Discussion Topic |
|
This thread has been locked |
Rhodo-Router
Gym climber
looking nervously up at Foresta
|
|
|
Aug 29, 2009 - 03:31pm PT
|
Tony, you're right in that *those* rocks will never fall again. But spontaneous rockfall sites tend to leave a lot of smaller litter behind which is still poised to go- have a look up above the Folly for evidence of this.
|
|
jstan
climber
|
|
|
Aug 29, 2009 - 03:41pm PT
|
Maybe rather than trying to use seismographs they can afix sensitive microphones to the rock. Small battery powered computer to send data by wireless only when a threshold has been exceeded. With a small amount of DRAM the part of the event prior to exceeding the threshold could even be sent. The NPS already has the guy most able to knock together a rig like this. Knowing him he could have five units in place before the end of September.
Hardest part would be experimenting to see what interface gives the best energy transfer with the rock. Also need to select the bandwidth. Piece of cake.
Have to get a highly skilled climber with a lot of experience to take it up on the face for him tho.
|
|
powderdan
Social climber
mammoth lakes
|
|
|
Aug 29, 2009 - 05:39pm PT
|
more boulders!!!!!!!!!!!
|
|
Mighty Hiker
Social climber
Vancouver, B.C.
|
|
|
Aug 29, 2009 - 05:47pm PT
|
If you placed microphones near identified rockfall zones, how would you filter out extraneous noise - snow, ice, and rainfall, and so on?
|
|
TwistedCrank
climber
Ideeho-dee-do-dah-day
|
|
|
Aug 29, 2009 - 07:48pm PT
|
The harmonics of rocks are different than the harmonics of water, ice, rain, snow, etc.
It would be harder filtering out the ambient noise like cars and trucks.
The seismology guys are always trying to snag the signal and bag the noise.
|
|
WBraun
climber
|
|
|
Aug 29, 2009 - 07:52pm PT
|
You don't use microphones to begin with.
You use piezoelectric elements as strain sensors.
Then SCADA to deliver the info.
|
|
cleo
Social climber
Berkeley, CA
|
|
|
Aug 29, 2009 - 11:09pm PT
|
These are all good ideas, btw... microphones, strain sensors, and studying how much flakes are moving. And yes, there are some great folks on NPS staff that I've meet (not just Yosemite). It is partly a matter of getting proposals funded - crossing my fingers!
(Also, I'm fairly confident that we can differentiate rock cracking from snow, ice, etc. Gunshot detectors in major cities are a similar concept).
|
|
jstan
climber
|
|
|
Aug 30, 2009 - 12:17am PT
|
In my lack of practical knowledge that is why I thought picking up sound in the rock would be more sensitve than detecting movement. Piezoelectic sensors could be the best interface. Sound should have a high velocity and where the integrity of the rock is good it should travel long distances. And by using a Fast Fourier Transform signal processing you can filter out all the band widths not germane to the sounds created in the rock.
Sounds like Cleo has been doing this.
Come to think of it oil drillers send pressure waves into the formations and from the reflections figure out what is below just as in ultrasound equipment. When you put a sensor on a face at a sufficient sensitivity you can surely study the time evolution of the reflections and calculate the underlying rock structure. Then if you know that beforehand when there is rock failure you might be able to locate its position, again by looking at the time delays of the reflections.
Not that I know how to do any of this.
|
|
Tom
Big Wall climber
San Luis Obispo CA
|
|
|
Aug 30, 2009 - 03:30am PT
|
The race is on: who's gonna get the FA of the new RA variations????
|
|
Peter Haan
Trad climber
San Francisco, CA
|
|
|
Aug 30, 2009 - 09:35am PT
|
Or for that matter Tom, who will do the FA of the Folly Continuation???
|
|
gstock
climber
Yosemite Valley
|
|
|
Aug 30, 2009 - 10:55am PT
|
Lots to respond to here.
First, Cleo is the best one on the Forum to address how to monitor rock fall "noise", as that is the thrust of her dissertation at UC Berkeley. In addition to seismometers at the Brothers, she had an acoustic sensor set up (in my back yard) for most of the winter and spring, and recorded the Ahwiyah Point rock fall in March. I think there is a lot of potential for combined seismic/acoustic monitoring of rock fall precursors; we are working on that, as are others, but it is still in the experimental phase. Even if a reliable method is developed, the sheer scale of Yosemite Valley will make monitoring beyond site-specific areas challenging.
I don’t claim to be able to predict exactly when and where rock falls will occur. In some cases, including a few events here in Yosemite, geologists have warned of imminent rock falls that did in fact occur. Usually these situations involved obvious changes such as cracking sounds, propagating fractures, etc. In other cases these “obvious” changes have not produced rock falls, even ten years later. Over half of the documented rock falls in Yosemite Valley do not have recognized triggers, meaning that we cannot say with certainty exactly what triggered the rocks to detach from the cliff. This is true even in cases where the events were closely observed. This speaks to the inherent uncertainty in rock-fall assessment. Prediction is still a goal but, as with earthquakes, we still have a long way to go.
It is possible to evaluate the stability of a rock mass by mapping the discontinuities (joints, faults, bedding planes, etc.) bounding that rock mass and making assumptions about the physical properties present. However, these sorts of analyses are better suited to small bedrock outcrops (e.g., roadcuts) than they are to the 1 kilometer-tall cliffs of Yosemite Valley. We have recently employed laser scanning (LiDAR) to map discontinuities, but have so far only scanned a small fraction of the 40 square kilometers or so of cliff area in the Valley. Another significant challenge is that many rock falls, including the recent one behind the Ahwahnee, fail along sheeting joints, or exfoliation joints, which parallel the cliff surface and are therefore not resolvable by remote sensing techniques. I am collaborating with the US Geological Survey this next year to instrument several flakes in the Valley to see if they move with daily and/or seasonal temperature fluctuations.
Regarding the apparent increase in rock fall activity in the past decade or so, I am not convinced that this is happening. As I posted in an earlier thread, the 1980’s had much larger and more consequential rock falls (in terms of injuries and fatalities) than this decade, but people are quick to forget. The database of historic rock falls does show more rock falls now than, say, 50 years ago, but that is almost certainly the result of increased reporting of rock falls, not an actual increase in the number of rock falls. Furthermore, 150 years of data may not accurately portray geological process rates. We are presently investigating the talus in Yosemite Valley to get a better perspective on this issue. The approach is to use GIS to isolate a talus pile in three-dimensions, calculate the talus volume (accounting for porosity), determine the contributing surface area of the adjacent cliff, and create a volume-frequency relationship for that location (the frequency part is possible by assuming that the talus accumulated since the last glacier retreated from the Valley about 17,000 years ago). So far we have done this analysis for Glacier Point, Royal Arches, Middle Brother, and Middle Cathedral. Overall we do not see that modern rates of talus accumulation are significantly larger than prehistoric rates, although Middle Brother is an exception because of the huge (600,000 cubic meter) rock fall on March 10, 1987, the largest historic event by far. We are still investigating this issue, but media statements that rock-fall activity has increased should be viewed skeptically.
Anyone wanting more information on this stuff is welcome to contact me. As always, I also appreciate hearing any information you may have about rock falls, possible precursor activities, etc.
Greg Stock
Yosemite Park Geologist
(209) 379-1420
greg_stock@nps.gov
|
|
jstan
climber
|
|
|
Aug 30, 2009 - 01:05pm PT
|
Thanks Greg. Fascinating what all the new tools make possible.
If we would disarm all nuclear capabilities worldwide GPS precision could be opened up to the max. Don't know whether that can possibly give precision in the millimeter range.
Might add another argument for getting rid of that stuff.
|
|
BMcC
Trad climber
Livermore
|
|
|
Aug 30, 2009 - 05:28pm PT
|
Photos?
Did this rockfall really come off the Royal Arches Route? (... and tumble over to the Ahwahnee Hotel?)
Seems like the RA route is a bit E of the Ahwahnee; although, part of the (valet?) parking extends E of the hotel.
Can Greg or someone tell us where on the cliff the rockfall originated? What did it hit on the way down and where did it land? What routes were affected?
Thanks. Be careful and lucky up and out there!
|
|
cleo
Social climber
Berkeley, CA
|
|
|
Aug 30, 2009 - 06:14pm PT
|
fattrad, are you hoping they'll close the Ahwahnee and you'll get to move in as Emperor of Yosemite?
|
|
gstock
climber
Yosemite Valley
|
|
|
Aug 30, 2009 - 06:15pm PT
|
The rock fall came from midway up the Rhombus Wall, directly behind the Ahwahnee and west of Royal Arches and Serenity/Sons. In the other rock fall thread Werner posted a photo that shows the source area. No popular climbs were affected, but please be aware that the approach to Serenity/Sons and Super Slide is within the rock-fall runout path. The approach to Royal Arches is east of the runout path, and the route itself is well clear of that.
Greg
|
|
neebee
Social climber
calif/texas
|
|
|
Aug 30, 2009 - 07:45pm PT
|
hey there greg stock, say, this is very interesting, thanks so very much for taking the time to share all this with us...
love rocks... sad to see them fall on folks, though...
but,as we all know, rocks, do DO that...
good that folks can take to study it all...
|
|
BMcC
Trad climber
Livermore
|
|
|
Aug 30, 2009 - 08:52pm PT
|
Greg -- thanks for the info and mention of the other thread (I had missed it). Just checked it out.
Werner -- thanks for the info and for the pointer on the photo.
Looks impressive on the MSNBC link.
Bill
|
|
|
SuperTopo on the Web
|
Let us know!