By LOUISE S. DURHAM
EXPLORER Correspondent

Passive Seismic

Listen: Is It the Next Big Thing?

It looks like this: Schematic of a sonde currently deployed downhole on a long-term monitoring project in the Middle East.

Graphic courtesy of Input/Output

Data Charted As You Read This

Given the plethora of expendable boreholes on land for receiver placement versus the marine environment, it comes as no surprise that most passive seismic applications thus far have been land-based.

Still, there have been a few noteworthy applications in water.

Perhaps the most highly documented offshore project took place in 1997 at Ekofisk in the North Sea, where micro-seismic recording identified a fault pattern under a gas cloud, which hinders normal seismic views. Locations of micro-seismic events recorded over 18 days indicated the seismicity clustered along lineations parallel to major structures in the field, according to S.C. Maxwell at ESG Canada.

The events were attributed to induced movement along pre-existing fractures.

One of the advantages of micro-seismic is that faults with small throws can be directly detected. Vertical faults ordinarily are mapped indirectly with reflection seismology by offset horizons.

Passive seismic monitoring at Ekofisk is taking place today, with the sensor hooked up to the power and telemetry distribution system in the cable to deliver data in real time back to the client's office.

"You can see things going on on the seafloor," said Input/Output's Dave Ridyard, "and also listen for what's happening in the subsurface."

-- LOUISE S. DURHAM

... CLICK ON GRAPHIC TO ENLARGE.

Passive Vp/Vs at Well Location

Zoom of Passive Vp/Vs at Well Location

FIGURE 1a

FIGURE 1b

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 waste repositories.
• 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 (Vp) image."

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 the medium.

Problem-Solving Potential

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 reasons, including:

• 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 wave imaging.

"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 geophone.

"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."

Passive data can be used to complement the information ... click to enlarge.

Graphic courtesy of LandTech Enterprises

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 passive results.

"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.