Getting the lay of the land is the most
basic building block for virtually any oil and gas operation, but
mapping the terrain isn't always easy — particularly in hostile,
remote and ever shifting places like the Sahara Desert.
This is a story of how one project dealt with the
challenge — and how getting a little distance up and away from
the problem provided positive results.
Infoterra, an airborne laser scanning company, was
asked to tackle a challenging survey in Algeria's Sahara Desert
region to prove the worth of laser scanning applications for the
petroleum business.
Despite some logistical nightmares, sandstorms, massive
sand dunes and a remote location over seven hours from the nearest
village, the project team — along with the vital support of WesternGeco
— successfully gathered geographic information that will aid the
service company and its clients operating in the hostile desert
landscape.
The story starts in 2000, when WesternGeco, on behalf
of several oil companies, hired Infoterra to acquire digital elevation
and terrain models over a large area of the Sahara using airborne
laser scanning, also known as lidar.
"This was a huge survey area covering about 1,500
square kilometers — about the size of Houston," said John Murtagh
with Infoterra, "and to acquire data via traditional means over
that large an area, which is also extremely inaccessible, would
have been very difficult.
"You can't even get a 4X4 vehicle into much of the
survey area," he continued. "Also, the sand dunes in this region
of the Sahara are the highest in the world, and conducting ground
surveys would have been too time consuming and costly.
"Lidar offered a safer, easier and more cost effective
solution."
First Steps
The project was designed to provide geographic data
for detailed engineering development planning, well site locations
and general infrastructure design. The survey's initial objectives
were to test the method and technology under these difficult desert
conditions — and if successful, use the results to survey the entire
area.
A secondary application of the data was for access
planning and identification of problematic high gradient regions,
which would be inaccessible for seismic source vehicles. The survey
was a success, providing WesternGeco with a dataset useful for many
applications.
But about those obstacles ...
After setting up camp in mobile trailers provided
by WesternGeco, Infoterra's acquisition team first had to modify
and fit its equipment into an aircraft that was in-country.
"Logistically it wasn't feasible for us to take one
of our aircraft into this part of the world, but installing the
equipment in a new plane took a great deal of work and some imaginative
problem solving," said Jason Stain, an operator for Infoterra. "After
installation we have to go through a calibration phase to make sure
the equipment is working properly — and that process typically
is done in a large flat-roofed building. We needed a Home Depot
in the middle of the Sahara Desert, but failing that we had to come
up with the next best thing, which was the hangar where the aircraft
was based.
"Although it was half the size we needed and had
a slightly pitched roof, we managed to get the equipment calibrated."
In addition to the lack of facilities, the weather
plagued the calibration process. During installation, high winds
blew up a sandstorm, delaying the project for several days since
the hangar was just a roof with no sidewalls.
Following installation and calibration of the instruments,
the main obstacle of the survey was the dramatic changes in the
terrain over relatively small areas.
"We ran flight checks to ensure we were getting coverage
with no gaps in the data and to determine the best parameters for
the overall acquisition," Stain said. "Most of our work has been
concentrated in places like the UK or northern Europe, so this project
was definitely a challenge from many perspectives."
Talk About Problems…
Infoterra acknowledges that without the logistical
support of WesternGeco this survey wouldn't have been possible.
"(They) helped us at every stage — from getting
our instruments through customs, to providing accommodations, data
processing facilities and essential ground survey components," Murtagh
said. "A crucial part of any lidar project is acquiring ground GPS.
Typically we do that ourselves, but in this case WesternGeco had
the ability in-country to handle that in a secure manner.
"This was a real team effort."
The need for accurate GPS data created another wrinkle:
The crews could only acquire data in the morning because solar activity
would interfere with the GPS.
However, even with this delay the airborne lidar
was substantially faster than conventional ground survey methods.
Infoterra acquired data for 5,000 individual points every second
and derived an individually heighted point every two and a half
meters.
"Data with that level of detail would take a lifetime
to acquire with ground methods," Stain said.
The airborne laser scanning project took five weeks
to acquire, and data processing was done in conjunction with the
acquisition phase to ensure complete coverage. But data processing
in the middle of the Sahara Desert posed its own problems.
"First, we had to get the computer equipment on site,"
said Mark Senior, an Infoterra processor. "It was difficult to ship
and we personally carried quite a bit of the components with us.
One of our concerns was having enough hard drive space for processing,
so we carried blank CDs and tapes with us. There was no local computer
store where we could pick up extras if we ran out."
After the standard PCs used for processing were installed
at the desert camp, the most critical issue became the power source.
"All the power was supplied by two generators and
one back up generator, and this did provide a consistent power supply,
which was critical for the processing stage," Senior said.
"The volumes of data we were dealing with took up
to 10 hours to process, so you can imagine if the power dies down
halfway through the night, we would lose a large volume of data."
A difficulty, Senior said, was the flies.
"There were thousands of flies in the area," he laughed.
"Apparently there is a certain time of year when flies hatch and
we hit it. Thankfully this region of North Africa doesn't have malaria
or anything.
"I now know why Australians have corks hanging off
their hats to keep the flies off of the them," he said. "Unfortunately
we couldn't find any corks in the middle of the Sahara Desert."
Selling Points
Infoterra's personnel said this survey gave them
a new appreciation for the level of expertise service companies
and oil firms have developed in such extreme, inhospitable regions.
"A typical onshore geophysical survey is usually
about 10 to 100 times more expensive than the same work offshore,
and we saw firsthand why that is," said Peter Elliott, a geophysicist
with Infoterra.
Airborne laser scanning is a relatively new remote
sensing technique that's advanced topographic terrain mapping. This
technique:
- Offers a high data rate with acquisition of 33,000 individually
heighted points per second.
- Measures over 700,000 points per square kilometer in a typical
survey mode.
- Is unaffected by poor contrast in areas like mud flats or beaches,
and lighting conditions have no impact.
- Can obtain ground elevations even in dense forest cover since
the instrument can penetrate vegetation (not a problem in the
Sahara).
- Makes possible the processing of data entirely automatically,
within hours of acquisition (all data is digital).
A laser scanner operates on a similar principle to
radar. A laser ranger in the aircraft transmits a series of laser
pulses, each of which is returned from targets in the laser path.
The target may be vegetation or a manmade structure as well as the
ground. The laser beam is scanned across a swath beneath the aircraft
using an oscillating mirror.
Target elevation and horizontal coordinates are obtained
by combining aircraft to target:
- Range from the laser ranger.
- The scanner angle.
- The aircraft position from GPS measurements.
- Aircraft roll, pitch and yaw orientation from an Inertial Navigation
System.
Infoterra, formally known as the National Remote
Sensing Centre in Farnborough, United Kingdom, has acquired laser scanner
data for a variety of commercial projects as diverse as beach monitoring,
3-D city mapping and insurance flood risk mapping. In the last few
years it has investigated the uses of laser scanning technology
for a multitude of applications in the oil industry.
"This is an exciting new technology and applications
are still being discovered," Elliot said.
Financial Considerations
Not only can this data aid the seismic contractor
in designing its survey on the ground; according to Elliot, it can
actually help set subsurface parameters.
"For example, if a seismic contractor needs to image
a specific zone in the subsurface, they need to design the geometry
of the survey to precisely focus on that specific depth," he said.
"Without a detailed terrain model there is no guarantee that a contractor
can acquire the precise geometry necessary."
During the seismic survey planning stage, lidar digital
elevation and terrain models help in positioning source and receivers.
The data can identify areas where the gradient is too steep to take
Vibroseis trucks as well as identify potentially hazardous areas
and obstacles in the seismic survey path.
"For example, the terrain models acquired in the
Sahara Desert provided WesternGeco with data on potential hazards
like unstable dune faces susceptible to collapse and areas where
the gradient up the dunes was too great for source vehicles," Elliott
said.
Seismic contractors are not the only potential users
of airborne laser scanning data. Oil companies also can use the information
for developing well sites, installing pipelines and flowlines, and
positioning other infrastructure.
"This data has enormous longevity from the seismic
acquisition planning stage all the way through to field development,"
he said.
"The name of the game through every stage of oil
and gas operations is to reduce costs and avoid problems before
they occur. Airborne laser scanning technology addresses these needs.
In the future this technique should become a standard approach for
petroleum operations. The more we use this new method the more economical
it will become."