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Acoustic Zoom - working with diffused seismic energy

Thursday, November 21, 2013

Acoustic Zoom Inc., a company based in Newfoundland, Canada, is developing a new type of seismic survey - to work with non-specular, or diffused seismic energy - which promises a much better understanding of rock properties, including fractures

Acoustic Zoom Inc, a company based in Newfoundland, Canada, is developing a completely new type of seismic survey – based around diffused, or non-specular seismic energy.

It promises to provide a much better insight into the amount of fracturing, mapping of the facies (a body of rock with specified characteristics) variations, natural fractures and micro-fault systems within and around the reservoir zone in the rock being studied, rock physics properties (such as sound velocity, stress and strain), and other rock properties, such as homogeneity.

Conventional seismic surveys are entirely geared around specular reflections, where the sound waves are reflected up like a light on a mirror. The survey and subsequent processing is designed to increase the signal of these reflections and treats everything else as noise.

Instead, the Acoustic Zoom seismic method analyses the sound waves which get diffused (split in many different directions) because it is this diffracted energy which best characterizes the true subtle complex character of geology thus providing a great deal of useful information on the internal nature of formations, not just their presence.

Radio telescopes work in a similar way, says Dr. Jacques Yves Guigné, president of Acoustic Zoom, based on understanding how beamforming of radio waves can be used to capture images of the galaxy.

Professor Guigné originally studied Geology and Geomorphology at the University of Winnipeg in Winnipeg, Canada, and then went on to do Oceanography and a Masters in Marine Geophysics at the University of Wales, Swansea University/College, UK, followed by a PhD in physics, specializing in nonlinear acoustics at Bath. He is a professor in the Department of physics at University of Bath (UK) and a specialist in acoustic physics having recently been internationally honoured by his peers with the Rayleigh Medal by the Institute of Acoustics.

The company recently completed a pilot acquisition on the Eagle Ford formation in San Antonio, Texas, in a formation with Austin chalk above it and limestone below it.

Specular and non-specular

To explain a bit more about the difference between specular and diffuse reflection:

Specular reflection is like reflecting from a mirror, at the angle you would expect it to.

Diffuse reflection is where the incoming wave is reflected in a broad range of directions.

If you consider a game of snooker (or pool or billiards) – the balls bounce off the cushion with specular reflection. If balls bounced with diffuse reflection (for example, if the cushion was made of a sponge) it would be impossible to play because you couldn’t predict where the balls would go.

Another example is the difference between gloss and matte paint. Matte paint reflects light with diffuse reflection – whereas gloss paint reflects mainly with specular reflection.

Complementing traditional seismic

The Acoustic Zoom method is a complement to conventional seismic, not an alternative.

Conventional seismic surveys are geared towards identifying seismic horizons (or areas where there is a big change in rock density).

It accentuates by stacking together seismic signals, spatially migrating the data in folds reflected in the same place on the subsurface to strengthen the signal and weaken the incoherent returns treating these as noise.

The conventional approach serves the industry very well for conventional reservoirs, because oil and gas reservoirs normally reside in stratigraphic traps, which are formed when you have a change in facies.

All major hydrocarbon finds show up in seismic as a continuous boundary between different types of rock (or rock and oil if it is a very porous reservoir).

Not so simple

The weakness in this method is that the subsurface is not made up of wide blocks of homogeneous rock. A conventional seismic survey gives a very simplified view of what the subsurface is like.

You can see this for yourself any time you look at some subsurface rock outcrop – for example, a cliff face. The rock layers are enormously complex in its composition, not in thick solid layers like on a seismic diagram.

The Acoustic Zoom survey makes it clear how complicated the subsurface actually is. “We’re dealing with the reality of the complexity of the earth as it should be,” says Professor Guigné.

Rock physics

By analysing and modelling the diffused energy, you can get a much better understanding of the rock’s engineering properties (rock physics), including how it is internally composed .

From the imagery and iterative modelling it uses in its processing to predict how the sound waves have been redistributed, you can find out by the resulting imagery the internal micro fissures, fractures, and discontinuities in the different parts of the rock formation, which indicates that there is some kind of structural change going on. To get sensitivity to geological property changes, you need high frequency seismic, Guigné says.

“With seismic it is very difficult to get engineering parameters, because the purpose of migrating the data is to remove what isn’t dominantly coherent and continuous in the boundaries—you’re not going after discontinuities of the scale of geological variability, roughness , micro-structural information,” he says.

Benefits

Seismic waves will reflect in a non-specular way if there is what the company terms ‘roughness’ in the rock – lots of fractures, fracture or rubble fields and non-homogeneity.

By analysing the non-specular reflection you can get an understanding of the rock properties, for example, by imaging the fissures, fractures, subtle discontinuities. Broken geological structures in the subsurface will diffuse the energy in all directions.

“In our case, we are not interested in the major boundaries. That’s what seismic does so well. We’re interested in capturing the subtle complex stratigraphy. We talk about unconventional reservoirs – this is what its geology looks like.”

As the oil and gas industry moves to more complex reservoirs (including unconventionals) there’s much more emphasis on understanding the characteristics of the rock – including the rock physics or engineering properties (stress, strain, permeability, secondary porosity).

If the rock is heavily fractured it is very hard to ‘frac’ – since fracing is mainly about forcing apart existing linear stratas, fractures redirect the energy unpredictably, not always where the intent for fracing was meant to be placed and not necessarily creating the effective opening up of the shales.

“You have to know the internal distribution of the character of the geology,” Guigné says, “ to understand how best to exploit such unconventional reservoirs. For the most part conventional seismic just tells you there’s a top and a bottom.”

Survey structure

Acoustic Zoom Inc. typically runs surveys with a seismic source (a modified Vibroseis truck to push higher frequencies then typically used) within a dense network of sensors around it, like spokes of a bicycle wheel around a hub.

The seismic receivers typically are arranged in 8 lines, all going through the centre hub (like 16 spokes around a wheel), with each spoke extending out 2kms.

The system uses the same geophones as in conventional seismic recording, but the layout is different, with more care in the placement of the receivers in step with the need to preserve the very high frequency returns. The final receiver configuration results in an irregular, more closely spaced set of geophones towards the centre.

After making about 500 recordings, the vibroseis truck is moved about a fraction of the transmitting signal’s wavelength – 4-5 metres. This is repeated 5 times to create a large “virtual” or “synthetic” transmitting lens.

4,000 receivers were used, and 2 terabytes of data were acquired.

The recording was made with frequencies of 5 Hz up to 170 hertz. At a depth of over one and half kilometres in the earth at the Eagle Ford structure 140 Hz signals were collected compared to a maximum 70 Hz with a conventional seismic survey at the same location.

With conventional seismic, it is very hard to stack seismic responses of above 70 Hz, Professor Guigné says – because if you move the seismic truck every 50m over many kilometres of seismic profiling, you find the spatially variable and subtle high resolution signal responses do not migrate and stack or fold well but rather become incoherent noise with very little similarity with the previous response until the more coherent lower resolution boundaries coherently appear. But with the Acoustic Zoom system the truck is stationary with little movement only to form its transmitting lens.

Acoustic Zoom’s unique and possibly game-changing non-specular diffused energy data capture has the potential to unmask the finer internal geological structures that control the fluid migration within unconventional reservoirs. This is a big prize for today’s growing dependency on such reservoirs!

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» Acoustic Zoom

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