Wednesday, June 17, 2009

Rotational Seismology Explained - II

Part 1

If you want to measure a gravity wave, you just need a big block of concrete embedded in an underground cavern of jello, fully instrument it, and wait. This block has accelerometers and ring gyros to measure the rotational components, as it shakes in the jello.

Problem is that this nasty earth has all those earthquakes! And you can't easily filter out the effects of seismic waves. And really, you need a whole bunch of blocks in jello to detect a gravity wave. Thus, they developed the sensitive rotational sensors that you need for the task, both for the block, and the rock all around it.

Look at this guy just preparing the glass block for a ring laser! These things are expensive!

Ok, they got the cost down by using fiber loops, and other little tricks. Now they can be dispersed in the field, but what are we looking for?

One reason that a ring laser may be quite good for gravity waves is that we generally can't expect rotational waves in the rock. A real torsion wave is something we propagate down a cylinder by twisting one end rapidly. The pure rotation can only be measured at the centre, and out from there we get a mixture of translation and rotation. Torsion waves are almost impossible to propagate in an elastic half-space (which is the earth).

Nope, our seismic waves are plane waves, which set up a front and propagate rapidly. For the shear wave there will be out-of-plane rotations, and they might be interesting to measure, but the amount of rotation falls directly from the plane wave equations, and translational measurements are most likely enough.

You can get complex rotational waves and effects right near a complex fault, where all the plane waves intersect, and I can imagine a tight rotational wave propagating for a short time. This makes life more deadly near a fault, and would be a good place for these new sensors.

In fact, any complexity will start things 'spinning'. A soil basin could easily hold a rotational wave, imagine a bowl of jello that you suddenly twist. Like a cylinder wave, the maximum vertical axis rotation would be at the centre of the basin. There would be horizontal axis rotation with surface waves.

It is with surface structures, that there is observable rotation. Images just as these have haunted 'rotation fans' for generations.
I don't think much of this, but some people do. Nevertheless, when a building starts to rock, things become much worse. Rotational sensors could sort some of this out.



Anonymous said...

The Ire of Bob I & II
Well they say you can’t teach old dog new tricks. But you are slowly coming around. At least you realize the need for rotational monitoring in structures. Good, the BSSA edition deals with civil engineering. In fact Prof. Trifunac has been writing papers for years. The community has finally accepted it after about 9 years. Now they are all chairmen’s of international committees and editors (i.e. the BSSA special edition).

You need to retract the need of field rotational monitoring as “bunk”. Then the Ire of Bob will become Happy Bob. It is unfortunate that others cannot view my comments. Only you.

Obviously you are familiar with Dr. Grazier wave formula. Every axis includes a rotational component, which forever has been ignored because it could not be measured. The rotational motion is much smaller amplitude than translational components. So when doing the math trying to derive rotations the error becomes lager than the motion. Not to mention the errors from not having clone translational seismometers (electrical and mechanical which is impossible).

Now throw out the math theory and concentrate on the real world. Actually measure pure rotational motion immune to translational motion. Combine that rotational sensor with a translational sensor and wal-la you have all 6 DOF.

Correct for earthquake applications you need to be close to the source because of the attenuation factor. BUT that is a function of frequency. The lower the frequency say 20 seconds the less the attenuation distance. Think of teleseisms

Hence the introduction of the R-1 field seismometer that can be deployed in all seismic networks one would be able to record all 6DOF relatively close to the source.

Therefore the math of double integration is gone (not counting the errors of the difference in translational sensors), and proven by actual measurements in the field. Read the BSSA.

The ring gyro photo was nice. But the one at LMU in Germany is 5 times as large. They are BIG and expensive in the $100,000 USD range. Not made for the field (close in). The R-1 is a low cost field rotational seismometer designed for field use. The R-2 has even better specifications and self-calibration that should be released soon.

Face it; this is the future and the next level of research in seismology and civil engineering in the last 20 years.
That is why the BSSA published the special edition which you said “bunk”.

Harold Asmis said...

Whatcha talkin' 'bout Bob? Everybody sees your comments, and see what an idiot I am. I wouldn't have it any other way!