Geophone
Encyclopedia
The term geophone derives from the Greek word "geo" meaning "earth
" and "phone" meaning "sound".
A geophone is a device which converts ground movement (displacement) 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.
Geophones have historically been passive analog devices and typically comprise a spring-mounted magnetic mass moving within a wire coil to generate an electrical signal. Recent designs have been based on Microelectromechanical systems
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 microelectromechanical systems devices usually respond proportional to
acceleration
. Microelectromechanical systems have a much higher noise level (50 dB velocity higher) than geophones and can only be used in strong motion or active seismic applications.
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 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.
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 travelling. 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 ll, 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/(meter/second), so they are in general not a replacement for
broadband seismometers.
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.
Classical element
Many philosophies and worldviews have a set of classical elements believed to reflect the simplest essential parts and principles of which anything consists or upon which the constitution and fundamental powers of anything are based. Most frequently, classical elements refer to ancient beliefs...
" and "phone" meaning "sound".
A geophone is a device which converts ground movement (displacement) into voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
, which may be recorded at a recording station. The deviation
Deviation
Deviation may refer to:* Deviation , the difference between the value of an observation and the mean of the population in mathematics and statistics** Standard deviation, which is based on the square of the difference...
of this measured voltage from the base line is called the seismic response and is analyzed for structure of the earth.
Geophones have historically been passive analog devices and typically comprise a spring-mounted magnetic mass moving within a wire coil to generate an electrical signal. Recent designs have been based on Microelectromechanical systems
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
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 microelectromechanical systems devices usually respond proportional to
acceleration
Peak ground acceleration
Peak ground acceleration is a measure of earthquake acceleration on the ground and an important input parameter for earthquake engineering, also known as the design basis earthquake ground motion...
. Microelectromechanical systems have a much higher noise level (50 dB velocity higher) than geophones and can only be used in strong motion or active seismic applications.
The frequency response of a geophone is that of a harmonic oscillator
Harmonic oscillator
In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x: \vec F = -k \vec x \, where k is a positive constant....
, fully determined by corner frequency (typically around 10 Hz) and damping (typically 0.707). Since the corner frequency is proportional to the inverse 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.
The majority of geophones are used in reflection seismology
Reflection seismology
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
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 travelling. 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 ll, 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/(meter/second), so they are in general not a replacement for
broadband seismometers.
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
Unattended Ground Sensors
This article describes Unattended Ground Sensors under development as part of the United States Army's Future Combat Systems Program. For information on currently fielded UGS systems, refer to the Current Force UGS Program or CF UGS....
(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.
External links
- PSR-1 Seismic Intrusion Detector (Vietnam era military device)