Recording device in geology

If you are looking for more details, kindly visit What Are Geophones.

This article is about the electronic seismic recording device. For the percussion instrument, see Geophone (musical instrument)

Geophone (SM-24), frequency band 10 Hz to 240 Hz, standard resistance 375 Ω

A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of the Earth.

Etymology

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The term geophone derives from the Greek word "γῆ (ge) " meaning "earth" and "phone" meaning "sound".

Construction

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Geosource Inc. MD-79—8 Hz, 335 Ω geophone

Geophones have historically been passive analog devices and typically comprise a spring-mounted wire coil moving within the field of a case-mounted permanent magnet to generate an electrical signal.[1] Recent designs have been based on microelectromechanical systems (MEMS) technology which generates an electrical response to ground motion through an active feedback circuit to maintain the position of a small piece of silicon.

The response of a coil/magnet geophone is proportional to ground velocity, while MEMS devices usually respond proportional to acceleration. MEMS have a much higher noise level (50 dB velocity higher) than geophones and can only be used in strong motion or active seismic applications.

Man using a geophone

Frequency response

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The frequency response of a geophone is that of a harmonic oscillator, fully determined by corner frequency (typically around 10 Hz) and damping (typically 0.707). Since the corner frequency is proportional to the inverse square root of the moving mass, geophones with low corner frequencies (< 1 Hz) become impractical. It is possible to lower the corner frequency electronically, at the price of higher noise and cost.

Although waves passing through the Earth have a three-dimensional nature, geophones are normally constrained to respond to single dimension - usually the vertical. However, some applications require the full wave to be used and three-component or 3-C geophones are used. In analog devices, three moving coil elements are mounted in an orthogonal arrangement within a single case.

Uses

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Single End Spread[2] & Split Stread[3] Source Receivers single fold-3 Fold

The majority of geophones are used in reflection seismology to record the energy waves reflected by the subsurface geology. In this case the primary interest is in the vertical motion of the Earth's surface. However, not all the waves are upwards traveling. A strong, horizontally transmitted wave known as ground-roll also generates vertical motion that can obliterate the weaker vertical signals. By using large areal arrays tuned to the wavelength of the ground-roll the dominant noise signals can be attenuated and the weaker data signals reinforced.

Analog geophones are very sensitive devices which can respond to very distant tremors. These small signals can be drowned by larger signals from local sources. It is possible though to recover the small signals caused by large but distant events by correlating signals from several geophones deployed in an array. Signals which are registered only at one or few geophones can be attributed to unwanted, local events and thus discarded. It can be assumed that small signals that register uniformly at all geophones in an array can be attributed to a distant and therefore significant event.

The sensitivity of passive geophones is typically 30 volts per (meter per second), so they are in general not a replacement for broadband seismometers.[clarification needed]

Conversely, some applications of geophones are interested only in very local events. A notable example is in the application of remote ground sensors (RGS) incorporated in unattended ground sensor (UGS) systems. In such an application there is an area of interest which when penetrated a system operator is to be informed, perhaps by an alert which could be accompanied by supporting photographic data.

Geophones were used on the Moon for a number of active and passive experiments as part of the Apollo Lunar Surface Experiments Package.

See also

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References

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• PSR-1 Seismic Intrusion Detector (Vietnam era military device)

What do I need to know about geophones?

Geophones are used in seismic investigations to transform the vibration in the ground to a voltage. The geophones are analogue instruments, with a spring-mounted wire coil moving past a magnet. This creates the electrical signal which is recorded by a seismograph such as the Terraloc. 

Further on, it is also good to have a hint on the following. 

 

Different geophones

 

There are three types of common geophones: vertical, horizontal and multi-component. And these can have different frequencies. Typically, we use vertical geophones for refraction and surface wave studies and horizontal geophones for reflection and multi-component for some advanced studies of the ground motion. Also see 3C and Omnidirectional below. Note that for survey in water we can use sensors that detect pressure changes rather than vibrations and these are called Hydrophones.

 

Geophone frequencies

 

For more information, please visit 3D Seismic Data.

The choice of geophone frequency will depend upon the type of seismic investigation being undertaken and the application. Surface wave methods must use low frequency geophones below 5 Hz, while refraction typically uses 10-28 Hz geophones and reflection surveys will normally use 10-40 Hz geophones. High frequency ratings on the geophone give greater resolution but limit the depth/distance that the seismic energy will be detected at. 

 

Frequency rating of a geophone

 

Geophones have a “resonant frequency” – this is the frequency of vibration at which they prefer to oscillate and therefore give the biggest response. Low frequency geophones are good at detecting low frequency vibrations and these are the one which travel deep and far. A higher frequency geophone will respond best to faster oscillations which are the ones which create more detail. For ground vibrations at frequencies either side of the resonance rating, a geophone will record those above the rating better than those below so, if in doubt, it is better to use a geophone with too low frequency than too high.  

 

Horizontal geophones for reflection surveys

 

Although reflection surveys can be undertaken with vertical geophones, for near-surface survey we recommend horizontal units because: 

• They minimize the effects of refracted waves.

   

• Random noise tends to be dominated by P-wave motion.

   

• Shear wave velocities can help differentiate between similar V

  P

   velocities.

   

• They will be measuring shear movement and S-waves travel slower, equating to shorter wavelengths and thus better resolution.

   

 

Using the same geophone for refraction, reflection, and surface wave methods

 

Under specific circumstances you can use the same geophone for all three different types of investigations, but your data would be compromised. 

The 4.5 Hz vertical geophones used in surface wave studies could be used to collect refraction and reflection data, but the resolution would be reduced. This will be a bigger problem for reflection investigations where the aim of the survey may be to map stratigraphy in detail. 

And compromising on use of a single geophone will not work the other way round: in other words, higher frequency geophones – suited to good quality refraction and/or reflection work – will not be appropriate for surface wave surveys, as these require frequencies below 5 Hz. 

 

3-component (3C) geophone

 

Regular geophones have a single sensor in them which is orientated to be sensitive to either horizontal motion or vertical motion. A 3-component geophone has three sensors inside – one mounted vertically and two mounted horizontally at 90° to each other (to measure ‘left-right’ and ‘forward-backward’ horizontal motion). These are commonly used for HVSR (Horizontal-to-Vertical Spectral Ratio) surface wave studies and monitoring projects. 

 

Omnidirectional geophone

 

An omnidirectional geophone is a single geophone sensor which will operate equally well regardless of its orientation (right way up, sideways, upside down, at an angle).  

 

More to read

 

Application areas 

https://www.guidelinegeo.com/application-areas/  

Methods 

https://www.guidelinegeo.com/seismic-methods/  

Case Stories 

https://www.guidelinegeo.com/solutions/case-stories/  

Products 

https://www.guidelinegeo.com/abem-resistivity-seismics-tem/  

 

 

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