PSN-L Message - Subject: Re: Re: what is your advice?

I think we all should petition Chris to write a book on seismometer = construction. I have learned more from reading his postings than just = about any other source. Larry Conklin lconklin@............ ----- Original Message -----=20 From: Meredith Lamb=20 To: psn-l=20 Sent: Wednesday, July 21, 2004 1:16 AM Subject: FW: Re: what is your advice? The following msg is being forwarded and was reply written by Chris = Chapman: Hi All,=20 A few more comments on seismometers.... HELPFUL HINTS FROM A FUSSY ELDERLY SEISMIC DATA LOGGER: 1. Be sure that your STATION CO-ORDINATES are correct. You can use = Microsoft Streets and Trips, or online MapQuest for the purpose. Also = align horizontal sensors to true North or East, or else give true = direction in sensor comments box. OK on the alignment and reporting. Full details would often be = most helpful. As far as I can see, the Lat/Long search option has now been = removed from Mapquest.=20 The Microsoft Streets and Trips CD only gives information for = the USA and parts of Canada.=20 Has anyone got a reference for a world wide map programme which = you can use with Lat + Long? 2. LOCK your timing to GPS or WWV or a crystal clock slaved to = WWVB. Fine, but it would be helpful if Larry included a programme = which DECODED the WWVB minute long signals directly and could also = update the clock at user defined intervals. WWVB modules are available. = The receive limit is set by local radio interference / noise, but can be = expected to be >2,000 miles at night. Keep the receiver well away (10 ft = +) from a CRT display. The effective signal coverage is shown at = http://www.boulder.nist.gov/timefreq/stations/wwvbcoverage.htm =20 3. CALIBRATE YOUR SENSOR. You would not have much use for a = voltmeter with no scale on it, would you? For open loop sensors, I have = two methods. One uses the raw sensor output of pendulum movement between = fixed stops, the other measures the force exerted by the pendulum in = response to a known current. I can tell you more if you ask. Remembering that seismometers are used which measure = displacement, or velocity or acceleration, so we need several = calibration methods.=20 Remember also that the signal which you actually see can depend = quite strongly on the local sub surface geological structures. If you = are sitting on a thick layer of water saturated alluvial soil, it can = shake much like a jelly. If your seismometer is calibrated in say volts = / metre / sec, you may see some variation in signal with the reception = angle and there may be large errors in the calculated amplitude / = distance measurement. An alternative method is to measure the recorded = amplitude of a series of 'known' quakes and then make up a rough = correlation table.=20 4. DO NOT submit files with high sample rates on distant = teleseisms. High frequencies are attenuated with distance, and files = with excessive sample rate only take up bandwidth and archival storage = space. Decimate before submitting. One to five samples per second should = handle most teleseisms without loss of waveform detail. Also, do not = cover an overly large time span after the L wave onset. You will have difficulty in separating out the P and S waves = if your system cannot reproduce 0.5 to 1 Hz quite accurately.=20 I would suggest that 10 sps is a more reasonable compromise. = Local quakes also have P & S components of higher frequency. Some locations have relatively high damping down to below about = 1/2 Hz. If you also have high environmental noise, you may have = difficulty in resolving quake arrival times. =20 5. DO NOT use any more FILTERING than absolutely necessary. Let = some microseisms come through. Leave it to the downloader of files do = more filtering if they wish. Sure, but it is helpful if you can at least recognise the = outline of the earthquake signal. I don't find a seismic signal which = completely masked in either high frequency environmental or in = instrument noise, very helpful. 6. If possible, adjust your sensor's NATURAL PERIOD to least 16 = seconds if you record and report teleseisms. If that is not practical, I = have written an application program for WinQuake files which can = digitally extend the effective period of your sensor by up to a factor = of five. I use it routinely on my sensors which have natural periods of = 5, 8, and 14 seconds to extend their response to 24 seconds. This does depend on what sort of seismometer you use and on it's = natural period. Local environmental noise may become serious by 10 Hz. = The Ocean background will give quite large signals somewhere between 2 = and 10 seconds, which need to be filtered out. Twin Tee rejection = filters have been used successfully and can give over 50 dB peak = rejection. The frequency and the amplitude may change with time. Using beam type seismometers, it is desirable to have the period = between 20 and 30 seconds. A 10 second to 30 second range allows good = detection of L & R waves.=20 Simple damped pendulum seismometers with a 1.4 sec period are = popular in Europe and pickup P & S waves quite nicely. 7. Control your sensor DAMPING. The barest amount of overshoot on = a displaced pendulum is about right. I agree, but this is the ONE FACTOR in seismometer construction = which is likely to give MOST PROBLEMS. I found it DIFFICULT to set up an = oil damped system to about 0.8 x critical. I found it NEAR IMPOSSIBLE to = keep it that way without oil temperature control, or frequent = measurement / re-setup. Oil is messy, attracts dust, creeps over all = surfaces, drowns insects (which then give 'bug quakes') and the = viscosity is strongly temperature dependant. The surface tension also = varies, which can give beam drift.=20 For oil systems, you need to measure the damping for very small = displacements - use your SDR recording programme to monitor and display = this. If you pull the mass 1/2" to one side and then release it, you may = observe a significantly higher damping due to swirl in the oil and the = system may still be under-damped for normal seismic signals.=20 I honestly don't know why anyone would try to use oil damping = these days. EM damping using NdFeB magnets is simple, cheap, clean, easy = to set up and to adjust. For a Lehman, the damping required is also strongly dependant on = the period you choose and hence on the suspension adjustment. You need a = fair range of damping adjustment available. There are two ways of providing electromagnetic damping. You can = either use a flat Al or Cu plate in a pole gap with a strong magnetic = field, or you can use a coil of wire half in the field and put a loading = resistor across it. After having set up the period, you choose a load = resistor (by experiment) which gives near critical damping. This was = done with the cylindrical coils and magnets in the Sprengnether = seismometers. However, the sensitivity then depends on the value of the = damping resistor, which complicates matters.=20 I prefer to keep the damping and sensor coil functions separate. = With two N+S pairs of opposing flat magnets 1" x 1/2" x 1/4" on 1/4" = baseplates, you just move the high central field over a 1/16" to 1/8" = thick Al or copper tongue till you get the desired damping. Electro magnetic damping is far easier to adjust than oil = damping and with the ready availability of strong NdFeB magnets, it is = cheap and simple to implement. Suppliers are Sedona2 on Ebay, Amazing = Magnets (occasional E-Bay), "Emovendo" on ebay only (perhaps the = cheapest N48 supplier), K&J magnetics ( wide range) www.kjmagnetics.com = and www.wondermagnet.com 8. Use as little AMPLIFIER GAIN as possible to avoid clipping on = major events. Most of the files I see on the seismicnet site have been = recorded at far more gain than necessary. You may have to make component = value changes in your amplifier to accomplish this. Agreed, but the practical choice is likely to depend on the = local environmental noise and on the Ocean background. You want to be = able to resolve the background signals at times of "low noise", but you = don't want strong quakes to saturate the sensor. Most amateurs do not = have the option of siting their seismometer in quiet rural area. If you use 'period extending' software, you will need more = amplification to cope with the lower amplitude of the longer period = waves. You are likely to get uncertain readings if you are amplifying = noise. 16 bit A/D converters, with three bits of converter noise, are = not too helpful in this respect. Perhaps we could agree on a rough amplitude for the ocean = background signals?=20 The gain you can use also depends on the noise and resolution in = your A/D Converter. A/D Converter boards with +/-1/2 LSB resolved = internal noise are available. >> Subject: feasible maximum mass for a Lehman seismometer At the risk of ruining a good discussion going on the last = couple of days -- I would like to have some input on what the best / = maximum mass for Lehman horizontal seismometer. The best mass is the lowest which gives you clear low noise = signals. This advice 'begs the question'. The 'kt' thermal excitation noise sets the minimum seismic mass = at about an ounce, so you are better with 1/2 lb, minimum.=20 If you use a solid metal beam, as opposed to a tube or a U = channel, the moment of inertia of the beam can actually reduce the = 'radius of gyration k' of the combined beam + seismic mass, giving a = shorter period than you would get from the seismic mass at the end of a = weightless arm. The beam needs to be light but rigid (aim for a tube = weight less end fittings 1/4 the weight of the seismic mass or less). I = have found the light 1/2" nominal welded stainless steel water pipe to = be very satisfactory. The thermal expansion coefficient matches that of = a piano wire suspension quite well. It is also cheap and you can buy = brass compression fittings on which to mount the suspension, the seismic = mass and the damping components. This makes the construction quite easy. This said, the period of a simple pendulum is independent of the = mass.=20 It is advisable to keep the boom length between 70 cm and 100 = cm. This is because you are using the garden gate type of suspension and = shorter lengths require you to set up the side to side level position = with rapidly increasing precision. This can make a 12" beam not only = exceptionally difficult to adjust, but very sensitive to tilt drift, = either from the suspension system or from natural earth movements. =20 Using a 30/60/90 degree triangle suspension is fine. Try to = keep the boom / wire angle above 20 deg, or the suspension loading will = be large. You can buy nickel coated 8 thou steel wire from a music shop, = for stringing mandolins. To clamp wire, I drill a 1/16" hole just under = the head of a bolt. Then I 'dish' a couple of washers by putting them on = a wood block and hitting the centre hole with a large centre punch + = hammer. You put the two washers on the bolt with the outer cup edges = touching and feed the wire between these edges and through the hole in = the bolt. This gives a good 'edge clamp' on the wire. For adjustment screws, I use stainless steel nuts and bolts = bought from a marine / boat-builder supplier. I drill out the threaded = end of the bolt with a centre drill and stick a stainless ball bearing = in the conical hole. I drill a plain hole in the baseplate and stick a = nut onto the lower side with methacrylate or epoxy cement. It is = essential that the bolt and the nut are made of the same material, or = the adjustment will drift with change in temperature. I usually use a = second nut + a spring washer on the lower side of the mounting nut to = provide tensioning / alignment in the thread. I stick stainless steel = mounting plates onto the concrete floor, either with the special = concrete 'pool adhesive' or with epoxy. If you use epoxy, it is a good = idea to dry out the top of the cement thoroughly with a warm air blower. Lastly: >> Hi gang, There has been some recent discussion about calibrating = seismometers. Several years ago, my article on a calibrator using a = meter movement as a force transducer was put on our 'home' = www.seismic.com. Go to "Build Your Own Seismographic Station" then to = "Article by Bob Barns". ?? Can anyone help me find this article, please? www seismic.com = seems like a huge website with lots of advertising, but only a very old = article on seismology. 'Earth Science' seems to be just advertising = vitamins...=20 If you want a force calibration system with much higher forces = than a meter movement, you can use a small NdFeB cube magnet with a = Maxwell coil. These look similar to a Helmholtz coil, but with double = the winding spacing and the windings connected in opposition. This gives = a constant field gradient. You can calibrate it using a pendulum of a = known length and mass, by measuring the deflection / coil current. Regards,

I think we all should petition Chris to write a book on seismometer = construction. I have learned more from reading his postings than = just=20 about any other source.

Larry Conklin

lconklin@............

----- Original Message -----
From:=20 Meredith Lamb
To: psn-l

Sent: Wednesday, July 21, 2004 = 1:16=20 AM

Subject: FW: Re: what is your=20 advice?

The following msg is being forwarded and was reply = written=20 by Chris Chapman:

Hi=20 All,

A few more comments on = seismometers....

HELPFUL HINTS FROM A FUSSY ELDERLY SEISMIC DATA = LOGGER:

1. Be sure that your STATION CO-ORDINATES are correct. You = can use=20 Microsoft Streets and Trips, or online MapQuest for the purpose. = Also=20 align horizontal sensors to true North or East, or else give true=20 direction in sensor comments box.

OK on the alignment and reporting. Full = details=20 would often be most helpful.

As far as I can see, the Lat/Long = search=20 option has now been removed from = Mapquest.

The Microsoft Streets and Trips CD only = gives=20 information for the USA and parts of Canada.

Has anyone got a reference for a world = wide map=20 programme which you can use with Lat + Long?

2. LOCK your timing to GPS or WWV or a crystal clock slaved = to=20 WWVB.

Fine, but it would be helpful if = Larry=20 included a programme which DECODED the WWVB minute long signals = directly and=20 could also update the clock at user defined intervals. WWVB modules = are=20 available. The receive limit is set by local radio interference = /=20 noise, but can be expected to be >2,000 miles at night. Keep = the=20 receiver well away (10 ft +) from a CRT display. = The effective=20 signal coverage is shown at h= ttp://www.boulder.nist.gov/timefreq/stations/wwvbcoverage.htm &n= bsp;

3. CALIBRATE YOUR SENSOR. You would not have much use for a = voltmeter=20 with no scale on it, would you? For open loop sensors, I have two = methods.=20 One uses the raw sensor output of pendulum movement between fixed = stops,=20 the other measures the force exerted by the pendulum in response = to a=20 known current. I can tell you more if you = ask.

Remembering that seismometers are used = which=20 measure displacement, or velocity or acceleration, so we need = several=20 calibration methods.=20

Remember also that the signal which you = actually see can depend quite strongly on the local sub surface = geological structures. If you are sitting on a thick layer of water=20 saturated alluvial soil, it can shake much like a jelly. If your = seismometer=20 is calibrated in say volts / metre / sec, you may see some variation = in=20 signal with the reception angle and there may be large errors in the = calculated amplitude / distance measurement. An alternative method = is to=20 measure the recorded amplitude of a series of 'known' quakes and = then make=20 up a rough correlation table.

4. DO NOT submit files with high sample rates on distant = teleseisms.=20 High frequencies are attenuated with distance, and files with = excessive=20 sample rate only take up bandwidth and archival storage space. = Decimate=20 before submitting. One to five samples per second should handle = most=20 teleseisms without loss of waveform detail. Also, do not cover an = overly=20 large time span after the L wave onset.

You will have difficulty in = separating out the P and S waves if your system cannot reproduce 0.5 = to 1 Hz=20 quite accurately.=20

I would suggest that 10 sps is a = more=20 reasonable compromise. Local quakes also have P & = S components=20 of higher frequency.

Some locations have relatively=20 high damping down to below about 1/2 Hz. If you also have high=20 environmental noise, you may have difficulty in resolving quake = arrival=20 times.

5. DO NOT use any more FILTERING than absolutely necessary. = Let some=20 microseisms come through. Leave it to the downloader of files do = more=20 filtering if they wish.

Sure, but it is helpful if you can at = least=20 recognise the outline of the earthquake signal. I don't find a = seismic=20 signal which completely masked in either high frequency = environmental or in=20 instrument noise, very helpful.

6. If possible, adjust your sensor's NATURAL PERIOD to least = 16=20 seconds if you record and report teleseisms. If that is not = practical, I=20 have written an application program for WinQuake files which can = digitally=20 extend the effective period of your sensor by up to a factor of = five. I=20 use it routinely on my sensors which have natural periods of 5, 8, = and 14=20 seconds to extend their response to 24 = seconds.

This does depend on what sort of seismometer = you use=20 and on it's natural period. Local environmental noise may become = serious by=20 10 Hz. The Ocean background will give quite large signals somewhere=20 between 2 and 10 seconds, which need to be filtered out. Twin = Tee=20 rejection filters have been used successfully and can give over 50 = dB peak=20 rejection. The frequency and the amplitude may change with = time.

Using beam type seismometers, it is = desirable=20 to have the period between 20 and 30 seconds. A 10 second to 30 = second range=20 allows good detection of L & R waves.

Simple damped pendulum seismometers = with a 1.4=20 sec period are popular in Europe and pickup P & S waves quite=20 nicely.

7. Control your sensor DAMPING. The barest amount of = overshoot on a=20 displaced pendulum is about right.

I agree, but this is the ONE FACTOR in=20 seismometer construction which is likely to give MOST PROBLEMS. = I found=20 it DIFFICULT to set up an oil damped system to about 0.8 x = critical. I found it NEAR IMPOSSIBLE to keep it that way = without oil=20 temperature control, or frequent measurement / re-setup. Oil is = messy,=20 attracts dust, creeps over all surfaces, drowns insects (which then = give=20 'bug quakes') and the viscosity is strongly temperature = dependant.=20 The surface tension also varies, which can give beam drift.

For oil systems, you need to measure = the=20 damping for very small displacements - use your SDR recording = programme to=20 monitor and display this. If you pull the mass 1/2" to one side and = then=20 release it, you may observe a significantly higher damping due to = swirl in=20 the oil and the system may still be under-damped for normal seismic = signals.=20

I honestly = don't=20 know why anyone would try to use oil damping these days. EM damping = using=20 NdFeB magnets is simple, cheap, clean, easy to set up and to=20 adjust.

For a Lehman, the damping required is = also=20 strongly dependant on the period you choose and = hence on=20 the suspension adjustment. You need a fair range of damping = adjustment=20 available.

There are two ways of providing = electromagnetic=20 damping. You can either use a flat Al or Cu plate in a pole gap = with a=20 strong magnetic field, or you can use a coil of wire half in = the field=20 and put a loading resistor across it. After having set up the = period,=20 you choose a load resistor (by experiment) which gives near = critical=20 damping. This was done with the cylindrical coils and magnets in the = Sprengnether seismometers. However, the sensitivity then = depends=20 on the value of the damping resistor, which complicates matters. =

I prefer to keep the damping and sensor = coil=20 functions separate. With two N+S pairs of opposing flat magnets = 1" x=20 1/2" x 1/4" on 1/4" baseplates, you just move the high = central field=20 over a 1/16" to 1/8" thick Al or copper tongue till you get the = desired=20 damping.

Electro magnetic damping is far easier = to=20 adjust than oil damping and with the ready availability of = strong NdFeB=20 magnets, it is cheap and simple to implement. Suppliers are Sedona2 = on Ebay,=20 Amazing Magnets (occasional E-Bay), = "Emovendo" on=20 ebay only (perhaps the cheapest = N48=20 supplier), K&J magnetics ( wide range) www.kjmagnetics.com = and www.wondermagnet.com

8. Use as little AMPLIFIER GAIN as possible to avoid = clipping on=20 major events. Most of the files I see on the seismicnet site have = been=20 recorded at far more gain than necessary. You may have to make = component=20 value changes in your amplifier to accomplish this.

Agreed, but the practical = choice is=20 likely to depend on the local environmental noise and on the Ocean=20 background. You want to be able to resolve the background signals at = times=20 of "low noise", but you don't want strong quakes to saturate the=20 sensor. Most amateurs do not have the option of siting their=20 seismometer in quiet rural area.

If you use 'period extending' software, = you=20 will need more amplification to cope with the lower amplitude of the = longer=20 period waves. You are likely to get uncertain readings if you are = amplifying=20 noise. 16 bit A/D converters, with three bits of converter noise, = are not=20 too helpful in this respect.

Perhaps we could agree on a rough = amplitude for=20 the ocean background signals?

The gain you can use also depends on = the noise=20 and resolution in your A/D Converter. A/D Converter boards with = +/-1/2 LSB=20 resolved internal noise are available.

>> Subject: feasible = maximum=20 mass for a Lehman seismometer
At the risk of ruining a good = discussion=20 going on the last couple of days -- I would like to have some input = on what=20 the best / maximum mass for Lehman horizontal = seismometer.

The best mass=20 is the lowest which gives you clear low noise signals. This = advice=20 'begs the question'.

The 'kt' = thermal=20 excitation noise sets the minimum seismic mass at about an ounce, so = you are=20 better with 1/2 lb, minimum.

If you use = a solid=20 metal beam, as opposed to a tube or a U channel, the moment of = inertia of=20 the beam can actually reduce the 'radius of gyration k' of the = combined=20 beam + seismic mass, giving a shorter period than you would get = from the=20 seismic mass at the end of a weightless arm. The beam = needs to be=20 light but rigid (aim for a tube weight less end fittings 1/4 the = weight of=20 the seismic mass or less). I have found the light 1/2" nominal = welded=20 stainless steel water pipe to be very satisfactory. The thermal = expansion=20 coefficient matches that of a piano wire suspension quite = well. It is=20 also cheap and you can buy brass compression fittings on which to = mount the=20 suspension, the seismic mass and the damping components. This makes = the=20 construction quite easy.

This said, the period of a = simple=20 pendulum is independent of the mass.

It is advisable to keep the boom length = between=20 70 cm and 100 cm. This is because you are using the garden gate type = of=20 suspension and shorter lengths require you to set up the side to = side level=20 position with rapidly increasing precision. This can make a 12" beam = not=20 only exceptionally difficult to adjust, but very sensitive to = tilt=20 drift, either from the suspension system or from natural earth = movements.=20

Using a 30/60/90 degree triangle=20 suspension is fine. Try to keep the boom / wire angle above 20 deg, = or the=20 suspension loading will be large. You can buy nickel coated 8 thou = steel=20 wire from a music shop, for stringing mandolins. To clamp wire, I = drill a=20 1/16" hole just under the head of a bolt. Then I 'dish' a couple of = washers=20 by putting them on a wood block and hitting the centre hole with a = large=20 centre punch + hammer. You put the two washers on the bolt with the = outer=20 cup edges touching and feed the wire between these edges and through = the=20 hole in the bolt. This gives a good 'edge clamp' on the = wire.

For adjustment screws, I use stainless = steel=20 nuts and bolts bought from a marine / boat-builder supplier. I drill = out the=20 threaded end of the bolt with a centre drill and stick a = stainless ball=20 bearing in the conical hole. I drill a plain hole in the baseplate = and stick=20 a nut onto the lower side with methacrylate or epoxy = cement. It is=20 essential that the bolt and the nut are made of the same material, = or the=20 adjustment will drift with change in temperature. I usually use a = second nut=20 + a spring washer on the lower side of the mounting nut to provide=20 tensioning / alignment in the thread. I stick stainless steel = mounting=20 plates onto the concrete floor, either with the special = concrete 'pool=20 adhesive' or with epoxy. If you use epoxy, it is a good idea to dry = out the=20 top of the cement thoroughly with a warm air blower.

Lastly:

>> Hi gang,

There has been some recent discussion about calibrating=20 seismometers. Several years ago, my article on a=20 calibrator using a meter movement as a force transducer was put on = our=20 'home' www.seismic.com. Go to "Build Your Own=20 Seismographic Station" then to "Article by Bob Barns".

?? Can anyone help me find this = article,=20 please? www seismic.com seems like a huge website with lots of = advertising,=20 but only a very old article on seismology. 'Earth Science' = seems to be=20 just advertising vitamins...

If you want a force calibration system = with=20 much higher forces than a meter movement, you can use a small NdFeB = cube=20 magnet with a Maxwell coil. These look similar to a Helmholtz = coil, but=20 with double the winding spacing and the windings connected in=20 opposition. This gives a constant field gradient. You can calibrate = it using=20 a pendulum of a known length and mass, by measuring the = deflection /=20 coil current.

Regards,

Chris=20 = Chapman

<= /HTML>

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