Now that the E&P community's
appetite for the last new big thing, i.e., 3-D seismic data, appears
to be satiated, the seismic industry has a new big question:
What's the next big thing?
Remember, technological innovations — blockbuster
or otherwise — are seldom recognized as gotta-have-it gizmos until
the rank-and-file E&P players decide "everyone" is using this
new tool advantageously.
The "new" technology, more often than not, has been
incubating for years.
Here, then, is a good guess for the next potential
big thing: passive, or "sourceless" seismic. It has been used for
some time for an array of applications, including:
- Monitoring mine fractures for safety purposes.
- Nuclear blast detection.
- Determining excavation stability in nuclear
- Geothermal reservoir performance.
- Probably the most highly publicized application
of the technology was on the moon during the Apollo space program,
where detectors measured the surface impact of meteorites and
man-made objects to evaluate lunar crustal structure.
Passive seismic has maintained a somewhat shadowy
presence in the E&P industry. In fact, it has been evaluated
by various companies for select applications for more than a decade
without creating any significant stir.
It's a fairly simple concept, based on the basic
principle that all the little creaks and groans in the earth are
actually seismic sources. These naturally occurring micro-seismic
events are accompanied by smaller scale man-made noise created by
production activity in the oil or gas reservoir.
"Passive seismic is 3-D seismic imaging of the target
geology without the use of artificial surface sources," said Peter
Duncan, president, MicroSeismic Inc. "Locally occurring micro-earthquakes
and induced seismic emissions from E&P activity are used instead.
"It uses multi-component seismic receivers to take
advantage of shear wave energy generated by the micro earthquakes,"
he continued, "thereby delivering a shear wave (Vs) velocity distribution
estimate of the subsurface in addition to the conventional compressional
In contrast, the man-made sources used in 3-D reflection
seismic methods do not produce large shear waves. The result, according
to Duncan: These conventional methods do not adequately provide
material parameter information related to the shear velocities in
Passive seismic technology has the potential to solve
a number of industry problems.
Consider, for instance, the continuing inefficiency
and expense of land-based conventional 3-D activity. It never got
up to speed economically with its marine counterpart for several
- It is labor intensive: Large crews must
set and retrieve geophones manually, with miles of cable laid
out on ground.
- Surface access is needed for vibrator buggies
or shot hole rigs — and this can be expensive and messy.
- Permitting, remediation and other environmental
concerns = $$$$.
In comparison, look at what passive seismic brings
to the table:
- Naturally occurring or production-activity-induced
micro-seismic events as sources: In other words, no need for vibrators,
dynamite or airguns.
- No heavy vehicle support — this lowers
costs and allows activity in otherwise inaccessible terrain.
- Recording station density reduced — this
reduces crew size and equipment costs.
The micro-array technology used in passive seismic
also has application in marine geophysical investigations — with
the potential to provide a substantial environmental bonus. Micro-arrays
spaced at 1-3 km intervals would provide an economical alternative
to Ocean Bottom Cable (OBC), according to Duncan, who noted the
elimination of the airgun source array would allow operations even
in the midst of fish or marine mammals.
Whether on land or in the marine environment, micro-seismic
technology applications until now have been limited by the lack
of available specialized equipment and processing technology to
make the application viable, i.e., cost effective.
Other recent innovations, such as the MEMS (micro-electro-mechnical-systems)
technology of Input/Output, offer high resolution P-wave and shear
"The (MEMS) sensor is built on a silicon wafer as
opposed to a conventional geophone," said Dave Ridyard, business
development manager reservoir operations group at Input/Output,
"and the performance properties are out of the league of a conventional
"It's ideal for passive seismic," he said, "because
you're dealing with relatively small events, and you need a high
performance sensor to pick them up."
Defining the Modes
Passive seismic technology for oil and gas can be
used in three different modes, according to Ridyard:
- Structural imaging: It's theoretically
possible to build a 3-D structural image equivalent to what you
build with conventional surface seismic sources. Big problem is
length of time required to sit and listen for data, which has
no guarantee of even distribution.
It's pie-in-the-sky now, but some people are working on it.
- Direct analysis of cause of seismic event:
A listening device detects location of event and tells where things
are changing. Can detect location of pressure front moving through
reservoir by looking at micro-seismic generated by front's passage.
Preferable to use sensors downhole, because it requires such
detail about tiny events.
- Tomographic approach: Look at travel time
associated with the micro-seismic events to infer information
about what is happening in the reservoir.
The majority of passive seismic applications employ downhole
receivers for reservoir monitoring and other production applications,
utilizing emission tomography — but there's increasing interest
in using transmission tomography as an exploration tool to look
at large areas using surface receivers.
"One of the things I looked at in passive seismic
was the monitoring of earthquakes as an exploration tool," said
geophysicist Dave Monk. "Somewhat surprisingly, it can be used quite
successfully as long as an area is quite seismically active — a
lot of earthquakes.
He noted there are three variables:
- Number of earthquakes.
- Number of detectors put out.
- Length of time you listen.
"If all of these are very big, it's possible to get
a very accurate velocity model," Monk said. "How long you listen
is very important.
"If you lay enough receivers on the surface and listen
for a long, long time, you can build tomography from the arrivals,"
he noted, "and build quite a good velocity model of the subsurface."
One of the projects in which Monk participated was
to determine the feasibility of using passive seismic in Vietnam
where it would be difficult to get equipment and crews into the
seismically-active delta area.
"It worked out that in one area if you placed a certain
number of geophones on the surface over certain spacing and listened
for a year, you could build a velocity model as accurate as one
from conventional seismic," Monk said.
"From a theoretic standpoint, we demonstrated it
should work," he noted, "but I don't know in this case if the company
went through with the experiment."
It's Greek to Him
The Enterprise Oil project he looked at in the Epirus
region of northwest Greece did come to fruition — with results
to validate the technology.
"It was decided to use passive seismic for the project
because the surface carbonates and difficult topography resulted
in poor seismic images that could not guide any exploration activity,"
said Sotiris Kapotas, chairman and CEO, LandTech Enterprises, S.A.
& Earth Research (UK) Ltd.
"The area is on a thrust belt with relatively small
earthquake activity in relation to the rest of the country," Kapotas
said. "Bear in mind that we seek to use very small earthquakes in
order to have plenty of them and within the area of interest."
To implement the project, a Passive Network (PATOS)
was installed and operated by LandTech in collaboration with the
Seismological Laboratory of Patras University, Greece. PATOS was
made up of 40 3-component high resolution 24-bit seismometers, which
were buried to improve signal-to-noise-ratio. The network recorded
micro-earthquakes continuously for 11 months.
Using P and S-wave travel times, the tomographic
inversion experiment at Epirus yielded a 3-D model for Vp (structure)
and Vp/Vs (lithology).
The Demetra X-1 well was drilled based in part on
"Unfortunately, due to high pressure, the well was
abandoned a few feet above the reservoir," Kapotas said. "But what
was interesting at the end is that the passive data fit the well
data quite good."
The passive results were compared to pre-existing
geological observations, to other geophysical methods and with the
VSP data after drilling in the study area, confirming the reliability
of the passive application.
Kapotas noted LandTech is wrapping up another passive
seismic project in the same general area, and setting up the final
stages of a new project in Italy for a joint venture between TotalFinaElf,
Shell and ExxonMobil.
The Jury is Out … For Now
Passive seismic technology clearly is gaining a foothold
in both exploration and production, but the jury is still out regarding
its potential real value as an exploration tool.
Despite the relatively easy operation, especially
in mountainous areas, and its environmental-friendly appeal, there
clearly are obstacles to surmount before this technology becomes
the exploration tool-of-choice.
For instance, you really can't control nature. Natural
seismic sources have the drawback of sporadic distribution in time
and space, and monitoring these sources may take an unpredictable
amount of time to record sufficient data to construct an image that
is as good as conventional seismic, according to Ridyard.
And time is money.
"It provides a valuable supplement to conventional
techniques," Ridyard said, "and it could be a substitute for conventional
in a limited number of areas.
"There's some use for it in very environmentally
sensitive areas," he said. "For example, in California it might
be easier to do passive seismic than get a permit for conventional."
Still, it's not always easy to take the pulse of
the next new/old big thing.
As Ridyard noted, just as companies tend to keep
quiet about their failures when dabbling in a new technology, they
tend not to talk about successes either.