How to select a suitable geophone for certain seismic method?

Geophones are used in seismic investigations to transform the vibration in the ground into a voltage. The geophones are analogue sensors, with a spring-mounted wire coil moving past a magnet. This creates the electrical signal with is recorded by a seismograph such as the ABEM Terraloc, PASI GEA 24, Geode 24 or DAQ Link 3 system.

Geophone Types

There are three types of common geophones: vertical, horizontal, three-component and omnidirectional geophone. And these can have different frequencies.

Typically, we use vertical geophones for refraction&surface wave and horizontal geophones for reflection. Three-component geophones and omnidirectional geophones are used for some advanced studies of ground motion. For survey in water, you may choose piezo sensors that detect pressure changes rather than vibration and these are called hydrophones.

low frequency geophone 4.5hz Seis Tech

Geophone Choice by Frequencies

The choice of geophone frequency will depend on the type of seismic investigation being undertaken and the application. Surface wave methods must use low frequency geophones below 5Hz, while refraction typically used geophones with a natural frequency 10Hz -28Hz. High frequency geophones give greater resolution but limit the depth/ distance that seismic energy will be detected.

Geophone Response Range

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 that travel deep and far. Higher frequency geophones will respond best to faster oscillations. This produces more detailed oscillations.

For ground vibrations at frequencies on either side of the resonance rating, the geophone will register frequencies above the rated value better than those below the rated value. When in doubt, it is better to use a detector with a frequency that is too low rather than too high.

Horizontal Geophones for Reflection Surveys

Although reflection surveys can be undertaken with vertical geophones, for near-surface surveys, we will recommend horizontal geophones for the following reasons:

-Horizontal geophone will minimize the effects of refraction waves. Random noise tends to be dominated by P-wave motion.

-Shear wave velocities can help differentiate between similar VP velocities.

-Horizontal geophones will measure shear movement and S-waves travel slower, equating to shorter wavelengths and thus better resolution. horizontal geophone Seis Tech

Is it possible to use the same geophone for Refraction, Reflection and Surface Wave Methods?

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

The 4.5Hz 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 the 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 5Hz.

Three Component Geophone (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.

3c geophone 4.5hz Seis Tech