I'm working on a project with Denny Ambrisco and Thom DiGeronimo to build a Lehman-style horizontal seismometer. So far we have the mass suspended and swinging, so the magnet and coil are then next step. I am using a wire for the hinge point between the boom and the post. Here's how it works:
The Lehman consists of a vertical post that is connected to a horizontal boom via diagonal wires from the top of the post to the far end of the boom, where the mass, coil, and magnet are located. The near end of the boom is pushing toward the bottom of the post and there are two ways to create a lower hinge:
1) Form a sharp point on the near end of the boom that rests in a dimple on the front of the post.
2) The near end of the boom can have a solid ring attached to it. The ring goes around the post and a short piece of wire is attached from the back of the post to the inside of the ring. This wire is in tension and acts like a frictionless hinge. The wire should be thin so that it does not prevent the boom from swinging freely.
This is the approach that I've taken, as it eliminates problems associated with friction at the hinge between the boom and the post.
Drawing of vertical view of seismometer
Photo of boom, mass, coil, and magnet
Drawing of horizontal view of seismometer
Closeup of lower hinge
Images of some of the earthquakes recorded.
3/20/1999: Since there was so much PSN Email on using a speaker magnet and coil, I decided today to take apart a 12"-diameter speaker that I got last summer at a garage sale. I got it down to a coil and the magnet assembly, but the coil had so little clearance that I decided to remove the center pole from the magnet and replace it with one of slightly smaller diameter.
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB nnnnnnnnn CSCSCS nnnnnnnnn nnnnnnnnn CSCSCS nnnnnnnnn sssssssss CSCSCS sssssssss sssssssss CSCSCS sssssssss FFFFFFFFFFFF CSCSCS FFFFFFFFFFFF
Speaker magnet assembly. B = back plate, nnss = ceramic magnet, CS = center slug, F = front plate.
Easier said than done. I drilled it out and then hammered on it, but no luck. I even tried a 20 lb sledge hammer. The back plate (B) and center slug (CS) were ONE CAST UNIT, so of course all of my efforts to break the center slug loose were for naught. I also boiled the whole magnet assembly, but the glue did not loosen between the back plate(B) or the front plate (F) and the ceramic doughnut magnet (nnss). Finally I got my lathe motor mounted (the lathe has not been run since I moved down from Alaska in November of 1997) and cut through the back plate (B) to remove the center slug. It turns out that the center pole was not glued or riveted to the back plate, but was in fact an integral part of if!!!
3/21-22/1999: Denny and I widened the magnetic gap. This was not too difficult once Denny adjusted to lathe to eliminate excess play.
The next challenge was getting the center slug (CS) mounted and centered again. The magnet is so strong that I had to use a wood clamp and wedges to force the center slug away from the edge of the front plate (F).
The coil is mounted on the flat side of the rectangular stainless steel piece that is being used for a mass. A current flaw is that the coil should have been rotated in a horizontal plane so that its center axis was perpendicular to a line extending from the coil to the vertical post.
MMMMM Top View
MMMMM M = mass B = boom C = center post
| | | = coil
With help from a number of PSN Emails and web sites, I completed a prototype amplifier board yesterday. The coil has been remounted to the boom so that movement of the coil is parallel to the coil's axis. The whole system has been moved from a wooden table to a brick fireplace hearth. I've connected the amplifier output to a DaTaq AD module that sends data through the serial port to the DaTaq data acquisition software. The software is set up to close a file after it reaches a set size and open another one with an incremented name. Right now I'm opening a new file every hour.
I've completed a beta version of a program that converts from the DaTaq format to the PSN format used by WinQuake. This program, wdq2psn.exe, is available if anyone wants to try it. It is included in the pgms.zip file (see below). The program reads a file named staparam.dat that includes, among other things, your stations coordinates. The version of staparam.dat in pgms.zip will need to be modified for your seismic station. The easiest way to run the program is to use the batch file, runwdq2.bat from pgms.zip. For example, to run the program at the DOS prompt to convert a DaTaq event file named dataq01.wdq to dataq01.psn, the command would be:
|April 4, 1999||Oaxaca, Mexico||24.7||5.1|
|April 6, 1999||Wyoming||2.7||4.3|
|May 7, 1999||Kodiak Is. Region, Alaska||35.2||6.1|
|June 15, 1999||Mexico||22.0||6.7|
I must say that I'm a bit frustrated at not detecting very many earthquakes. It's probably due to the local noise that is obscuring all but the largest signals. I have made some progress, however. For an amplifier/filter I'm now using one that Sean-Thomas Morrissey generously sent me to try out. It uses four LM4250CM cmos op amps (although one was bad and has been temporarily replaced with a 741 op amp. Two op amps are for gain, adjustable by switches, and the other two low-pass filters with 2 Hz corner frequencies.
To aid in earthquake identification, I have been generating one-page daily seismograms. These have reasonable resolution when printed out on my HP DeskJet 722C printer. This reduced-size .jpg image will give you an idea of how they look.
My recording is being done with the external DI-150RS serial-port AD unit and WinDaq data acquisition program sold by Dataq Instruments (which is no longer sold by Dataq). The acquisition program is set up to record at 5 samples per second into a series of one-hour long files. The file names are numerically incremented, so that if the first file specified when recording is begun is 03000001.wdq, then the next one will be 03000002.wdq, etc. The acquisition PC is running Windows95, so I can copy the files to a floppy disk each day without interrupting acquisition. Timing is still a big problem. The start of each file is time stamped with the PC time, and on the particular PC I'm using the clock drifts in a non-uniform way by as much as 1 minute per day! More on this later.
To process the data I'm using a second Windows98 computer. The files for each day are put into their "own" directory. For example, the hourly files for April 30 are in the directory \psn\data\990430. Prior to making a daily seismogram, the data are converted to one day-long SUDS format file using the program wdq2asc which I wrote and asc2sud program that Robert Banfill wrote for the International Association of Seismology and Physics of the Earth's Interior (IASPEI). Then Banfill's IASPEI programs sudsfilt and sudsdrum are then used to create the postscript formatted file. Finally the program GSView is used to display and print the seismogram.
If you want to try out this software, the programs I'm using are available. A readme.txt file describes the use of a few batch files that implement the routine operations. These batch files assume the following:
|The directories \psn, \psn\pgms, and \psn\data exist|
|The data files are in directories such as \psn\data\990430|
|The file pgms.zip has been downloaded and unzipped in the directory \psn\pgms..|
|The DOS path statement includes c:\psn\pgms (or d:\psn\pgms, depending on the disk in use)|
|The code for each of the FORTRAN programs that I wrote is in a separate zip file.|
Slowly I'm getting the bugs worked out of my system. For quite a while the boom was pegged to one side and the "background" noise that I thought I was seeing was due to electronic noise! I've moved my sample rate to 6 Hz and moved the low pass filter corner from about 2 to about 3 Hz. The amplifier gain is set at 16,384. A magnitude 6.1 at a distance of 35.2 degrees should generate surface waves with a peak velocity of about 1.06 micrometers/s at 20 s period, or 0.74 micrometers at 14 s period. The peak on my May 7th Kodiak record was 58 counts, my AD has 10/2048 V/count, so the peak was 0.28 V for a signal of 0.74 micrometers/s. This gives 0.38 V/micrometer/s, or 380,000 V/m/s. Dividing by the amplifier gain, this implies 23 V/m/s from the coil-magnet system.
PSN/CoPEPP Meeting - May 21, 1999
Plans are shaping up for the meeting of the PSN, with, I hope, some participants of the Colorado Princeton Earth Physics Project (CoPEPP). The meeting will begin with a tour of the USGS facilities, including the NEIC earthquake operations area, conducted by Waverly Person, and continuing with John McMillan showing us the electronics area and some interesting seismic instruments. We will then spend some time getting to know each other in the entry level meeting room. I'll set up a few of my hands-on seismic-education demonstrations there. When hunger strikes, we'll send out for some pizza. At around 7 pm or so we'll make the short drive over to my house to see my fledgling seismic system and have some coffee and dessert. This map shows where the USGS (1711 Illinois St.) and my house (1925 Foothills Rd.) are located. Anyone with an interest in seismology and seismology education is welcome to attend.
Today I found a bug in my programs to convert from DaTaq DI-150 data format to ascii and to PSN formats. Ken Spikowski at DaTaq has been very helpful and he let me know that when sampling at a low rate, multiple samples are averaged for each stored point. In this situation, the data is stored in a 14-bit integer instead of a 12-bit integer.
I've been out of town lately, but am lucky that my system is still recording and has not tilted off scale. The Mexico earthquake that occurred today was well recorded.
My seismic station has been out of commission for about a year, as my back "went out" and I had to have surgery last summer. I've recently set up an AS1 seismic system that I've borrowed from IRIS. Here are the details.
I've tried two different configurations of horizontal sensors.
I've recorded a large event with my "Romberg-style" horizontal pendulum. Details here.
The simplest system yet, based on the magnets and coil from a hard disk drive.
As progress is made (or not), I'll add to this page. See also the pages on the use of levitated graphite as a horizontal sensor.
Pictures of a wooden sensor.