Seismographic Instrumentation

	A variety of devices or systems of devices for measuring the motion of the Earth.  The ground motion is generally the result of passing seismic waves, gravitational tides, atmospheric processes, and tectonic processes.  The instrumentation typically consists of a sensing element (seismometer), a signal conditioning element or elements (galvanometer, mechanical or electronic amplifier, filters, analog-to-digital conversion circuitry, telemetry, and so on), and a recording element (analog visible or direct, frequency modulation, or digital magnetic tape or disk).  Seismographs find a wide range of applications including earthquake studies, investigations of the Earths gravity field, nuclear explosion monitoring, petroleum exploration, and industrial vibration measurement.

	Design Requirement.  In its wide range of uses, seismographic instruments may be required to measure ground motions accurately over a range approaching 12 orders of magnitude, from as small as 10-11 m (the Earth's background noise level in the 2 mHz to 100 Hz band at very quiet sites) to as large as several meters (strong ground motion in the near-field of a very large earthquake).  Similarly, seismographic instruments may be required to measure frequencies as small as ~10-5 Hz (the semi-diurnal gravitational tides), and even lower frequencies that are involved in tectonic strain monitoring, to as high as ~104 Hz (as observed from acoustic emissions from rock failures in mines at distances of a few meters).  Seismic waves from earthquakes are observed in the bandwidth of ~3x10-4 Hz (the gravest free oscillations of the Earth) to ~200 Hz (a local earthquake recorded by a seismometer installed in a 100+ m deep bore hole).  In exploration seismology the frequency range of interest is typically 10 Hz to 103 Hz.  No single instrument can operate over such a large dynamic range and frequency bandwidth.  Thus a great deal of variety is seen in seismometry.

	Seismometers.  The seismometer is the basic sensing element in seismographic instruments and there are two fundamentally different types: inertial and strain.  The inertial seismometer generates an output signal that is proportional to the relative motion between its frame (usually attached to the ground or a point of interest) and an internal inertial reference mass.  The strain seismometer (or linear extensometer) generates an output that is proportional to the distance between two points.

	In the inertial seismometer (Fig. 1), the inertial mass is an element of a damped mechanical oscillator, in either a spring-mass or a pendulum configuration.  The restoring force may be either an elastic (including the inertial elasticity of a piezoelectric device), an electrical spring, or it may be gravity.  Damping may be effected by the use of viscous fluids or by electrical feedback to provide a force that resists the relative velocity of the suspended mass.

	Regardless of the design details, inertial seismometers are described closely by the basic second-order differential equation for the harmonic oscillator:

	

where x is the relative motion between the mass and the frame, z is the damping factor (z = 1 is critical damping), wn is the natural frequency (in radians per second), and z is the frame (ground) motion.  The relative displacement, x, or the relative velocity, dx/dt, is sensed by an appropriate transducer.  Displacement transducers are typically optical, variable capacitance, or linear vector differential transformer (LVDT) devices.  Velocity transducers invariably used coil-magnet systems until recently.  A new type of velocity transducer, however, has recently been developed which uses a molecular electronic transfer cell to detect the motion of an electrolytic fluid.  In the case of piezoelectric (lead zirconate titanate, or PZT) seismometers, the voltage generated across the piezoelectric element is proportional to acceleration at frequencies below the resonant frequency (normally very high) of the piezoelectric device.  See HARMONIC OSCILLATOR; PIEZOELECTRICITY; TRANSDUCER.  

	The output of a seismometer is a signal, usually electrical, that is proportional to some function of the ground motion, z.  It is possible for this signal, over some frequency range, to be directly proportional to the ground displacement (z), the ground velocity (dz/dt), or the ground acceleration (d2z/dt2).  Within such frequency ranges, seismographs are commonly referred to as displacement seismographs, velocity seismographs, or accelerographs, respectively.  That is, the recorded amplitude represents the appropriate ground motion quantity multiplied by a frequency-independent constant (the magnification, the velocity sensitivity, or the acceleration sensitivity of the seismograph) for motion in the particular frequency range.

	The strain seismometer (or linear extensometer) (Fig. 2) in an instrument capable of providing a continuous precise measurement of the distance between two points, spaced typically 1 m (3.3 ft) to 1 km (0.6 mi) apart.  A wide variety of designs have been implemented, utilizing as reference lengths (L) from one point to the vicinity of the other point such standards as fused silica rods or light beams.  Rod-based extensometers normally use a capacitance displacement transducer to sense variations in the position of the reference rod end with respect to the other reference point.  Light-beam extensometers invariably incorporate some type of interferometer to detect changes in the length of the optical path between the two reference points.  Longer paths require evacuated tubes and laser sources to achieve adequate sensitivity and stability in the measurement of the length change.  The frequency range of interest is from 0 to no more than ~1 Hz in most systems.  See INTERFEROMETRY.

	The quantity measured in these instruments is the change DL in the reference length L.  The ratio, DL/L gives the horizontal component (typically) of the strain between the two points.  With a displacement resolution of 10-9 m (3.3 x 10-9 ft), a 100 m (330 ft) extensometer can measure changes in strain of 1 part in 10-11.  However, practical limits in environmental controls (temperature, atmospheric pressure, and mechanical stability) yield long term stabilities no better than 10-7 per year for such instruments.

	Secular strains are also measured using Very Long Baseline Interferometry (VLBI) and geodetic Global Positioning System (GPS) techniques. VLBI uses phase differences in signals from extraterrestrial radio sources (quasars) observed simultaneously at points separated by continental dimensions.  Accuracies of a few centimeters in a few 1000 km are observed, for a resolution of 10-9 in relative distance change between the two remote points.  The GPS system is used to observe geodetic strain by absolute position measurement on the Earth's surface.  The geodetic deformation monitoring is being done with both continuous (permanent) stations and campaign measurements (temporary deployments at existing benchmarks) over a period of a few years.  Accuracies of a few mm are attained.  See GEODESY; SATELLITE NAVIGATION SYSTEMS.

Seismoscopes.  A seismoscope is a device which indicates only the occurrence of relatively strong ground shaking and not its time of occurrence or duration.  A typical seismoscope inscribes a hodograph of horizontal strong ground motion on a smoked watch glass.

Dilatometers.  One variation on the linear extensometer is the dilatometer which is capable of giving continuous precise measures of volumetric strain.  The quantity measured is the change DV in the reference volume V and the ration DV/V gives the volumetric strain.  Dilatometers are typically installed in a bore hole in competent rock (preferably granite) at a depth of 100-300 m (330-1000 ft).

Tiltmeters.  Another variation on the linear extensometer is the tiltmeter where the relative change in the elevation between two points is monitored, usually with respect to a liquid-level surface.  Horizontal distance between the reference points may be as little as a few millimeters or as large as several hundred meters.  A displacement transducer is typically employed to sense the vertical separation between the two liquid-level surfaces.  Environmental effects limit the long-term stability to about 10-6 radian.

	Tilt at a point can be measured by an inertial seismometer.  Tilt of the horizontal surface is indistinguishable inertially from a horizontal acceleration of magnitude g sinq, where q is the angle and g is the acceleration due to gravity.  Any seismometer with DC response to acceleration (for example a horizontal pendulum equipped with a displacement transducer) is therefore a tiltmeter with constant tilt sensitivity throughout the frequency range where its output is proportional to acceleration.  See ACCELEROMETER.

Gravimeters.  The gravity meter is no more than a vertical-component accelerometer, that is, a pendulum sensing ground motion and equipped with a displacement transducer, analogous to the above inertial tiltmeter.  In its most widely used form, the pendulum is small for portability and ease of thermal stabilization, with a natural frequency of ~0.1 Hz.  Transducer technology and environmental control methods allow the best gravimeters to have a repeatable accuracy of ~10-8 g.  Gravimeters are widely used in geophysical exploration, in the study of earth tides, and in the recording of very low-frequency (0.0003 to 0.01 Hz) seismic waves from earthquakes.

	Recording Systems.  The complete seismograph produces a record of the properly conditioned signal from the seismometer, along with appropriate timing information.  The recording system may be a simple as a mechanical stylus scratching a line on a smoke-covered drum in a portable microearthquake seismograph, or as complex as a multichannel computer-controlled system handling 25,000 24-bit digital words per second in a modern seismic reflection survey for petroleum exploration.  The range between these extremes includes many special-purpose seismographs, all designed to record ground motion in a particular application.  See EARTH TIDES; EARTHQUAKE; GEOPHYSICAL EXPLORATION; SEISMIC EXPLORATION for OIL AND GAS; SEISMOLOGY.

	Deployment.  The methods used for deploying a seismograph are highly varied and they depend upon whether the site is temporary or permanent and also on whether the site is land-based or on the ocean bottom.  Seismic station installations are susceptible to several types of noise which contaminate or even mask the desired signals, effectively reducing the operating sensitivity of the instrument.  These noise sources are of several types: atmospheric, cultural, ocean currents, microseismic, and ambient temperature variation.  The atmosphere affects the seismometer through pressure fluctuations directly on the mechanical system and indirectly on the surrounding ground.  Cultural and industrial vibrations from human activities can be seen near any population center.  In the case of seismographs deployed on the sea floor, ocean currents are a large noise source.  Microseisms, the natural background vibrations of the Earth, are generally larger near continental margins than inland, a result of wave action on the coastline.  For modern fedback broadband seismometers, ambient temperature variation is also a significant source of noise.

	In the conventional land-based permanent seismographic station, the seismometer is placed upon a stable (usually concrete) pier in an environment as noise-free as possible.  Two deployment methods have been devised to mitigate against noise sources, and they are widely used.  One technique uses seismometers in closely spaced arrays, relying on the coherency of the signal of interest over the array dimensions, and the lack of correlation of the noise components in the seismic wave field.  This method is applied successfully in seismic reflection surveying and in the arrays deployed to monitor from large distance the seismic waves from detonation of underground nuclear explosions.  A second noise-reduction method uses bore hole or deep mine installations to take advantage of the natural attenuation with depth of much of the noise field due to shallow propagating surface waves, to attenuate the effects of the surface ground motion due to atmospheric pressure fluctuation, and to provide temperature stability.

	In the deployment of a temporary array of portable seismographic stations, as required for aftershock studies or research investigations, the siting criteria also include obtaining permission from the land owners and concerns about vandalism.  The portable seismographs are typically housed in a small enclosure which can be placed on the surface or buried in the ground a few feet.  The stations are generally battery and solar powered and they include sufficient  storage capacity to run unattended for, typically, from weeks to months.  Siting the seismometer on hard rock and away from cultural noise sources is preferable but not always possible.  Instrument pools of portable seismographs exists for a wide variety of seismic studies.  The Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) instrumentation center, for example, maintains a large stockpile of portable seismographs which are used by researchers.

	In the deployment of an Ocean Bottom Seismograph (OBS), the logistics and installation problems are a formidable challenge.  However, there is considerable incentive to place OBS's on the sea floor since it represents the largest uninstrumented area of the Earth's surface.  A number of specialized methodologies have been devised deploying, servicing, and retrieving OBS instrumentation.  Borehole installations in the sea floor produce the lowest noise levels.  Most OBS's use retrievable instrument capsules with on-board recording and sufficient battery capacity for unattended operation from a few weeks to a year or more.  Some permanent OBS stations have been installed along cables which link them to shore and provide telemetry and power.  Transoceanic telephone cables, decommissioned as a result of increasing reliance on satellite communications, are being considered for the installation of permanent OBS stations on the sea floor.  OBS instrument pools exist for a wide variety of seismic studies.  For example, the Ocean Bottom Seismograph Facility of the Scripps Institution of Oceanography and the Woods Hole Oceanographic Institution Marine Seismology Group both maintain a pool of OBS instrumentation for use by researchers.

Original by Thomas V. McEvilly, Extensively Revised by Robert A. Uhrhammer.


Bibliography.

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Related URL's.
	The U.S. Geological Survey Albuquerque Seismological Laboratory (ASL)  seismic background noise modeling and reduction (URL:  http://aslwww.cr.usgs.gov/Publications/index.htm). 

	The Incorporated Research Institutions for Seismology (IRIS) Global Seismic Network (GSN) (URL: http://www.iris.washington.edu/GSN/).

	The Incorporated Research Institutions for Seismology (IRIS) Program for the Array Seismic Studies of the Continental Lithosphere (PASSCAL) (URL: http://www.iris.washington.edu/PASSCAL/passcal.htm).

	Office of Naval Research (ONR) Ocean Bottom Seismograph (OBS) Facility at the Scripps Institution of Oceanography (SIO) (URL: http://www.mpl.ucsd.edu/obs/history.html#sensors).

	Woods Hole Oceanographic Institution (WHOI) Marine Seismology Group (URL: http://obslab.whoi.edu/seismic_instrumentation.html).




	Berkeley Seismological Laboratory (BSL) Berkeley Digital Seismic Network (BDSN) guidelines for installing broadband seismic instrumentation (URL: http://www.seismo.berkeley.edu/seismo/bdsn/instrumentation/guidelines.html).

	University of Stuttgart Institute of Geophysics: Seismic sensors and their calibration:  (URL: http://www.geophys.uni-stuttgart.de/seismometry/man_html/man_html.html).

	Precision Measurement Devices (PMD) description of Molecular Electronic Transfer (MET) sensor (URL: http://pmdsci.home.att.net).

	Use a search engine and keywords such as seismometer, broadband, dilatometer, geophone, gravimeter, etc. to find additional URL addresses.