Illustrated History of the Seismometer Project, 1986 - 1991:
Strip Chart to Apple IIe
My interest in constructing seismometers began in 1986 when I attended a workshop at Kean College in Union, NJ, which was taught by Richard (Dick) Kroll. Dick had made a seismometer similar to one previously built by James Lehman of James Madison University. Jim Lehman published the design in The Science Teacher in 1977. Lehman's design was subsequently published in the "Amateur Scientist" portion of Scientific American by Jearl Walker in 1979, and Dick Kroll invoked some design changes that made construction easier and published the modifications in the Journal of Geological Education in 1987 . The workshop run by Dick prior to the appearance of his article emphasized the relative ease and cost effectiveness of constructing such a device. Encouraged as I was from what I heard and saw at Dick Kroll's workshop, I returned to Moravian College and constructed a Lehman seismometer of my own. Joe Powlette, a physicist at Moravian College, taught me techniques for building electronic circuits and helped me obtain materials that I could assemble into completed amplifiers. He also helped me trouble-shoot electronic problems when they occurred.
The Lehman seismometer consists of a weighted horizontal rod that is allowed to pivot from near the bottom of a frame at one end and, at the other end, is suspended by a metal guitar string which is attached to the top of the frame. A strong horseshoe-shaped magnet is mounted on the rod near its free end and is permitted to move relative to a coil of wire that contains approximately 12,000 to 14,000 turns on a Plexiglas core. Motion of the magnet relative to the coil generates a small voltage which is amplified by the relatively simple electronic circuit. Motion of the horizontal rod is damped by attaching to the free end of the rod a blade that is partially immersed in motor oil. The period of oscillation of the free pendulum arm can be adjusted to 18 seconds by tilting the frame forward or backward. In the original configuration, the amplified output was directed to a strip chart recorder that had been reclaimed from an older piece of equipment. A case made primarily of Plexiglas sheets keeps air currents from disturbing the equipment. Although it seemed to be too simple to actually work, I recorded my first earthquake on a strip chart with this setup on February 28, 1987. It was a magnitude 6.7 earthquake from the Aleutian Islands.
Students reacted very favorably to the seismometer, which was placed on a table in the rear of the classroom, and the strip chart recorder, which was placed along the side of the room closer to the front so it could be easily seen during class. The movements of the instructor and the students within the room disturbed the seismometer and caused the strip chart recorder pen to deflect. The recorder was run at rate of 5 minutes per inch of strip chart paper, which is one foot per hour. Every morning I had to unroll and examine 24 feet of paper, and on Monday mornings I had to examine 72 feet of paper. Because paper rolls were very expensive, I would reverse each roll to get multiple traces (4, 5, or maybe 6) on one roll, offsetting the pen each time so the traces did not overlap. But I discovered I would need to come in on weekends to reverse the rolls and tend other mechanical problems with the paper or the pen. Computer monitoring, rather than mechanical monitoring, seemed to be the answer.
Gary Barker had published a BASIC program which ran on the Apple IIe to monitor Lehman seismometers in a 1987 issue of The Journal of Geological Education. His article and Dick Kroll's article were published back-to-back in this same issue. I contacted Gary and he graciously supplied me with a copy of the code. Barker's program monitors the input and saves it to a floppy disk (Apple IIe's do not possess hard drives). If the input varied by less than a user-defined amount, the file on the floppy disk would be overwritten, but if it varied by more than the set amount, the software treated it as an earthquake and the file would not be overwritten. All subsequent files would likewise be saved on floppy disk until the disturbance being monitored would cease to exceed the limits which the user set. This procedure created a lot of wear on the disk drives and disk drive failure became an issue.
I obtained an Apple IIe computer and an A/D - D/A card by my participation in an NSF grant at the time (NSF award number CSI-8650225), so I was equipped to switch to a computerized system, but I did not wish to remove the seismometer from the classroom and the easy vision of the students because it was always interesting to them. Thus, I decided to make a second seismometer and install it in the basement beneath the classroom and to modify Gary Barker's BASIC program to manage and display both inputs. In this way, the data from both seismometers is continuously displayed, but my reprogramming of Becker's software involved a "trigger" to save the data based only on the signal from the seismometer in the basement. I moved the television/monitor to a high location in a front corner of the classroom to make it more conspicuous, and I moved the computer to an equipment storage area adjacent to the room. All of this worked successfully, and the results were reported at the 65th annual meeting of the Pennsylvania Academy of Sciences in 1989. Click here to see the abstract.
In 1988 Ken Kodama, a faculty member of the Geology Department of Lehigh University, offered to me, on an indefinite loan basis, the use of a long-period vertical seismometer of unknown origin, manufacture, and history. The unit was broken, had no amplifier and was without supporting materials. I repaired it and used a Fluke meter as an amplifier, but when I attempted to further modify Gary Barker's BASIC code to manage three rather than two inputs, I could not make it successfully run in the limited 128 K environment of an Apple IIe. It was at this point that Ron Jackson, a Computer Science major at the time, undertook an independent study project of rewriting the code to support more than two inputs. Ron wrote the new program in 6502 assembly language, avoiding BASIC almost entirely, and called his new program "Seismological data Acquisition Software," or SAS for short. The SAS code ran so efficiently on the Apple IIe that we could manage not only three, but four simultaneous seismometer inputs. The SAS code has many additional user-selectable options available both for managing the input and displaying the output. In addition, it buffered internally all data, which allowed the floppy drive to be used only when an actual earthquake was detected. Mechanical drive wear was no longer a persistent problem. Ron and I wrote a 64-page SAS User's Manual for the operation of the program. Running with these three seismometer inputs, we recorded a 7.7 magnitude earthquake from Iran in 1990 and used this seismogram to illustrate our jointly-authored article, "Classroom Utilization of a Multi-Axis Lehman Seismograph System," which was published in the September 1991 issue of the Journal of Geological Education. I gave a paper on this system at the joint Northeastern / Southeastern Sectional Meeting of the Geological Society of America in March, 1991. Click to read the abstract of that presentation.