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.
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.
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 VP 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.
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).
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