Advances in Applications and Methodology for Aerial Infrared Thermography
By: Gregory R. Stockton
AITscan (Aerial
A division of
(800) AIT-SCAN
Cell Phone (336) 689-3658
http://www.AITscan.com
Abstract
Aerial infrared applications
can be divided into two types; those where a straight-down view and/or a large
area view is needed and those where long distances must be covered in a limited
amount of time. Selection of aircraft, aircrew, navigational aids, infrared
imaging system, data acquisition and image processing system are all important
to a successful survey. This paper discusses the ongoing advances in
methodology, platform and equipment required to produce high quality usable data
for the end-user.
Author Biography
Gregory R. Stockton is
President of Stockton
Keywords
Aerial, infrared,
thermography, environmental impact, pollution, stormwater,
animal counts, geothermal, forest fires, subsurface fires, structural fires,
landfill fires, peat, coal and wood chip piles, Indian trails, high voltage
electrical transmission lines, high voltage electrical distribution lines,
pipelines, search and rescue (SAR), roof moisture surveys, steam lines, high
temperature hot water system (HTHW), computer aided design & drafting
(CADD), global positioning system (GPS), geographical information system (GIS).
Introduction
Aerial
Equipment & Crew
Both helicopters and light
airplanes can be used to perform aerial infrared surveys. A helicopter is best
used if the number of targets or distance between targets is low because there
are inherent problems with vibrations, slower ferry speeds and higher operating
costs. These problems can be offset by being able to use relatively inexpensive
small format (~65,000 pixels) focal plane array imagers. If a light airplane is
used, the imager must have a higher spatial resolution (~262,000 pixels or more),
because it must operate at higher altitudes and therefore farther away from the
target, allowing the same resolution from four times the distance. The
advantage of using a large format imager is that the field of view (FOV) is
larger, making report preparation much easier and the report product superior. Larger
lenses can improve the needed ground resolution element (GRE) or the size of
one pixel on the ground, if some signal strength degradation is acceptable, but
the FOV suffers as a result. One might mosaic or ‘paste’ many images together
but this is very labor-intensive and often cannot be accomplished without
greatly distorting the imagery. It is always better to have more pixels.
It is nearly impossible to get professional results with hand held imagers. The
imager should be fixed mounted solid (See
Figure 1), turret-mounted or fixed manually articulated. In any case, a
well maintained aircraft, experienced aircrew and an imager capable of the
resolution required for the intended task should be used.
Figure 1) Large
format infrared imager, fixed-mounted.
The type of infrared imager
used will dictate how images are recorded and saved. Modern infrared cameras
have a variety of storage media, but must be within reach and/or have remote
controls so that the camera can be moved, lenses adjusted and the images
stored. Since
the pilot and thermographer are extremely busy during the flight, one or both
may not see every possible anomaly, so no
matter what type of imager or storage medium, a videotaped record of all the
‘raw’ infrared imaging should always be made.
Precise navigation is
important in any aircraft and particularly so in nighttime aerial infrared
operations. A global positioning system (GPS) is a necessity. Combining the GPS
with a mobile mapping program on a computer and a video encoder-decoder (VED)
is very useful, especially when multiple targets need to be imaged. The VED
encodes the videotape with a continuous stream of GPS derived data (latitude/longitude,
altitude, date, time and speed, etc.) and displays the information through the
video signal (See Figure 2). All
equipment in the aircraft must be secured with wires labeled, shielded from
electromagnetic interference and out of the way.
Figure 2) Video encoder-decoder (VED) annotation
guide.
Most aerial infrared imaging
is performed at night because reflected and direct daylight solar radiation
usually adversely affects the imagery. Nighttime infrared imaging is NOT a job
for amateur pilots or airsick prone infrared equipment operators. The professional
pilot should be specifically trained
and experienced in aerial IR operations. He/she will be flying low, slow and
maneuvering without much room for recovery...in the dark.
Upon returning from the
mission, the report must be prepared. The office equipment needed to analyze
imagery and produce reports is the same as that used by ground-based infrared thermographers.
This includes a computer workstation complete with digital photographic and
thermographic imaging peripherals for handling infrared images, daylight
photographs and capable of producing high quality reports in a popular format.
Also needed are specialized video capture, image processing, computer aided
design & drafting (CADD) software and mapping software necessary for
producing drawings and specialized parts of the final report product. A high
quality printer is required for printing the report with photographic quality.
Applications
Listed below are various
aerial infrared applications and descriptions.
Waterways and Drainage Systems
The flow of a liquid into the
body of another liquid can be identified using infrared thermography if there
is a temperature difference between the two liquids. Typically, liquids flowing
into a body of water appear warm as compared to the surface water in a creek,
stream, river or lake – particularly during cooler
times of the year, due to the relative warmth of the ground a short distance
below the surface. Leaks from nearby water, sewer and/or stormwater
lines and direct run-off from a sloped surface can be detected because the warm
plume flowing over the ground toward the water and the liquid joining and
flowing downstream with the body of water are visible in the thermal infrared
spectrum. In most parts of the US, late fall, winter and early spring are well
suited to this type of inspection because the difference between water temps
(ground and surface waters) is present and because interference to view due to
overhanging foliage is minimized. The waterway is flown and infrared images are
saved with exact location information of each thermal anomaly (See Figure 3). A map is created with
exact latitude/longitude of each marked area. The system operator then takes a
hand held GPS device to each location and tests the outfall for signs of
contamination. If necessary, the outfall is traced back to its source.
Figure 3) Stormwater
drainage system outfall flowing into a creek.
Aerial IR can also be used to
detect illegal dumping and/or discharges, track pollution such as waste spills
or oil spills, monitor sewage treatment plant discharges, manage heated water
from power plant cooling towers, monitor ground water seepage into rivers, streams
and lakes and measure the amount of fresh water from ground sources that are
introduced into an estuary.
Animal Census
Many warm-blooded animals can
be found and counted from the air. Aerial IR is far more accurate than any
other method and primarily used by government agencies. Animals such as deer (See Figure 4), moose and large migratory
birds are among the most popular counted. Population density information is
used to monitor and control the population of these animals on city, county,
state and federal lands.
Figure 4) Deer in a forested area – three in one group (left and slightly below image
center) and two in another group (right bottom edge).
Geothermal
When a road or building
complex is planned, the site can be flown over with aerial IR to determine if
any geothermal activity is present at the surface. This allows the planner to
route the road around the activity or decide the site is unsuitable for the
intended purpose.
Pipes and Pipelines
Pipes and pipelines are
usually difficult to survey. Trees, shrubs, brush, water and man-made
structures like bridges, roads, sidewalks and buildings often cover pipes. If a
liquid is leaking from a pipe and the location of the leak is unknown, an
aerial IR survey can be used to find the leak. Even if the pipe itself cannot
be seen on the surface, it may be possible to see the leaking liquid and narrow
the search to a relative small area. The best results are found when the pipe
is not buried deeply, has a high flow and when the difference in temperature
between the liquid and the ground above is high.
We have flown over known
leaks on natural gas pipelines and not been able to measure any temperature
difference on the pipe or the sounding surfaces. Usually, color IR (CIR) and not thermal IR is more effective. CIR is
used to look at the damage to vegetation around the natural gas pipeline leak.
The U.S. Forest Service uses
aerial infrared imaging to monitor forest fires. Very accurate mosaic infrared
maps of active fires can be made to help with fire management and suppression
efforts. This information can be sent immediately to those in charge of
controlling fire lines. Thermal intensity is resolved to classify the hottest
sections of the active fire, therefore pinpointing the areas of most intense
thermal energy. These digital aerial
maps are loaded to hand held GPS devices to enable ground teams to
navigate directly to the hot spots rapidly, either by walking, driving or
flying in a helicopter. Thermal IR provides an important visual reference
locator by identifying the hot spots with respect to terrain features in the
thermal imagery. Positive identification of hot spots is rapid even through
dense smoke.
Structural Fires
Aerial infrared can be
helpful to the firefighters of structural fires especially on large, single
story buildings. Often the smoke escapes the building from a different location
than the hottest part of the fire. These areas can be imaged and the
firefighters informed as to the location of the hottest areas.
Landfill Fires
Subsurface fires can also be
monitored using aerial infrared thermography. Landfill fires (See Figure 5), can be hazardous to the
surrounding environment. Knowing where, how many and the extent of underground
fires is useful to those in charge of containing and/or extinguishing them. Similarly,
peat, coal and wood chip piles, which combust spontaneously, can be monitored.
Figure 5)
Indian Trails
Where ancient Indian trails
cross the desert, the land under the trails has been compacted. By using
nighttime aerial infrared imaging, the thermographer can see this higher
density differentiated from the lower density adjacent to the trails.
Search and Rescue (SAR)
SAR operations are often
‘rush’ jobs where conditions are less than ideal. Aerial infrared SAR is better
than ground-based SAR in most instances however it is still very overrated.
People targets either do not want to be seen, are disabled and unable to move
to an area where they can be seen, or are trying to keep the warmth of their
body close by insulating themselves, so they cannot be seen.
Steam and High Temperature Hot Water (HTHW) Systems
Commonly referred to as
district heating systems, steam (See
Figure 6) and HTHW systems can be imaged to find leaks and other thermal
anomalies. Even from high altitudes, steam line inspections are one of the
easiest applications for the aerial infrared thermographer. Thermal contrast
between active underground steam lines (especially leaks) and the surrounding
ground are usually good. HTHW loops, while not as brilliant as steam systems,
can be flown in the same manner. Sometimes leaks appear as cool spots because
the water has come to the surface and is being cooled by evaporating. In both
cases, the systems can be flown and problem areas pinpointed and documented. Along
with other data sets, geographical information system (GIS) maps can be
produced for the system operator.
Figure 6)
High Voltage Electric Utility Transmission and
Distribution Lines
Detecting
electrical faults on high voltage electrical transmission lines is fairly easy
and can be accomplished rapidly from a light aircraft. However, even from short
distances, accurate temperatures of electrical faults are impossible to measure [quantify]. There are several
problems associated with temperature measurement from the air. These include
spot size to target distance ratios, reflection of the objects surveyed, having
a sufficient load on the line at the time of the survey, among others. The spot
size to target distance ratio is the number one problem with respect to temperature
measurement. Specification writers have not yet realized the seriousness of
this problem and continue to ask for quantitative data on fault areas. The fact is that infrared cameras that are in
general commercial use today cannot measure accurate temperatures on small objects
from distances of 50 feet...much less from reliably safe flying distances. A
one-inch (relative size of a transmission line splice) target cannot be measured from that distance, plain and
simple, although it can be detected.
These spot sizes are unmanageable and inaccurate on any target that does not
have a large homogeneous heat signature. The GRE is critical to the measure of
spatial resolution in aerial infrared thermography. Nyquist's
frequency theorem states that an object less than two times the size of a
sensor's GRE cannot be resolved for measurement, so a 3x3 pixel or GRE spot is
needed for reliably obtaining measurements.
This
shortcoming may be addressed by using a more powerful lens to reduce the GRE
for a given distance, but then the sensor's FOV is reduced, limiting the area
covered over a given period of time. So, if one is using a small format IR
camera (256x256 pixels) in a helicopter only 50 feet away from a 1 inch hot
spot, it is impossible to obtain accurate temperatures using a standard lens.
The smallest hot spot that could be accurately measured with one of these
imagers is over 2", even at that extreme short distance. Also, from the
air, using a more powerful lens does not work well because vibration is more
evident in the form of image 'shaking'. Image 'smearing' may also occur due to
an increase in the apparent speed of the sensor's view across the ground. In
the air, there are few substitutes for a large pixel array, but even using
large format detectors, one cannot and should not profess to measure temperatures on very small
objects. These anomalies can be seen, and by comparing them to similarly loaded
phases or equipment, potential problem areas can be identified, saved and
marked on a map. For ‘good’ measurements, a ground verification team should be
used to inspect suspect hot spots from the ground (cloudy nights are best) and
verify the findings of the aerial IR survey. They will be closer to the target
and with a powerful lens on a stable surface, much more accurate.
Because they are smaller,
lower to the ground and often run through populated areas, high voltage
electrical distribution lines are much more difficult to see against all the
thermal clutter on the ground such as trees, street lights, people, animals,
etc., than transmission lines. Therefore they are best left to ground-based infrared
thermographers.
Roof Moisture Surveys
No other application better
illustrates the advantage of aerial infrared thermography over ground-based
infrared, than infrared roof moisture surveys. Regularly scheduled infrared
surveys quantify areas of moisture (water) contamination in insulated flat or
low-sloped building roofs (See Figure 7).
This helps the building owner assess the roof’s general condition at all stages
of its service life and make repair decisions based on actual data instead of biased
opinions. Straight down aerial imagery is much more useful to the owner than
on-roof imagery because aerial infrared images are plan view and because large
areas can be seen in one image, allowing the slightest temperature differences
to be noted.
Aerial is the best platform
for performing infrared roof moisture surveys for these reasons:
Q
Straight down,
high-resolution aerial imagery captures large areas at once, making the report
easier [less expensive] to produce, while lessening reflection and perspective
problems and eliminating all of on-roof infrared’s logistical evils, such as
access to multiple levels and dealing with security, manpower and safety
issues.
Q
The data
collection process is fast. Aerial infrared thermographers can survey many
roofs in one night while conditions are very good. Processing the data is done
not in a hurry at night on a roof, but under comfortable conditions at a
consistent pace in an office setting.
Q
Plan view imaging
allows for the precise, accurate marking of areas of suspect roof moisture
contamination.
Q
The report can be
provided at various levels of completeness, detail and complexity. These
different levels are unedited videotape, edited videotape, printed
thermographs, aerial photographs, AutoCAD™ drawings, database creation, or any
combination. The buyer has the advantage of obtaining any one or all of these
report components, with the costs determined by the report level needed.
Q
Trending roof infrared
imagery is only possible using aerial imagery. Proportional images can
be digitally overlaid and compared to future surveys of the same roof at a
later date. Drawings are easily updated with the new data.
Figure 7) Illustration of how roof moisture is
detected using infrared thermography.
Perhaps the biggest advantage
of aerial infrared is not its use on roofs that have well-defined areas of
moisture at all, but on those roofs that are the most difficult to image from
any distance or angle. I am referring to the roofs that, for instance, have a
lot of ballast, are covered with reflective coatings, have multiple layers or
that for whatever reason are impossible to image while standing on the roof.
With high-resolution aerial imagery, slight nuances of temperature differences
can be seen from far enough away to actually trace the patterns of heat.
There are two advantages to
on-roof infrared. First, it can be cost-prohibitive to fly a small roof far
away from the aerial infrared thermographer’s
operational area. Second, since on-roof verification does have to take place at
some point by a qualified professional, if a roof consultant is on the roof
with the thermographer and helper on the night of the survey, areas that
exhibit suspect heat patterns can be tested right then, so that only verified
wet areas are marked.
In the
Figure 8) Visual photograph of a roof.
Figure 9)
Figure 10) CADD drawing of a roof.
Conclusions
As outlined above there are many commercial
uses for aerial infrared thermography. The aircraft, imager and crew must be
capable of performing the tasks and providing professional results. With
improvements in camera quality (IR and visual), methodology, platform and
software, aerial infrared thermography has a bright future.