that may be displayed on a monitor.
[0051] FIG. 3 shows a manner of use of the GFCR 101 shown in FIG.
1. Detecting a leak of a hydrocarbon requires traversing a target
area with a gas filter correlation radiometer having a field of
view oriented towards the target area. A helicopter 350 traverses a
pipeline 354 with a GFCR 101 having a field of view 352 oriented
towards the pipeline 354. The GFCR 101 is tuned to detect for
example ethane by appropriate selection of the bandpass of the
filter 116, and the gas filter 114 contains a sample of ethane. If
a leak 356 exists in the pipeline 354, the presence of ethane in
the resulting plume 358 that may be moved by the presence of wind
360 will be detected using the GFCR 101. There is a further step of
identifying a leak upon the gas filter correlation radiometer
detecting the hydrocarbon. The presence of a leak is indicated by
for example displaying the received signal using a monitor that is
provided as part of the computer 108. Pixels on the monitor display
corresponding to detected ethane may be coloured to enhance the
image. Other methods of indication of the presence of a leak may be
used such as detecting a concentration of ethane in the path
between helicopter 350 and the ground that exceeds a predetermined
threshold, and providing an alarm. The threshold is determined
readily by calibration of the radiometer and experimentation. Due
to the remote sensing capabilities of the device, the GFCR does not
have to fly through the plume in order to detect leaks. The GFCR
measures the integrated column concentration of natural gas between
the helicopter and the ground, regardless of where in this column
the natural gas occurs.
[0052] In one embodiment as shown in FIG. 4, the field of view 352
covers an area of 128 m.sup.2, representing a swath 64 m long by 2
m wide. The long but narrow swath of the field of view 352 leads to
an overall view of the pipeline 354 or target area through the use
of a technique known as pushbroom imaging. As the helicopter 350
advances along the helicopter path 464 over the pipeline 354 or
other target area, successive swaths below the helicopter 350 and
perpendicular to the helicopter path 464 are detected by the GFCR
101. At a first time interval, the detectors 102A and 102B would
sample signals from the field of view 352A, followed moments later
by 352B, followed again by 352C and so on.
[0053] In FIG. 4, the field of view 352F represents the current
swath of the target area being detected by the detectors 102A and
102B. Detectors 102A and 102B have corresponding pixels having
collocated fields of view 352F where each 2 m.times.2 m cell of the
field of view 352F is sampled synchronously by detectors 102A and
102B. Therefore, the cell marked P1 would be detected by a first
pixel representing a portion of the field of view collocated and
synchronized on detectors 102A and 102B. The cell marked P2 would
be detected by a second pixel collocated and synchronized on
detectors 102A and 102B. The same can be said for the cells marked
P3 and P4 and so on. All cells P1 to P32 along a line would be
detected simultaneously.
[0054] In an exemplary embodiment, the GFCR 101 operates using
ambient background radiation that passes through the plume 358 of
natural gas. The upwelling radiation field is comprised of
reflected solar radiation, radiation emitted from the surface, plus
upwelling emission from the atmosphere. For operation during cloudy
periods or at night, a source of illumination 362 may be used. For
example, a powerful 1600W Tungsten Halogen bulb may be mounted on
the helicopter 350, with an IR transmitting window (not shown) and
a focusing mirror (not shown). This mirror focuses the emission
from the illumination source 362 to a 5 m spot on the ground.
Assuming a lambertian reflective surface and a reflectivity of 5%,
the reflected intensity at the surface would be 0.048 W m.sup.-2.
This is roughly equivalent to (or slight greater than) the
reflected intensity of sunlight. The illumination source 362 should
be mounted to reduce vibrations that could increase the signal to
noise ratio of the detected signal. In an alternative embodiment,
the GFCR 101 may be mounted on a different type of vehicle, such as
a truck, and driving the vehicle along a pipeline or other possible
source of a gas leak. The GFCR 101 may also be tuned to detect
other gases by selection of the bandpass of the filter 116.
[0055] The detected instrument signal is a function of the height
of the natural gas column. For an atmospheric background
concentration of 1 ppb of C.sub.2H.sub.6, the equivalent total
atmospheric column thickness is approximately 8.5 .mu.m. The
equivalent CH.sub.4 column thickness would be approximately 1700
times thicker.
[0056] A linear regression of the signal sensitivity between 0 and
4 mm of natural gas shows that the change in signal per mm of
natural gas is -1.69.times.10.sup.-3 mm.sup.-1. The measurement is
actually detecting C.sub.2H.sub.6 which is assumed to be 2.5% of
natural gas. Therefore, the detected columns of pure C.sub.2H.sub.6
are 40 times shorter than that of methane. Maximum sensitivity to
C.sub.2H.sub.6 occurs at the lowest concentrations. This is the
most desirable for detecting the smallest leaks.
[0057] Uncertainties may be introduced into the measurement by
spectral interferences by other gases in the atmosphere
(principally H.sub.2O and CH.sub.4), variations in the surface
emissivity, temperature variations in the atmospheric temperature,
and variations in the altitude of the airplane. These uncertainties
tend to reduce the sensitivity of the measurement to concentrations
of natural gas, and variations may result in false signatures of
leaks. The combined uncertainty is about +/-19 .mu.m. This level of
accuracy places a minimum limitation on the measurement's accuracy.
Given a measurement resolution of -1.69.times.10.sup.-3 per mm
natural gas, to measure a column height of .+-.19 .mu.m a
measurement precision of .+-.3.2.times.10.sup.-5 (i.e., a
signal-to-noise ratio of 31,000) is required. Such a measurement
precision may be obtained from the GFCR 101, and may be adjusted by
for example varying the length of the absorption cell 114.
[0058] The sensitivity of the instrument is ultimately a function
of the amount of energy that is collected and focussed onto the
detector element. This in turn is a function of the field-of-view
(FOV) of the instrument (which determines the surface resolution),
the size of the collector optic 124, the size of the detector pixel
in the photodiodes 102A, 102B, the transmission of the instrument,
and the observation period (frequency) of the instrument. The FOV
and the collector optic size directly affect the energy collected,
as the larger the optic and FOV, the more photons collected.
However, they also directly affect the detector pixel size, due to
the principle of etendue (AC)) conservation in an optical chain.
The transmission of the instrument directly affects the energy
collected as any losses in the system directly reduces the number
of photons incident on the detector. And finally, the pixel size
and observation period directly affect the noise-equivalent power
(NEP) of the detector.
[0059] Two versions of a 2.3 .mu.m instrument based on the detector
system are described below. In the first version the detector may
be an off-the-shelf 320.times.256 Focal Plane Array (FPA) detector;
in an example, the detector is a Xenics Xeva-2.5-320 extended
InGaAs camera. The pixels may be "binned" so as to increase the
signal-to-noise ratio and to produce the same (on-the-ground) Field
of View (FOV) pixel size and shape as a realSens.TM. detector. The
wider 320 pixel dimension may be the across track dimension. Thus
to achieve a 32.times.1 sub-array of binned pixels with the same
relative size as a realSens.TM. detector each "binned-pixel" may be
10.times.14 pixels in extent (140 pixels total). FIG. 22 shows a
schematic of the FPA, showing a mapping of the proposed
binned-pixels arrays. Given a desired on-the-ground FOV of
2.times.2.8 m realSens.TM. detector, this may result in a focal
length for the 2.3 .mu.m realSens.TM. detector of 45 mm.
[0060] The second version of 2.3 .mu.m instrument may use a custom
detector built as an FPA consisting of two 32.times.1 linear arrays
with pixels of dimension 1.times.1.4 mm.
[0061] To detect leaks from hydrocarbon liquids pipelines, the
detector system may be adapted using a method to make the detector
system sensitive to a specific chemical by putting the vapour of
the chemical into the correlation cell(s) of a gas filter
correlation radiometer, which may be a realSens.TM. radiometer. The
instrument was originally designed to measure ethane and methane in
the 3.3 .mu.m (3000 cm.sup.-1) region. FIGS. 5-7 show the spectral
absorption bands for methane from the Pacific Northwest National
Laboratory (PNNL) database and FIGS. 8-10 and show the spectral
absorption bands for ethane from the PNNL. This spectral region was
originally chosen as it has the strongest spectral absorption
features in the infrared, and therefore should theoretically
provide the greatest sensitivity to methane and ethane.
[0062] However, although the spectral absorptions are very strong
in the 3.3 .mu.m region for methane and ethane, this is a spectral
region with low radiances in the environment. Detected radiances
are a combination of reflected solar radiation and terrestrially
emitted radiance, both of which are low energy at these
wavelengths. Also, the surface reflectivity for most surface types
is quite low in this spectral region. These factors limit the
sensitivity of the detection system using the 3.3 .mu.m region, due
to spatial variations in surface temperature and reflectivity.
Also, since the environmental radiances are so low, the optics were
designed to maximise the energy gathered (F/0.75, .apprxeq.12''
fore-optic, and 1.4.times.1.0 mm InSb pixels), and thus required a
large and heavy instrument.
[0063] Instead, ethane and methane may be detected using the
detection system at absorption bands in the 2.3 .mu.m region (4350
cm.sup.-1). The 2.3 .mu.m region has lower spectral absorption by
the leaked gases than the larger wavelength absorption band, which
initially suggests it would be unsuitable in the detection system.
The 2.3 .mu.m absorption bands of methane and ethane are
approximately 50 times weaker than at 3.3 .mu.m. However, the
radiance at longer wavelengths is entirely terrestrial, which means
a low spectral contrast between the background radiance and the
leaked gas, if the leaked gas is of similar temperature as the
background.
[0064] Methane Leak Sensitivity at 2.3 .mu.m
[0065] FIG. 11 shows the 2.3 .mu.m (4350 cm.sup.-1) transmission of
a 1 cm long gas cell filled with 1 atm of CH.sub.4 at 20.degree. C.
For the purpose of this analysis, two bandpass filters were
considered, a wide filter covering most of the band (half heights:
4150 to 4450 cm.sup.-1, 300 cm.sup.-1 wide), and a narrow filter
covering a portion of the band with strong absorption (half
heights: 4175 to 4275 cm.sup.-1, 100 cm.sup.-1 wide). At these
wavelengths, the terrestrial emission is negligible compared to the
reflected solar radiance. FIG. 12 shows a GenARTS.TM. model
calculation of the sensitivity of a 2.3 .mu.m realSens.TM. to
leaked methane, assuming a correlation cell of 10 cm and a pressure
of 1 atm. The results shown in FIG. 12 show the change in D2A
signal as a function of the leak concentration. It shows
.apprxeq.33% higher sensitivity for the narrow bandpass, over the
wide. Also included in the plot is the sensitivity of the 3.3
.mu.m. Quite surprisingly, FIG. 12 shows lower sensitivity for the
3.3 .mu.m realSens.TM.. This may be caused by the following
factors:
[0066] (1) The calculations for 3.3 .mu.m realSens.TM. assumes the
very wide bandpass filter profile,
[0067] (2) The absorption lines of methane at 3.3 .mu.m saturate
quickly, and
[0068] (3) The terrestrial emission at 3.3 .mu.m significantly
increases the AVG signal, lowering the D2A.
[0069] First, the 3.3 .mu.m passband of realSens.TM. is very wide
to maximise the energy gathering. This reduces the sensitivity, the
same as shown with the 2.3 .mu.m analysis. Second, although this
figure seems to show similar sensitivities to methane, the rapid
saturation of absorption lines in the 3.3 .mu.m band quickly
reduces the sensitivity at higher leaks (ppmm). Finally, the
increase in the AVG signal due to terrestrial emissions greatly
reduces the change in D2A due to leaks. To illustrate this effect,
a fourth line was added to FIG. 12, showing the sensitivity of a
3.3 .mu.m realSens if there was no terrestrial emission.
[0070] To further illustrate the sensitivities of 2.3 versus 3.3
.mu.m realSens, Table 1 shows the sensitivity (.DELTA.D2A per ppm-m
CH.sub.4) at low leak concentrations. This table shows: (1) the
large reduction in sensitivity for the 3.3 .mu.m realSens.TM. due
to terrestrial emission and (2) the advantages of using a narrow
bandpass filter.
TABLE-US-00001 TABLE 1 Sensitivity of realSens instruments to small
leaks of methane. Sensitivity Instrument (.DELTA.D2A per ppm-m
CH.sub.4) 2.3 .mu.m realSens (wide) -3.4 .times. 10.sup.6 2.3 .mu.m
realSens (narrow) -5 .times. 10.sup.6 3.3 .mu.m realSens -3.4
.times. 10.sup.6 3.3 .mu.m realSens -2.2 .times. 10.sup.5 (no
Terrestrial Emission)
[0071] Sensitivity to Rsurf Variations
[0072] One major impediment to maximising the sensitivity of a 3.3
.mu.m realSens.TM. was the difficulty of accounting for variations
in surface temperature, emissivity and reflectivity (Rsurf). An
advantage of a 2.3 realSens.TM. radiometer is the fact that the
terrestrial emission is very small compared to reflected solar
radiances, removing the influence of varying surface temperature
and emissivity. However, Rsurf variations across the passband are
still significant. FIGS. 13-14 show the reflectivity of a number of
"typical" surface types for the 2.3 and 3.3 .mu.m bands. The 3.3
.mu.m band shows significantly more structure in reflectivity than
the 2.3 .mu.m band. Also the 2.3 .mu.m band shows higher
reflectivities than the 3.3 .mu.m band. FIGS. 15-17 shows the
results for the modelled sensitivity to variations in Rsurf
(uniform over passband) for a wide passband 2.3 .mu.m realSens.TM.,
a narrow passband 2.3 .mu.m realSens.TM., and the 3.3 .mu.m
realSens.TM., respectively. There are a number of observations to
make comparing these figures.
[0073] The D2A signals for the narrow 2.3 .mu.m realSens.TM. are
higher than the wide 2.3 .mu.m realSens. This is may be due to the
narrower passband realSens.TM. having a higher absorption line
density over the passband. The sensitivity to variations in uniform
Rsurf is similar for the narrow and wide 2.3 .mu.m realSens.TM..
The largest variations occur at low Rsurf. The sensitivity to
different surface types for the narrow and wide 2.3 .mu.m realSens
are similar.
[0074] The variations of D2A as a function of (uniform) Rsurf is
very large. This is due to the fact that the model assumes surface
emission varies with respect to Rsurf (Kirchoff's law: emissivity=1
reflectivity). As the Rsurf increases the emission from the surface
decreases, making the signal detector "more solar" in origin. The
model assumes the gas is the same temperature as the surface. Since
the surface and the gas are the same temperature, the emission by
the gas equals the absorption of the surface radiance by the gas.
The result is no radiative contrast for the terrestrial component
of the radiance.
[0075] This lack of radiative contrast is a very important
consideration for the data retrieval for the 3.3 .mu.m
realSens.TM.. Because the detected radiance is composed of
reflected solar and terrestrially emitted radiance, the variation
in D2A due to Rsurf variations (both uniform and within the
passband) complicate data retrievals. Further complications come
from the fact that (1) the Tsurf is also varying, (2) the leaked
gas temperature is likely lower than Tsurf due to decompression,
and (3) the relationship between Rsurf and surface emissivity used
in the model (Kirchoff's law) does not actually hold. This is where
the process of surface normalisation has been proposed to improve
data retrievals for real Sens.
[0076] For a 2.3 .mu.m realSens.TM., the magnitude of terrestrial
emission is negligible compared to reflected solar radiance. This
removes a major source of complications which arose from 3.3 .mu.m
realSens.TM. analysis.
[0077] Calculation Parameters
[0078] The results presented above are dependent on many parameters
about the environment and the instrument. To explore how variations
in some of these parameters may add noise to measurements, models
of changes to the values of a few important parameters were made.
They include, (1) instrument temperature, (2) atmospheric water
vapour, (3) surface emission, and (4) emission by the gas in the
correlation cell of the detector instrument. FIGS. 18-20 shows the
change in D2A signal as a function of Rsurf, for changes in the
four instrument and environmental parameters, and for the 3
different instrument configurations (wide and narrow passband 2.3
.mu.m realSens.TM. and the 3.3 .mu.m realSens.TM.). A number of
observations can be made from these results: [0079] (1) All
instrument configurations show sensitivity to Rsurf, however the
sensitivity of the 2.3 .mu.m instrument configurations is
significantly less than the 3.3 .mu.m instrument. This may be due
the detected emission at 2.3 .mu.m being almost all reflected
solar. [0080] (2) The 2.3 .mu.m instrument configurations do show
some sensitivity to Rsurf variations. This may be due to the fact
that although the terrestrial emission is extremely small, it still
does make up a tiny component of the upwelling radiance. [0081] (3)
Reducing the gas temperature of the correlation cell significantly
reduces the sensitivity of all configurations of the instrument to
variations in Rsurf. However, in the extreme case of no radiative
emission by the correlation cell gas (the equivalent of chopping
the input radiance), the D2A sensitivity to Rsurf is minimal.
[0082] (4) The narrow passband version of the 2.3 .mu.m instrument
is more sensitivity to variations in model parameters than the wide
passband. This may be due to the higher density of CH.sub.4
absorption lines in the narrow passband. [0083] (5) All instrument
configurations show increased sensitivity to Rsurf if there is no
emission from the surface. This is a small effect for the 2.3 .mu.m
configurations, but a massive effect for the 3.3 .mu.m
configuration (due to the different proportions of the upwelling
radiance being composed of surface emission). This may be due to
the gas in the atmosphere emitting at the wavelengths of the gas
lines [0084] (6) And finally, the effects of water vapour in
atmosphere on the instrument are small.
[0085] 2.3 .mu.m Band of Ethane
[0086] Ethane has an absorption band in the same 2.3 .mu.m region
as methane. FIG. 21 shows a transmission spectrum of a 100 ppm-m
sample of ethane and, for reference, methane. These spectrums were
calculated from the PNNL database and show the spectra at 1 atm
pressure. The data seems to show that the ethane spectra is very
dense, approaching a continuum absorption. It also seems to show
that the absorption by ethane is weaker than methane. However, it
should be noted that the resolution of these spectra is
insufficient to separate close spaced lines. As such the actual
ethane spectra may be more structured than this data shows. This
dataset provides moderate resolution spectral absorption features
of various gases and vapours at low concentration mixed in 1 atm of
air or N.sub.2. The ethane band mostly overlaps the methane band,
so an instrument designed for methane may be used to test ethane
sensitivity.
[0087] Energy Models
[0088] The 2.3 .mu.m system was modeled to determine the amount of
energy which can be gathered. The sensitivity of the system will
depend on the noise-to-signal ratio (NSR) it can achieve. Smaller
NSR means higher potential sensitivities of the instrument.
[0089] An energy model analysis is show for four different 2.3
.mu.m CH.sub.4 sensing systems, with the results listed in Table 2.
The first two systems listed assume an off-the-shelf InGaAs camera
system by Xenics (Xeva-2.5-320), with the wide and narrow
bandpasses (Wide: 4150 to 4450 cm.sup.-1, 300 cm.sup.-1 wide;
Narrow: 4175 to 4275 cm.sup.-1, 100 cm.sup.-1 wide). The next two
systems assume a system identical to the current realSens.TM., but
with InGaAs detectors tuned to 2.3 .mu.m, again with the wide and
narrow bandpasses. For reference the fifth system in the energy
model is the 3.3 .mu.m realSens.TM.
TABLE-US-00002 TABLE 2 Energy model for three models of realSens
type instruments 2.3 .mu.m realSens 3.3 .mu.m realSens Detector:
Xenic Xeva-2.5- Custom InGaAs InSb 320 (InGaAs) (real Sens-like)
Bandpass Wide Narrow Wide Narrow Altitude 300 m Swath 64 m Focal
Length 45 mm 150 mm FPA 320 .times. 256 32 .times. 1 InGaAs 32
.times. 1 InSb InGaAs Pixel Pitch 30 .mu.m 1000 .times. 1400 .mu.m
.sup. FPA Dimension 9.6 .times. 7.68 mm 32 .times. 1.4 mm .sup.
Pixel FOV 0.2 .times. 0.2 m 2.0 .times. 2.8 m .sup. Binned-Pixel 10
.times. 14 pixels b-Pixel Area 300 .times. 420 .mu.m b-Pixel FOV
2.0 .times. 2.8 m (b-)Pixel Area 0.126 mm.sup.2 .sup. 1.4 mm.sup.2
Fore-Optic 38 mm 200 mm Diam. F/# F/1.18 F/0.75 Angle on Pixel
22.9.degree. 33.7.degree. Fore-Optics .OMEGA. 0.495 sr 1.055 sr
.sup. Eff. Etendue 2.49 .times. 10.sup.-8 m.sup.2 sr 5.91 .times.
10.sup.-7 m.sup.2 sr REF Radiance 0.227 W m.sup.-2 sr.sup.-1 0.065
W m.sup.-2 sr.sup.-1 0.227 W m.sup.-2 sr.sup.-1 0.065 W m.sup.-2
sr.sup.-1 COR Radiance 0.156 W m.sup.-2 sr.sup.-1 0.041 W m.sup.-2
sr.sup.-1 0.156 W m.sup.-2 sr.sup.-1 0.041 W m.sup.-2 sr.sup.-1 REF
5.67 .times. 10.sup.-9 W 1.62 .times. 10.sup.-9 W 1.34 .times.
10.sup.-7 W 3.84 .times. 10.sup.-8 W COR 3.89 .times. 10.sup.-9 W
1.03 .times. 10.sup.-9 W 9.22 .times. 10.sup.-8 W 2.43 .times.
10.sup.-8 W D* 2 .times. 10.sup.11 cm Hz.sup.1/2 W.sup.-1 2 .times.
10.sup.11 cm Hz.sup.1/2 W.sup.-1 2 .times. 10.sup.11 Integration
0.01 sec Period Bandwidth 15.91 Hz.sup. NEP 7.08 .times. 10.sup.-13
W 2.36 .times. 10.sup.-12 W 2.36 .times. 10.sup.-12 W .sup. NSR
(REF) 0.00012 0.00044 0.00002 0.00006 NSR (COR) 0.00018 0.00069
0.00003 0.00010 NSR (D2A) 0.00022 0.00082 0.00003 0.00011 CH.sub.4
Sensitivity 64 ppm-m 164 ppm-m 9 ppm-m 23 ppm-m
[0090] The Xenics.TM. system assumes a Simultaneous-View
Correlation Radiometry (SVCR) system in which the FOV is focussed
onto two linear strips of the long axis (320 pixels) of the FPA. It
is also assumed that the fore-optics diameter was 1.5'' (38 mm). To
achieve a 64 m wide swath, a focal length of 45 mm was required. It
was also assumed that to achieve identical FOVs as realSens, pixels
would be binned. To achieve a 2.0.times.2.8 m FOV, 10.times.14
pixels would be "binned". Based on calculated REF and COR signals,
the energy model determines a NSR for a Xenics 2.3 .mu.m
realSens.TM. detector of 0.00022 and 0.00082, for the wide and
narrow bandpasses. This corresponds to a sensitivity of 64 and 164
ppm-m of CH.sub.4.
[0091] The next two systems modelled were the identical to real
Sens.TM. but tuned to 2.3 .mu.m. These models determined a
sensitivity of 9 and 23 ppm-m of CH.sub.4. The final system
calculated was the sensitivity of the current 3.3 .mu.m real Sens.
The model found a sensitivity of 32 ppm-m of CH.sub.4.
[0092] Because of the relatively high sensitivity of the models,
none of the modelled instruments are likely to be energy limited,
and thus a simple instrument made with off-the-shelf detectors and
optics may be effective.
[0093] It should be noted that the model assumed the same D* for
(extended) InGaAs and InSb detectors. The detectivities of these
two detectors are similar at 2.3 .mu.m. The main difference between
the two detectors is that the cut-off for InGaAs is .apprxeq.2.6
.mu.m versus 5 .mu.m for InSb. This allows InGaAs detectors to be
operated at room-temperature (or better cooled by TE-coolers) where
as InSb detectors must be cooled to LN2 temperatures.
[0094] Vehicle-Based Remote Sensing System
[0095] A system may be mounted on the roof of a vehicle looking
forward. The remote sensing unit and the electronics may be
separated. A visible camera may provide images of the locality of
leak. A GPS/INS system may provide position and direction
information. The system may be mounted on a vehicle to detect gas
leaks. The vehicle may be for example a small airplane, helicopter
or truck. A truck may for example be driven along a pipeline or
other possible source of a gas leak.
[0096] The system may be a passive or an active system. For
example, a scanning mechanism or an active mechanism using a
radiation source may be used. In an active system, a light source
may also be added to the system to allow the instrument to operate
in cloudy and nighttime conditions.
[0097] The system may provide some information about leak location
relative to vehicle. The system may also operate in real-time, and
may be autonomous. It may be able to look forward 50 to 200 ft (15
to 60 m) and operate at speeds up to 50 km/h. The system may also
provide significant across-track measurements.
[0098] The system may also be mounted low enough on the vehicle to
not cause height problems. It may also be small and light enough to
be easily handled. The system may be designed to minimise
sensitivity to vibrations. The system may be operable at low power.
The system may have a GPS to provide location information.
[0099] Instrument Concept
[0100] The instrument may be mounted on an airborne instrument, due
to a series of factors that may allow the instrument to be small,
light and relatively inexpensive to build. These factors include:
[0101] (1) At 2.3 .mu.m, the upwelling reflected solar radiance is
much higher than at 3.3 .mu.m. This is due to (a) higher surface
reflectivities, as shown in FIGS. 13-14, and (b) solar radiance at
2.3 .mu.m is about 75% higher than at 3.3 .mu.m (Planck's Law).
[0102] (2) The spectral band at 2.3 .mu.m is denser than at 3.3
.mu.m (increasing the correlation depth). [0103] (3) The
terrestrial component of upwelling radiance in extremely small
(also increasing the correlation depth). [0104] (4) IR detectors at
2.3 .mu.m have higher potential detectivities (D*) than at 3.3
.mu.m. [0105] (5) Optical materials tend to be much less expensive
(glass instead of Si or Ge). [0106] (6) Detectors tend to be less
expensive (InGaAs instead of InSb), and potentially
off-the-shelf.
[0107] The below analysis for a vehicle-based remote sensing
instrument uses a 2.3 .mu.m realSens.TM. of the same optical
configuration as proposed for aircraft (in this document). FIG. 24
shows a schematic of the configuration of a vehicle-mounted
instrument. Table 3 lists the viewing angle (.phi.), the on-ground
binned-pixel FOV, and the swath width (Y), as a function of the
forward viewing distance for an instrument height (Z) of 3 m.
Assuming a 30 m (nominal) viewing distance and an instrument height
of 3 m above ground, the angle of the view relative to the surface
(.phi.) will be 5.71.degree. and the swath width (Y) will be 6.4 m.
Assuming that 10.times.10 pixels are binned, the FOV on the ground
of the binned pixel will be 0.2.times.2 m.
TABLE-US-00003 TABLE 3 Viewing angle (.phi.), binned-Pixel
on-ground FOV and swath width (Y), as a function of the forward
view distance (X) for a 3 m high vehicle mounted realSens. X .phi.
b-Pixel FOV 10 16.7.degree. 0.067 .times. 0.23 m 20 8.53.degree.
0.13 .times. 0.88 m 30 5.71.degree. 0.2 .times. 2.0 m 40
4.29.degree. 0.27 .times. 3.6 m 50 3.43.degree. 0.33 .times. 5.5
m
[0108] Another option that could be implemented would be to rather
than split the FPA into two linear arrays (as shown in FIG. 25);
the FPA could be split into two area arrays. For example, assuming
the binned pixels are 10.times.10 individual pixels, the FPA could
be split into 8 (or more) rows. FIG. 26 shows this splitting of the
FPA. This would allow a 32.times.8 low resolution image of the
leak. If the vehicle was stopped, images or video of a leak plume
could be gathered without moving. If a visible camera was part of
the system, images/videos of the plume superimposed on the visible
images/videos could provide further information to operators.
[0109] Regarding surface reflectivity for a passive instrument, the
energy detected by a vehicle-mounted 2.3 .mu.m real Sens would be
reflected solar radiance. The angle of reflection between the Sun
in the sky and the viewing direction of the instrument would vary
greatly, depending on the time of day, latitude, direction of
travel, and slopes in the surface. Assuming that the surface is
reflectively isotropic (i.e., energy is reflected equally in all
directions), the energy detected by the instrument would be
independent of the reflection angle. However, isotropic reflections
are unlikely. It is likely that backwards scattering will be more
significant than forward scattering (i.e., more signal will be
detected with the Sun behind the instrument). As such the signal
detected (and therefore sensitivity) will vary significantly when
using a 3.3 .mu.m instrument. However, surface emissions do not
complicate retrieval with a 2.3 .mu.m instrument, because the
thermal emission from the surface is tiny or negligible.
[0110] A 2.3 .mu.m instrument may be thermoelectrically cooled
(TE-cooled) as opposed to cooled by liquid nitrogen or sterling
cycle cooler. Real-time measurements may be possible since the
detected radiance will be only reflected solar, reducing the
complexity of data analysis.
[0111] A person skilled in the art could make immaterial
modifications to the invention described in this patent document
without departing from what is claimed.
* * * * *