F.A.Q F.A.Q

What Does A Geophone Measure?

Geophones, deriving from the Greek words “geo” meaning earth and “phone” meaning sound. The geophones can be deployed on the surface or near geothermal reservoirs in wells to gather detailed data. Geophones generate electrical signals proportional to particle velocity in acoustic waves.

 

How Geophones Measure

 

Geophones are primarily used to measure seismic waves propagating through the Earth, also known as seismic waves, and can also measure particle velocity in underwater acoustic waves. Geophones or hydrophones measure the seismic wave field, producing electrical signals. The voltage data from geophones are initially amplified at the pre-amplifier and then filtered through analog low-pass filters (applied to seismic signals to attenuate frequencies smaller than the cut-off frequency).

 

Geophone instruments rely on inertial mass suspended on springs or coils. Their response is proportional to ground velocity. They function more like microphones or speakers with magnets enclosed by coils. The system employs electromagnetic induction. In seismic applications, geophone casings are securely fixed to the Earth and move with passing seismic waves.

 

However, the mass mounted on the spring tends to remain stationary, hence moving relative to the casing. A permanent magnet attached to the casing and a coil wound around the mass block is used to measure the motion of the mass. When the wire passes through the magnetic field, a voltage proportional to the speed of the wire passing through the magnetic field is generated. The voltage is recorded.

What does a geophone measure

What need to know about geophones?

 

Different Types

Common geophone types include vertical, horizontal, and multi-component sensors, operating at various frequencies. Vertical sensors are typically used for refraction and surface wave studies, while horizontal sensors are employed for reflection and advanced research on multi-component seismic motion. Also, consider 3C and Omnidirectional sensors. Note that underwater measurements can utilize hydrophones, which detect pressure changes instead of vibrations.

 

Frequency

Geophone frequency selection depends on the seismic survey type and application. Surface wave methods require sensors below 5 Hz, refraction methods use sensors ranging from 10-28 Hz, and reflection measurements typically utilize sensors ranging from 10-40 Hz. Higher frequency-rated geophones offer greater resolution but may limit the detection depth/distance of seismic energy.

 

Frequency Ratings

Geophones have a resonance frequency, where they respond most effectively. Low-frequency geophones excel at detecting deep, low-frequency vibrations. Higher-frequency geophones capture more detail in faster oscillations. Geophones record vibrations better around their resonance frequency. Hence, when in doubt, it’s better to use geophones with frequencies too low rather than too high.

Frequency Ratings

Size Matters

The physical properties of a geophone, specifically the internal mass and spring stiffness, determine how quickly it responds to vibrations. Increasing the mass of the device and softening the spring system increases sensitivity to low frequencies, but decreases response to fast movements and increases sensitivity to tilt. For example, a 28Hz geophone with a strong spring has a stable linear response, while a 4.5Hz geophone requires specific vertical or horizontal alignment when installed.

 

As the sensitivity of the geophone decreases, the size and cost of the equipment increases exponentially. For example, a 2Hz geophone is typically six times larger than a 4.5Hz geophone and costs almost ten times more. A comparison of internal quality, closely related to cost, shows that the quality inside a 1Hz sensor is almost 100 times that of a 4.5Hz sensor.

 

Additionally, larger components produce higher voltage outputs, so small sensors produce speed readings around 20 V/m/s, while large sensors can exceed 200 V/m/s or even higher.

 

Choosing the appropriate type of geophone depends on frequency range, vibration monitoring needs, and location. In applications such as construction, demolition, and near-field blast events, high-frequency geophones are needed, while in long-distance monitoring, low-frequency geophones are needed. Choosing a proper geophone sensor can maximize monitoring station performance and minimize potential losses.

Size Matters

Related products:

Low-frequency geophone 1Hz:

Low Frequency Geophone 1Hz

 

2Hz high sensitivity geophone:

2Hz High Sensitivity Geophone

 

Omni geophone 14Hz:

Omni Geophone 14Hz

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