Experiments in Laser Seismology

At first you hear it. It sounds like the low rumbling of a woofer speaker, yet the sounds are coming from nowhere and everywhere. Then the windows join the strange session of music-making and begin to rattle, followed by the eerie creaks of the wooden beams in the house.

Then you feel it, a swaying and pumping motion like a carnival ride gone haywire. Within a few seconds, you realize it’s not a large truck passing outside or the heavy thump of a jet breaking the sound barrier, but an earthquake.

Earthquakes are among the most frightening natural phenomenon, feared most because of their stealthy suddenness. Even though geologists and seismologists have been measuring earthquakes for decades with sophisticated instruments on land, in the air, and even in space, predicting tremors is an inexact science. The best seismologists can do is warn that a “big earthquake is due soon” — expect it anytime between now and the next century.

Fortunately, massive earthquakes on land are rare. Many of the largest earthquakes occur out at sea, and while they can cause enormous tidal waves (such as the Japanese tsunami), earthquakes at sea seldom topple buildings or swallow up people. Earthquakes on the West Coast in Southern California are almost a dime a dozen, and most of them so faint that they cannot be felt.

Earthquakes can happen anywhere, and even tremors that occur hundreds of miles away can be detected with the proper instruments. Most seismographs use complex and massive electromagnetic sensors to detect earthquakes, both near and far, but you can readily build your own compact seismograph using a laser and a coil of fiberoptics.

When constructed properly, a laser/fiberoptic seismograph can be just as sensitive as an electromagnetic seismograph costing several thousand dollars.

This section covers construction details of a laser/fiberoptic seismograph as well as useful information on how to attach the seismograph to a personal computer.

THE RICHTER SCALE

A number of scales are used to quantify the magnitude of earthquakes. The best known and most used is the Richter scale, named after Charles F. Richter, a pioneer in seismology research. The Richter scale is a logarithmic measuring system ranging from 1 to 10 (and theoretically from 10 to 100), where each increase of 1 represents a ten-fold increase in earthquake magnitude. However, the actual energy released by the earth during the quake can be anywhere between 30 and 60 times for each increase of one digit. Each numeral is further broken down into units of 10, so earthquakes are often cited as 4.2 or 5.6 on the Richter scale.

To give you an idea of how the Richter scale works (and why it can cause confusion), consider the difference between a 3.0 and 4.0 earthquake. Both are difficult to detect without instruments (although some people say they can feel the swaying motion of a 4.0 earthquake). But because the magnitude is increased logarithmically by a factor of 10 per numeral and the actual energy released could be 60 times as great, the difference between a 3.0 and 5.0 earthquake is quite large. That is, the 5.0 earthquake is 10 X 10 (or 100) times more powerful than the 3.0 earthquake.

Similarly, a 6.0 earthquake can cause extensive structural damage and close down buildings for repair; and an earthquake measuring 7.0, if it continues for any length of time, can result in massive destruction of buildings, bridges, and roads. An earthquake measuring 9.0 or 10.0 would level any town.

Most earthquakes occur at a fault, which is a crack or fissure in the earth’s crust. The majority of earthquakes occur at the boundaries of crustal plates. These plates slip and slide over the earth’s inner surface. Sudden motion in these plates is released as an earthquake. Major faults, such as the San Andreas in California, create many thousand “mini-faults” or fractures that spread out like cracks in dried mud. These fractures are also responsible for earthquakes. In fact, two large earthquakes in southern California, occurring in 1971 and 1987, were caused by relatively small faults lying outside the San Andreas line.

Though faults are long—some measure thousands of miles—the earthquake occurs at a specific location along it. This location is the epicenter. The magnitude of earthquakes is measured at the epicenter (or more accurately, at a standard seismographic station distance of 100 km or 62 miles) where the amount of released energy is the greatest. The shock waves from the earthquake fan out and lose their energy the further they go. Obviously, there can’t be a seismograph every 50 or even 100 miles along a fault to measure the exact amplitude of an earthquake. When the epicenter is some distance from a seismograph, its magnitude is inferred, based on its strength at several nearby seismograph stations, past earthquake readings, and the geological makeup of the land in-between.

This accounts for the uncertainty of the exact magnitude of an earthquake immediately after it has occurred and why different seismologists can arrive at different readings. It takes some careful calculations to determine an accurate Richter scale reading for an earthquake, and the precise measurement is sometimes debated for months or even years after the tremor.

HOW ELECTROMAGNETIC SEISMOGRAPHS WORK

The most common seismograph in use today is the electromagnetic variety that uses a sensing element not unlike a dynamic microphone. Basically, the case of the seismograph is a large and heavy magnet. Inside the case is a core, consisting of a spool of fine wire. During an earthquake, the spool bobs up and down, inducing an electromagnetic signal through the wires. A similar effect occurs in a dynamic microphone. Sound vibrates a membrane, which causes a small voice coil (spool of wire) to vibrate. The voice coil is surrounded by a magnet, so the vibrational motion induces a constantly changing alternating current in the wire. As the sound varies, so does the polarity and strength of the alternating current.

To prevent accidental readings of surface vibration, the seismograph is buried several feet into the ground and is sometimes attached to the bedrock. In other cases, it's encased in concrete or secured to a cement piling sunk deep into the ground. Wires lead from the seismograph to a reading station, that might be directly above or several miles away. Telephone lines, radio links, or some other means connect the distant seismographs to a central office location.

The signal from the seismograph is amplified and applied to a galvanometer on a chart recorder (the galvanometer is similar to the movement on a volt-ohmmeter). The galvanometer responds to electrical changes induced by the moving core of the seismograph. The bigger the movement, the larger the response. Attached to the galvanometer is a long needle that applies ink to a piece of paper wound around a slowly rotating drum.

An advance over the chart recorder is the computer interface. The pulses from the seismograph are sent to an analog-to-digital converter (ADC), which connects directly to a computer. The ADC transforms the analog signals generated by the seismograph into digital data for use by the computer. Software running on the computer records each tremor and can perform mathematical analysis.

Laser/Fiberoptic Seismograph Basics

The laser/fiberoptic seismograph (hereinafter referred to as the laser/optic seismograph) doesn’t use the electromagnetic principle to detect movement in the earth. Though there are several ways you can implement a laser/optic seismograph, we’ll concentrate on just one that offers a great deal of flexibility and sensitivity. The system detects the change in coherency through a length of fiberoptics.

As discussed in Section 15, “Lasers and Fiberoptics,” when a laser beam is transmitted through a stepped-index optical fiber, some of the waves arrive at the other end before others. This reduces the coherency of the beam in proportion to the design of the fiber, its length, and the amount of curvature or bending of the fiber. Given enough of the right optical fiber, a laser beam could emerge at the opposite end that's totally incoherent.

It isn't our intent to completely remove the coherency of a laser beam, but just to alter it slightly through a length of 10 or 20 feet of fiber. Movement or vibration of the fiber causes a displacement of the coherency, and that displacement can be detected with a phototransistor. You can even hear this change in coherency by connecting the phototransistor to an audio amplifier. The “hiss” of the light coming through the fiber changes pitch and makes odd thuds, pings, and thrums as the fiber vibrates. The sound settles as the fiber stops moving or vibrating. In a way, the optical fiber makes a unique form of interferometer that settles quickly after the external vibrations have been removed.

Reducing Local Vibrations

The laser/optic seismograph is susceptible to the effects of local vibrations, movement caused by people walking or playing nearby, passing cars, trucks, and trains, even the vibration triggered by the sound of a jet passing overhead.

To be most effective, the seismograph should be placed in an area where it won’t be affected by local vibrations. Those living on a ranch or the outskirts of town will have better luck at finding such a location than city dwellers or those conducting earthquake experiments in a school or other populated area.

Even if you can’t move away from people and things that cause vibration, you can reduce its effects by firmly planting the seismograph in solid ground. Avoid placing it indoors, especially on a wooden floor. Most buildings are flexible, and not only do they readily transmit vibrations from one location to the next, they act as a spring and /or cushion to the movement of an earthquake, improperly influencing the readings.

The cement flooring or foundation of the building is only marginally better. Small vibrations easily travel through cement, so if you attach your seismograph to the floor in your room, you are likely to pick up the movement of people walking around in the living room and kitchen.

The best spot for a seismograph is attached to a big rock out in the back yard, away from the house. Lacking a rock, you can fasten the seismograph to a cement piling, and then bury some or all of the piling into the ground. You can also spread out four to eight cement blocks (about 75 cents each at a builder’s supply store), and partially bury them in the ground. Fill the center of the blocks with sand and mount the seismograph on top. Other possible spots include (test first):

* The base of a telephone pole.

* A heavy fence post.

* A brick retaining wall or fence.

* The cement slab of a separate garage, work shop, or tool shed.

CONSTRUCTING THE SEISMOGRAPH

Cut a piece of optical fiber (jacketed or unjacketed) to 15 feet. Polish the ends as described in the last section. Using a small bit (to match the diameter of the fiber), drill a hole in the top of a phototransistor. Be sure to drill directly over the chip inside the detector, but don't pierce through to the chip. Epoxy the fiber in place. Alternatively, you can terminate the output end of the fiber using a low-cost FLCS-type connector. You can also use a modified FLSC connector for the emitter end of the fiber (as detailed in Section 15) or one of the other mounting techniques described.

Put the fiber aside and construct the base following the diagram in ill. 16-1. A parts list is included in TABLE 16-1. You can use metal, plastic, or wood for the base, but it should be as dimensionally sturdy as possible. The prototype used s/is-inch-thick acrylic plastic. Cut the base to size and drill the post and mounting holes as shown. Insert four ¼-inch-20-by-3-inch bolts in the inside four holes. Starting at one post, thread the fiber around the bolts in a counterclockwise direction (see ill. 16-2). Leave 1 to 2 feet on either end to secure the laser and photodiode. If the fiber slips off the bolts, you can secure it using dabs of epoxy.

Mount the photodetector and laser diode (on a heatsink, as shown in ill. 16-3), in the center of the platform. Alternatively, you can use a He-Ne laser as the coherent light source. Mount the laser tube securely on a separate platform and position the end of the optical fiber so that it catches the beam.

You can use the universal laser light detector presented in Section 13 to receive and amplify the laser light intercepted by the phototransistor. You can use either a pulsed or cw drive power supply for the laser diode. Schematics for these drives appear in

ill. 16-1. Drilling and cutting guide for the laser/optic seismograph base.

Table 16-1. Fiberoptic Seismograph Parts List

• 6-inch-square acrylic plastic base (Y16-inch thick)
• 3-inch by ¼-inch 20 carriage bolt, nuts, washers
• Laser diode on heatsink
• Sensor board (see ill. 16-4)
• Misc. 15 feet (approx.) jacketed or unjacketed fiberoptics, connector for receiver photodiode, connector or attachment for laser to fiber
• Base
• Optical fiber wound around bolts

Section 11, “Laser Power Supplies.” In either case, be sure that the laser diode does not receive too much current. In pulsed mode, the laser doesn’t operate at peak efficiency, but it's not required in this application.

Note that when using a He-Ne tube, you don’t need excessive power—a 0.5 or 1 mW He-Ne is more than enough. This simplifies the power supply requirements and allows you to operate the seismograph for a day or two on each charge on a pair of lead- acid or gelled electrolyte batteries.

Test the seismograph by turning on the laser and connecting the output of the amplifier to a speaker or pair of headphones. A small vibration of the base will cause a noticeable thrum or hiss in the audio output. You might need to adjust the control knob on the amplifier to turn the sound up or down.

The mounting holes allow you to attach the seismograph to almost any stable base. Whatever base you use, it should be directly connected to the earth. Use cement or masonry screws to attach the seismograph to concrete or a concrete pylon. When you get tired of listening to earthquake vibrations, connect the output of the amplifier to a volt-ohmmeter. Remove the ac coupling capacitor on the output of the amplifier.

ill. 16-2. How to wind the optical fiber around the bolts. (bottom) ill. 16-3. Arrangement of fiber, laser (on heatsink) and sensor/amplifier board on the seismograph base.

Table 16-2. Commodore 64 Seismograph ADC Parts List

• Basic Setup:
• IC1 TLC548 serial ADC
• R1 10 k-ohm potentiometer
• Light-Dependent Resistor Setup:
• IC1 TLC548 serial ADC
• R1, R2 1 k-ohm resistor
• R3 22 k-ohm resistor
• Phototransistor Setup:
• IC TLC548 serial ADC
• R1, R2 1 k-ohm resistor
• R3 100 kilohm potentiometer
• Q1 Infrared phototransistor
• Misc. 12/24 pin connector for attaching to Commodore 64 User Port
• Computer Interface of Seismograph Sensor

A small handful of readily available electronic parts are all that's necessary to convert the voltage developed by the solar cell or phototransistor into a form usable by a computer. The circuit shown in ill. 16-4 uses the TLC548 serial ADC, connected to a Commodore 64. The Commodore 64 provides the timing pulses, so only a minimum number of parts are required. See TABLE 16-2 for a parts list for the circuit. Construct the TLC548 circuit on a perforated board using soldering or wire-wrapping techniques.

Software

The software is relatively simple. You might want to collect a number of samples and either print them out for future reference or graph them in a chart. Such programs are beyond the scope of this book, but if you are interested in pursuing the subject, you can find suitable charting programs using the Commodore 64 in Practical Interfacing Projects with the Commodore Computers, as well as a number of other publications. Check back issues of magazines that cater to owners of the Commodore 64.

LISTING 16-1

10 POKE 56579, 255

20 POKE 56577, 0

30 POKE 56589, 127

40 FORN=0T07

50 POKE 56577, 0

60 POKE 56577, 1

70 NEXT N

80 IF (PEEK (56589) AND 8) = 0 THEN 80

90 N = PEEK (56588)

100 PRINT N;

110 POKE 65677, 2

120 GOTO 40

ill. 16-4. Hookup diagrams for connecting the TLC548 serial analog-to-digital converter to a Commodore 64 computer. (A) Test circuit (vary Ri and watch change in values); (B) Interfacing the circuit with a photoresistor; (C) Interfacing the circuit with a phototransistor. Adjust R3 to vary the sensitivity.

CORRELATING YOUR RESULTS

I had just put the finishing touches on the coherency change seismograph prototype using a Commodore 64, when Los Angeles, my home town, was racked by a 6.1 Richter- scale earthquake (October 1, 1987). Although the epicenter of the earthquake was more than 50 miles distant with the Hollywood mountains in between, the entire area still shook violently. The earthquake could be felt for about 45 seconds but the remains of the tremor continued for several minutes (with lots of aftershocks).

I immediately checked the computer and found that it had recorded the full duration of the quake. I jotted down the results (I had not yet implemented a recording feature to save the data on disk or tape), and then waited until the seismologists in southern California could settle on an accurate magnitude for the tremor. At the epicenter, the earthquake measured 6.1 on the Richter scale. In my area, however, it was calculated that the earthquake measured only about 5.6 on the Richter scale. I used that information to “calibrate” the results from the computer. That way, with the digital data I recorded more accurately compared to a known value, I could better estimate the magnitude of future quakes.