Using the Earth's "creaks and groans" as a seismic source provides high-resolution images at a fraction of the cost of a traditional survey.

Have you ever seen a land 3-D seismic crew in action? A well-trained crew is like a well-oiled machine. It's like the circus coming to town and deftly turning an empty field into a bustling enterprise.

First come the team of permit agents visiting each landowner, securing access for the crew. Then come the surveyors leaving little trails of orange flags for guidance. Now the cable trucks arrive with the line crews. That's when the helicopters start flying, dropping 100-lb sacks of cables and phones down the line. If it's a dynamite crew there will be three or four small drilling units tearing about, drilling and loading the shot holes. If it's a vibrator crew, those big monsters will be sitting in the corner of the field, warming up just waiting to start shaking. Finally, enough line is laid out that recording can begin. For the next few weeks (or even months) each day will follow a prescribed pattern and the crew will settle into a confident rhythm: wake up the lines, fix the breaks, light up the patch, fire the shot, roll the spread, pick up in the rear, lay down out front. It's a huge undertaking and a marvel in its efficiency.

All this industry is not just for fun or even for science. Seismic data has become virtually indispensable to the petroleum industry. The seismic method is the "eyes" of the geologist and the reservoir engineer.
As you might imagine from the foregoing description, there are some places where it's too difficult and too expensive to operate such a crew. The landowner may not want lines cut and heavy vehicles tearing up his land. The terrain may be too rugged for the trucks and buggies. Or perhaps the field sits below a major urban area where it's hard enough to find a parking spot, let alone drill a shot hole. And yet more and more it is in just such places that seismic "eyes" are required.

Passive seismic

There is a solution to this problem. There is a way to get seismic data and create 3-D images like those gathered with the equipment described above, without using all the heavy gear. All one needs to do is lay out some geophones and . . . listen. The earth "creaks and groans" all the time. This is the natural result of the tectonic stresses that are present everywhere. Not all of these stresses are released in concrete-crumbling catastrophes. Much and probably most of the stress is released in microseismic events that may go unnoticed and unreported. It is well established that there are 10 times as many magnitude 3 earthquakes as there are magnitude 4, and 10 times as many magnitude 2 as there are 3, and so on. If one employs events as small as magnitude 2, that's a large number of events and a large number of "shots" to use as seismic sources. "Shot records" from these natural sources can be used to create 3-D images just as can the ones generated by dynamite, vibrators or air guns.

There is another source of seismic energy as well. That source is the earth's response to exploration and production activity. When we drill a well, draw down a reservoir, inject fluids or frac a reservoir, we create seismic noise. These too may be recorded and used to illuminate the subsurface.

The use of such microseismic sources as opposed to controlled sources like dynamite and vibrators to image the subsurface is generally referred to as passive seismic. This is not a new technology.
Researchers have used earthquakes to image the interior structure of the earth for decades. Passive seismic is a mainstay in the mapping of hydrothermal resources. Mapping of the microseismic events associated with fracs, reservoir compaction and steam injection using arrays of borehole seismometers is becoming quite common. Recently the method has been developed to the point of creating 3-D images of the subsurface at a scale and resolution that is useful to hydrocarbon exploration and development.

The technique

So how does one use passive seismic to generate 3-D images of the subsurface?

To understand this, let us first distinguish between those events that occur outside the image area and those that occur within the image area. The first group we will use as external energy sources, and we will image the subsurface by virtue of the way the sound interacts with the target area, jus as we do in conventional seismic. The other sources are themselves the target, and we will use their character and distribution to create useful images.

One way to use the external seismic events is termed transmission tomography (Figure 1). First an array of receivers is placed over the area to be imaged. This is a much sparser array than is used in reflection seismic, usually consisting of 20 to 100 receivers spread over several square miles. As well, three-component receivers are used so that both compressional (P) and shear (S) waves may be efficiently recorded. When a microseismic event occurs below the array, the P and S wave arrivals are recorded across the array. If one assumes a simple, initial velocity model, then one can calculate the location of the event. After many events are recorded, one can invert the process and use the timing of the multiple event arrivals to estimate a new velocity model. As more and more events are recorded over time, the resolution at which the new velocity structure is estimated can be increased. With enough events, a resolution of structure similar to that attained with reflection seismic is possible. The final processed cube details the estimated 3-D velocity distribution, both P and S, throughout the image volume.

The product of such a survey resembles the result of a 3-D inversion of reflection seismic except that the estimated parameter is velocity rather than impedance, and the cube is in depth rather than time. Another important difference is that both P and S velocity distributions are estimated, and Vp/Vs ratio distributions for lithologic discrimination are a standard deliverable. All this is at a fraction of the cost of conventional 3-D when working in unconventional environments.

The flip side of the passive coin might be termed emission tomography. In emission tomography the goal is to map the distribution of acoustic sources within the target zone itself. As illustrated in Figure 2, a sparse array of three-component geophones is deployed once again above the target zone. Commonly, downhole geophones might be used as well if a well bore of convenience is available. Downhole receivers provide a useful reduction in ambient noise levels, allowing for the detection of smaller signals, but the availability of well bores is usually problematic, and downhole equipment is several times more expensive than surface equipment, negating one of the advantages of the method over conventional reflection seismic.

In emission tomography we again assume a velocity model and then calculate the location of all the microseismic events detected in a period of time and originating within the target volume. A plot of the distribution of energy source locations is likely to have geologic significance. These distributions may track the progress of a frac, the advance of a flood front, or the location of active or reactivated faults. Acoustic monitoring of hydrofracs is one example of emission tomography. Another is the well-publicized monitoring of the microseismic events associated with the subsidence observed at the Ekofisk field. We can expect more case histories of this type of seismic imaging to appear as more fields are wired for continuous monitoring in what is becoming known as the electric oilfield initiative.

Passive techniques offer exciting opportunities to collect 3-D images in areas where conventional gear either can't go or can't go at reasonable cost. Passive seismic offers a way to collect seismic data with virtually zero environmental impact, and it offers an opportunity to reveal some of the dynamic activity going on at the reservoir rather than only the static, structural and stratigraphic images available with conventional 3-D methods. Passive seismic is an old technique with some exciting new applications only now being explored.

For more information, visit www.microsesmicinc.com.

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