World's Largest Wheatstone Bridge
in [Basics]
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From: Bret Cahill on 4 May 2010 11:25 > >> >> >> >>> String a wire back and forth across / along a fault line to measure > >> >> >> >>> very small displacements in the earth's surface. If the resistance > >> >> >> >>> and/or tensile strength needs to be higher than a common single alloy > >> >> >> >>> wire then structural steel cable could be wrapped around a insulated > >> >> >> >>> wire with a higher resistivity. It could be temperature compensated > >> >> >> >>> as usual, with another wire of the same length loosely supported > >> >> >> >>> nearby in another leg of the bridge. > >> >> >> >>> An abandoned power line may be good to go if it is properly located. > >> >> >> >>> Good info sometimes comes in small displacements. > >> >> >> >>> Bret Cahill > >> >> >> >> Two gps stations on both sides do the same trick > > >> >> >> > What's the smallest displacement -- not movement but actual change in > >> >> >> > _distance_ between two points -- they can measure? > > >> >> >> > Bret Cahill > > >> >> >> They measure continental drift with them in cm's per year.... > > >> >> >The warning might be in microns. > > >> >> >Bret Cahill > > >> >> What warning? Faults creep all the time. > > >> >At constant speed? > > >> >If that were true all the acceleration measurements published by USGS > >> >or Cal Tech on the web in real time would always be zero. > > >> >There may be some characteristic behaviour of certain faults that > >> >could be highly reliable early warning info. > > >> >> Knowing the rate of creep has > >> >> zero useful predictive value. > > >> >Has this been proven over long distances measuring displacements of a > >> >few thousandths of an inch? > > >> >Bret Cahill > > >> Nobody is making useful earthquake predictions. > > >That's the problem. > > >> It's probably > >> impossible. > > >That may very well be somewhat true. > > >> A superficial surface measurement is obviously > >> insufficient to understand an immensely complex and chaotic subsurface > >> 3D system. > > >Maybe. Probably. > > >But why leave any stone unturned when _my_ safety is at stake? > > Good point. Move to Mississippi and live in a tent and you will be in > somewhat less terror of earthquakes. At least they can predict hurricanes and oil slick movements. Anyone want to buy a used shrimper? Why is earthquake insurance was so expensive? Unlike floods or fire it's pretty hard to fake an earthquake to defraud the insurance company. Bret Cahill
From: Paul O on 4 May 2010 12:19 Bret Cahill wrote, On 5/4/2010 11:25 AM: >> <snip> > At least they can predict hurricanes and oil slick movements. Anyone > want to buy a used shrimper? > > Why is earthquake insurance was so expensive? Unlike floods or fire > it's pretty hard to fake an earthquake to defraud the insurance > company. > > > Bret Cahill > > > Bret, Take a wooden pencil and very slowly bend it. You will begin to hear small cracking noises (mini earthquakes) as the wood fibers begin to fail. Then suddenly snap - the pencil will break (major earthquake)! Now, run the experiment again. This time try to predict the exact millisecond that the pencil will break. Run the experiment a dozen times and see how you do. Its not easily. Every pencil is different. Even accurately knowing the load and the displacement on the pencil will not allow you to predict exactly when the pencil will snap. This experiment is very similar to predicting when an earthquake will occur. Also, geologists already use strain meters (sometimes called borehole extensometers) to measure displacements across fault lines. See: <http://www.springerlink.com/content/x0833xv4j4qq02qj/> <http://www.visionsmart.com/turtlemountain/extensometer.htm> <http://www.mhest.com/spotlight/earthquakes/articles/Seismographic_Instrumentation.pdf> -- Paul D Oosterhout I work for SAIC (but I don't speak for SAIC)
From: jimp on 4 May 2010 12:34 In sci.physics Bret Cahill <BretCahill(a)peoplepc.com> wrote: > Motion detectors are good down to a few microns and millihertz but the > really low frequency events are lost. A seismometer is a really low frequency motion detector. -- Jim Pennino Remove .spam.sux to reply.
From: Bob Eld on 4 May 2010 13:04 "Bret Cahill" <BretCahill(a)peoplepc.com> wrote in message news:cbd8dbe2-93eb-4ed1-ac8c-08b4e000ac69(a)11g2000prw.googlegroups.com... > > String a wire back and forth across / along a fault line to measure > > very small displacements in the earth's surface. If the resistance > > and/or tensile strength needs to be higher than a common single alloy > > wire then structural steel cable could be wrapped around a insulated > > wire with a higher resistivity. It could be temperature compensated > > as usual, with another wire of the same length loosely supported > > nearby in another leg of the bridge. > > > An abandoned power line may be good to go if it is properly located. > > > Good info sometimes comes in small displacements. > > > Bret Cahill > > What you are trying to make is a strain gauge. This scheme wont work because > the gauge has to be attached to the substrate along it's whole length not > just strung up like a power line. But attaching it to the earth in any > meaningful way over distance would be next to impossible. Tie it down every few feet. > Yes, you can > compensate for the temperature coefficient of resistance but how do you > compensate for the change in length due to temperature, coefficient of > expansion, a very different animal. If you know the coefficient of expansion then you can correct for it, either mechanically or later in the calculations. > I can't see a strain gauge being a > solution for seismic motions. Is there any other way to measure very low frequency / very small displacements? > The best way is a laser interferometer for small displacements and GPS or > gross measurements. Only two points need to be attached to the earth with > these schemes. >What's the accuracy? >Bret Cahill What is the accuracy of a laser interferometer? Fractions of a wave length of light: nanometers. How accurate do you want it? If you think you can make a strain gauge by hanging a some wire on poles space a few feet apart, then test your theory and set up such a scheme in your back yard. See if you can detect the vibration of nearby traffic or the earth tides every time the sun goes across the sky. I think you'll find it WONT WORK! But, prove me wrong. If you can show it works then great, more power to ya.
From: Bret Cahill on 5 May 2010 14:17
> > String a wire back and forth across / along a fault line to measure > > very small displacements in the earth's surface. If the resistance > > and/or tensile strength needs to be higher than a common single alloy > > wire then structural steel cable could be wrapped around a insulated > > wire with a higher resistivity. It could be temperature compensated > > as usual, with another wire of the same length loosely supported > > nearby in another leg of the bridge. > > > An abandoned power line may be good to go if it is properly located. > > > Good info sometimes comes in small displacements. > > > Bret Cahill > > What you are trying to make is a strain gauge. This scheme wont work because > the gauge has to be attached to the substrate along it's whole length not > just strung up like a power line. Mounting a conventional strain gage just at the ends would give you the same output assuming you had the dexterity to glue just the ends. dl/l should be const. over the entire area of the strain gage site. > But attaching it to the earth in any > meaningful way over distance would be next to impossible. Yes, you can > compensate for the temperature coefficient of resistance but how do you > compensate for the change in length due to temperature, coefficient of > expansion, a very different animal. Correcting for thermal expansion would require temperature data over the entire length. > I can't see a strain gauge being a > solution for seismic motions. > The best way is a laser interferometer for small displacements and GPS or > gross measurements. Adding a +/- nanometer error to a +/- meter error just gives you a +/- meter error. GPS wouldn't be useful for the actual data. > Only two points need to be attached to the earth with > these schemes. They claim laser interferometry won't work over long distances, i.e., 50 km, but they are probably thinking of measuring an entire unknown distance when all that is really necessary is a _change_ in distance. Maybe use several different meters at different wavelengths so that you don't skip a wave. Bret Cahill |