Over the past five years, Brisbane-based seismic services company Velseis has completed two ACARP-supported projects developing techniques to utilise the second wave type and convert it to usable imagery.

Broadly, the projects to date have been aimed at finding a way to use the second wave type to produce an auxiliary seismic image to complement the conventional image. This can potentially yield improved geological characterisation of coal structures.

Additionally, removal of the interference effects of the second wave type can yield a cleaner conventional image.

Velseis chief geophysicist Steve Hearn told International Longwall News that the basic approach in this type of seismic imaging was to integrate two sound wave images.

“One is the conventional P-wave image, and the other is a converted-wave, or PS-wave image,” he said.

“By looking at them as an integrated pair, in some circumstances we can get a better idea of what’s going on.

“Normally, the conventional image on its own can really just tell you what the shape of the geological structures are – it can’t tell you too much about what type of rocks they are.

“The combination of the two waves ultimately has the potential to give you more information regarding such things as rock types, fracturing, and the presence of gas.”

However, the initial primary commercial appeal of developing a method of accurately interpreting the PS-wave has been its ability to image shallower geological features.

“The conventional P-wave imagery generally can’t see things shallower than about 50 metres,” Hearn said.

“Basically, there are various types of noise that swamp the image. The different type of wave we’re using (PS) travels more slowly for part of its journey.

“This means it comes in a bit later, after the noise has died down a bit. We’ve got some good imagery up in the 20 to 25 metre range.”

Hearn said this advance in imaging would be particularly useful where there is potential for large opencut coal operations. Over the past two years, a number of successful commercial surveys have been recorded by Velseis for this purpose.

The success of this project in being able to generate usable imagery for shallow deposits has led to Velseis being awarded a follow-up ACARP project, Toward 3D Integrated P+PS Seismic Imaging of Coal Targets. The project kicks off this month.

“All the work we’ve done to date has been 2D, which provides cross-sectional images along single lines,” Hearn said.

“Now we’re exploring the issues involved in building a 3D image using this PS-wave.”

While 3D conventional P-wave imaging is common, Hearn said generating a 3D image using PS-waves was a much more difficult proposition.

“The acquisition phase is no problem,” Hearn said.

“We can record both wave types simultaneously. All we do is use a different receiver that picks up the PS-waves as well as the P-waves.”

He said the big challenge, however, is in processing the converted-wave data.

“The fundamental problem is this. If you send a P-wave down, it bounces off a coal seam and back to the surface. The point where it bounces is basically halfway between the source and the receiver, so it’s a symmetric reflection,” Hearn said.

“Now, this other wave, the PS-wave, goes down as a P-wave and bounces back as an S-wave, and because of the physics, the point where it bounces is not halfway between them, it’s over towards the receiver.

“It’s what we call asymmetric reflection, and in essence, that is the root of one difficulty as far as the data processing goes.

“When you’re processing conventional [P-wave] data, there are about 10 fundamental steps you take. It’s a bit like making a music recording. You’ve got to adjust the levels of the different instruments, adjust the bass and treble, put some reverb on it, and do various other tweaking before you can mix it all together into the final image.”

Hearn said that while a number of those processes work on both wave types, about half have to be done a different way that is specific to PS-waves. Many of the differences in processing relate to the asymmetry in the ray paths.

He said the one PS-wave-specific process that presents the biggest challenge is the one that corrects for variations in the softer weathering material near the surface.

“If the surface was perfectly constant right across the paddock – so a constant thickness and the same stuff – everything would be okay. But it changes in thickness and in type, and the wave that goes through one part of the shallow ground is going to be delayed in time relative to another wave somewhere else,” Hearn said.

“If you imagine the musical analogy, it’d be like building your recording with multiple vocals happening and one of them is delayed when you don’t want it to be. Different delays are happening as the waves are going through different thicknesses of weathering.

“PS waves are much more susceptible to these variations than P-waves. This is proving to be one of our biggest challenges in terms of PS processing.”

Hearn said that the immediate demand for the technology has come from its fortuitous ability to image shallower coal deposits.

However, he said he expects that, in the longer term, the additional information content available from integrated P+PS 3D imagery will also be exploited in underground coal mine planning.

He expects to be able to have some prototype PS-wave 3D imagery available within a year, while commercially applicable systems could be approximately three to five years away.