I. Q: When was the first geophone made? A: Real seismometers go back to the 1900’s. Geophones probably came shortly after.
II. Q: What is a geophone? A: A geophone is a small, cheap instrument for measuring ground motion. There are many different varieties for different applications. They are designed for earthquakes, machine vibrations, oil exploration, mining, etc.
III. Q: Who uses geophone? A: Oil companies, security companies, mining companies and seismologists, etc.
IV. Q: How do geophone work? A: Most of the models have a coil hanging from a spring in the center of some magnets. When the case is moved up and down the mass tends to stay put and induces small currents into the coil as it moves through the magnetic field. It measures velocity of motion.
V. Q: Is a geophone the same thing as an accelerometer or seismometer?
A: Not really. Geophones would sit in between accelerometers and seismometers in function and price. Seismometers are typically larger and more expensive. They usually detect extremely small movements at lower frequencies than geophones. Seismometers can be very fragile and sensitive, although they are designed to survive earthquakes, that may be from hundreds of miles away. Calibrating a seismometer might be critical to getting useful data out of it and a geophone or accelerometer would more likely to be used just to get a simpler signal. Accelerometers are nearly solid state and good at handling more violent motion. Video games and cell phones are now using accelerometers to interact with the users as another input device. All three might have a intended axis or orientation (XYZ or Mercator plus vertical) or they might be a bundle of more than one axis in the same package. Single axis instruments are limited in function off of intended axis.
VI. Q: What’s the difference between all of the different models of geophones? A: There are huge differences between the models and options for geophones. The exterior case is optional on a lot of geophones. Some have coaxial connectors and some have binding post connectors, but most have two little pins that you connect your leads to. The resonant frequency is one of the main factors in the price. Lower resonant frequencies are more difficult to achieve in a small box with a light weight and a low price. Basically you want the resonant frequency to be close to what your looking for in signals. Also your application should be a factor. You probably don’t need a 1 Hz resonant frequency to watch local earthquakes, but you would want one for distant earthquakes. The frequency response of an instrument is probably centered around the resonant frequency and is very narrow in width.
VIII. Q: What types of information can you get out of a geophone? – Natural Frequency – Coil Resistance – Moving Mass – Harmonic Distortion – Open Circuit Damping – Damping With Shunt Resistor – Sensitivity With Shunt Resistor – Open Circuit Intrinsic Voltage Sensitivity – Damping Calibration-shunt Resistance – Typical Case To Coil Motion p-p – Typical Spurious Frequency,etc.
