Contrary to what you might think, geophones are remarkably simple (yet ingenious) devices. Like gravity meters, the active element of the device consists of a mass hanging on a spring. When the ground moves, the mass (because it has inertia) wants to remain motionless. If you were watching the seismometer as the ground moved, it would look like the mass itself was moving. But, in reality, you are moving with the ground, and the mass is remaining motionless*.
For more information, please visit What Are Geophones.
Now for the part that I really consider ingeneous. Wrapped around the mass is a strand of wire. Surrounding the wire-wrapped mass is a magnet that is fixed to the Earth. As the Earth moves, the magnet moves up and down around the mass. The magnetic field of this moving magnet produces an electrical voltage in the wire. This voltage can be amplified and recorded by a simple voltmeter. It is relatively easy to show that the voltage recorded by the voltmeter is proportional to the velocity (speed) at which the ground is moving**.
Shown to the left is an example of a geophone that is representative of those typically used in seismic refraction and reflection work. A quarter is shown for scale. This particular seismometer has had its side cut out so that you can see its working parts. The wire- (copper wire in this case) wrapped mass can be clearly seen inside the geophone. The spring connecting the geophone to the case can not be seen but is just above the mass. The silver colored case just inside the blue plastic external case is magnetized. The black wires coming out from either side of the blue case transmit the variations in voltage to the recording system. The long silver spike below the blue case is used to firmly attach the geophone to the ground. This spike is pressed into the ground by stepping on the top of the geophone until it is completely buried.
Different styles of geophone cases are available for use in different environments. Several examples are shown to the right. The geophone shown to the far right (the one without the spike), for example, is designed for use on hard surfaces into which spikes can not be pushed.
Geophones used in exploration seismology are relatively inexpensive. Costs ranging from $75 to $150 per geophone are not uncommon. Although this cost per geophone is small, remember that many (1000's) of geophones may be used in the large reflection seismic surveys conducted for the petroleum industry. Near-surface investigations are typically much smaller in scale, both in terms of area covered and in terms of equipment needed. For a near-surface refraction survey, one could use as few as twelve or as many as a hundred geophones. Near-surface reflection surveys use only a moderately greater (24 to 150) amount of geophones at any one time.
*Obviously, this is a simplification of what really happens. Because the spring is not perfectly compliant, the mass does in fact move when the Earth moves. It moves in a very complex fashion that can be relatively easily quantified. For our purposes, however, we can make the assumption that the mass remains motionless without loss of generalization.
**This type of geophone was first invented in 1906 by a prince of the Russian empire by the name of B. B. Galitizin.
Seismic geophone is a device used in surface seismic acquisition, both onshore and on the seabed offshore, which detects ground velocity produced by seismic waves and transforms the movement into electrical impulses. Geophones detect motion in only one direction. Conventional seismic surveys on land use one geophone per receiver location to detect motion in the vertical direction or horizontal direction. Three mutually orthogonal geophones are typically used in combination to collect 3C seismic data. Hydrophones, unlike geophones, detect changes in pressure rather than motion.
How does a geophone work?
The geophones currently available in the market are highly sophisticated, and they operate based on the magnetic field generated by a coil suspended in a magnetic field. The current seismic industry market requires tight specifications to meet the standards of the seismic survey. 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 you’re 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.
Is a geophone the same thing as an accelerometer or seismometer?
If you want to learn more, please visit our website 3D Seismic Data.
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 an 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 the intended axis.
How to select a suitable geophone?
As we all know, Oil companies, security companies, mining companies and seismologists are the main users of seismic geophones and usually professional geophone suppliers (including but not limited to Seis Tech ) will recommend different types of seismic geophones as per client’s applications. A pro geophone supplier may ask quite a few of technical questions before recommending a geophone or quote prices but please be patient as maybe this is the most responsible attitude for the geophone users. Generally speaking, It is not a wise choice if you adopt a low frequency geophone lower than 10Hz for coal mining prospecting.
What are the advantages of Seis Tech in supplying geophones?
Seis Tech is a professional supplier for velocity geophones, piezo sensor hydrophones, ert cables, geophone spread cables and related coaxial connectors. Mostly, geophones, hydrophones and cables supplied by Seis Tech have been qualified with RoHS 2.0 and Seis Tech has also been certified with ISO 9001:2015 QMS, ISO 14001:2015 EMS and ISO 45001:2018 OHSMS.
If you are interested in conventional velocity geophones in low costs for main stream oil, gas or mining prospecting, please contact Seis Tech; If you are interested in very low frequency geophones or very high sensitivity geophones for applying in micro seismic monitoring or other minority fields, please contact Seis Tech. As always, Seis Tech is dedicated to supplying geophones and related products with reasonable prices and professional services.
Contact us to discuss your requirements of DIY Geophone. Our experienced sales team can help you identify the options that best suit your needs.