Forrest M. Mims III (forrestmims.org), an amateur scientist and Rolex Award winner, was named by Discover magazine as one of the “50 Best Brains in Science.” His books have sold more than 7 million copies.
Volcano eruptions, slippage of faults, explosions, landslides, drilling, and even traffic can create vibrations in the Earth’s crust known as seismic events or earthquakes. Seismometers are devices that detect seismic events, usually by mechanically or electronically detecting the movement of a suspended mass. Miniature accelerometers inside smartphones and video games also employ tiny movable masses to detect movements, and even seismic events.
Optical Fiber Seismometer
My son Eric Mims developed a novel seismometer in high school. His system was an optical fiber pendulum suspended from a steel frame bolted to the concrete slab under the carpet in his bedroom. A weight was attached to the free end of the fiber, which hung directly over an LED mounted under a pinhole. Seismic events caused the end of the fiber to oscillate back and forth across the pinhole. A photodetector at the opposite end of the fiber detected changes in light intensity, which were amplified and sent to a printer that Eric programmed as a chart recorder. This simple device detected earthquakes and two underground nuclear tests in Nevada, which earned Eric a record number of awards at the Alamo Regional Science and Engineering Fair in San Antonio.
Above is an outline view and circuit diagram of a revised version of Eric’s fiber optic seismometer, made with a fishing weight, a laser pointer or laser diode, and a photodiode or miniature solar cell connected to an amplifier. Insert the fiber into the open end of the laser pointer, and secure it with a hot glue gun. Insert the opposite end of the fiber through the central hole in a bullet-style fishing weight and glue it in place, as shown below.
The detector circuit is built on a solderless board like the one shown below. The laser light emerging from the weighted end of the fiber is detected by the photodiode. The photocurrent from the photodiode is amplified by a TLC271 operational amplifier, the gain of which is determined by the resistance of R1. The higher the resistance, the more sensitive the circuit is to light. A red LED glows to indicate when the photodiode is receiving light.
For initial tests, place the laser on a table and suspend the weighted end of the fiber over the table’s edge. Place the detector circuit on the floor, and adjust the fiber until it’s suspended just over the photodiode. Now darken the room completely, as external light will confuse the system. When the laser is switched on, the detector circuit’s red LED should glow. Gently pushing or even blowing on the weight will cause the fiber to oscillate back and forth like a pendulum. Each time it sweeps over the photodiode, the LED will glow.
To detect very subtle movements, move the circuit board until the end of the fiber is over just the edge of the detector. Or make a pinhole in a square of black tape and place the tape over the photodiode.
If you plan to detect earthquakes, install your seismometer inside a dark enclosure or closet, on the concrete slab or basement of a house. The LED can be connected to the circuit with wires long enough to allow it to be mounted outside the enclosure. The laser pointer will quickly exhaust its miniature batteries, so remove them and connect a 3-volt battery holder with a pair of AA or AAA cells to the pointer using tiny alligator clip leads. If your pointer has a pushbutton switch, close it with tape or a clothespin. Or you can substitute a laser diode and on-off switch, as shown below.
You can connect a data logger to the seismometer to provide a record of what it detects. The above chart shows a typical response of the seismometer when its weight is pushed slightly and allowed to swing until it is again stable. These data were logged by a 4-channel, 16-bit Onset Hobo UX120-006M Analog Data Logger.
Connect a piezo buzzer or tone generator to the output of IC1 to provide an audible signal when movement is detected.
If you’d like to detect the in-line direction of the seismic event, replace the photodiode with a quadrant photodetector. Center the free end of the fiber at the junction of all four quadrants of the sensor. Connect each quadrant to its own op-amp and their outputs to a 4-input data logger like the Hobo. You can make your own quadrant detector from four miniature solar cells cemented to a base.
While this seismometer is ultra-sensitive, it takes time to settle back to its neutral position after a seismic event. Dampening the pendulum reduces its sensitivity, while also reducing the time required to settle to neutral. Therefore, this improves the time resolution of the device.
The easiest way to dampen your seismometer is to simply shorten the fiber. Another way is to place a clear container with a flat bottom over the light sensor. Fill this container with clear vegetable oil and dip the weighted end of the fiber into the oil. The amount of damping is determined by the depth of the oil.
The simplest way to determine the magnitude and direction of movement of an optical fiber pendulum during a seismic event is to place a video camera under the moving end of the fiber. The above photo is a 15-second time exposure of an oscillating optical fiber pendulum. A video was made of the moving end of the fiber with a Panasonic Lumix camera in video mode. The video was played on a Surface Pro 3 tablet, and a 15-second time exposure of the video was made with a Sony a6000 (ISO 100 at f7). When Eric saw this image, he suggested using a webcam that records video only when movement is detected. That’s definitely on my list of things to do.
Forrest M. Mims III (forrestmims.org), an amateur scientist and Rolex Award winner, was named by Discover magazine as one of the “50 Best Brains in Science.” His books have sold more than 7 million copies.
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