A seismometer is a device that is sensitive to vibrations, and it is used to measure the motion of the ground. Movement may be caused by earthquakes, explosions, and volcanic eruptions. When combined with a timing and recording device, they are seismographs. The output of a device is a seismogram.
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What Is Seismic Sensing?
Seismic sensing is the process of measuring and analyzing seismic waves. Seismic waves are not restricted to the motions produced due to earthquakes; movements can be caused by any force generated from the earth's core or applied to the ground. There is a large range of ground motion that is sampled during earthquake monitoring applications as earthquake-generated motions that vary in intensity.
Ground motion is characterized by three parameters: displacement, velocity, and acceleration. Displacement measures the distance traveled by the earth's surface.
Position can be horizontal or vertical, ground velocity determines how far the surface of the ground has moved, and ground acceleration describes how the ground velocity changes with respect to time.
There are a range of seismic sensors that are applied depending on the context.
An Overview Of Current Day Seismometers and Ground Sensors
Initially, seismometers took the form of a traditional pen and pendulum. Since then, they have evolved to use electronic and electromechanical sensors. Technological advances have enabled a range of sensors that have varied operating frequency ranges mechanisms of sensing ma and with the capability of measuring different parameters of ground motion.
Strain Seismometers
Seismic sensors, historically, could only record ground displacement. Through technological advances, ground displacement measures have been possible. A strain seismometer is an instrument capable of recording and measuring the displacement between two points in the ground. Traditionally, a solid piece of metal can be used, which is highly sensitive to changes in both strain and length.
Another form of implement is the volumetric strainmeter. This uses a cylinder with a liquid-filled tube. Deformations of the cylinder cause changes in the liquid level; displacement transducers translate this into voltage. More recent forms of implementation use laser technology, producing the laser interferometer.
Laser interferometers operate at one point serving as a sensor, laser source, and short arm; at another point, a reflector is situated. Laser interferometers translate the change in the movement of the reflector, caused by ground displacement; strain sensors require deep underground installations as the sensitivity and accuracy of the displacement measurement is directly proportional to the measuring distance.
Strain seismometers have a precision of up to one part per billion. Despite their ability to measure the earth's deformation, they cannot sense ground motions to the accuracy required to detect those caused by earthquakes.
Inertial Seismometers
Inertial seismometers can characterize ground motion parameters with respect to a reference of inertia. This is typically a suspended mass, while the ground motion parameters measure other linear velocity and displacement of the suspended mass.
The ground motion that results is comprised of a linear and angular component; if the ground is displaced rapidly in the direction of freedom of the pendulum when the pendulum is not moving, it will remain in place through inertia.
When the movement of the ground occurs in the same time frame of the period of the pendulum, the seismograph will not record the movement accurately. This correction can be made mathematically, however,
There are three directions in which the ground can move; two horizontal and two vertical. Each kind of movement is separately recorded using three pendulums for each direction; this allows a complete seismograph to be generated.
In sum, a seismograph records the relative motion pendulum(s) and the ground. If the ratio between the deflection of the pendulum and the velocity (or acceleration) of the ground is recorded, the seismograph's velocity (or acceleration) sensitivity is produced.
If the free movement of the pendulum is not minimized, a proper recording of seismic waves cannot be detected. The easiest way to prevent this is to suspend the pendulum in a viscous liquid; the resisting force of the liquid is proportional to the pendulum's velocity.
Alternatively, a device called a damper is used; this produces a resisting force necessary using electromagnetics. In an electromagnetic damper, electrical currents generate the resisting force using copper plates moving in a strong magnetic field.
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Accelerometer-based sensors for seismic detection
Accelerometers used in earthquake sensors can detect noises with sensitivity 20 times greater than current-based seismometers. The use of an accelerometer is capable as seismic pressure waves can travel faster than corresponding land motions, enabling sensors to detect those waves.
Accelerometers detect vibrations and quantitatively measure acceleration, which is directly proportional to the force applied to an object that causes it to change speed or position. In accelerometer-based seismometers, the velocity of a point on the ground during the earthquake is measured.
In contrast to seismometers, accelerometers provide additional information about forces that an object a subject to during the seismic activity. They are usually less than 100 mm and are more easily incorporated into instrumentation.
Future of Seisomometer Sensing
While seismic waves have been traditionally recorded using the seismometer, accelerometer, or geophone, an emerging technology called distributed acoustic sensing (DAS) office further advantages over conventional sensors. It enables a larger number of sample locations and a greater bandwidth.
DAS uses short pulses of laser light and a fiber optic cable To measure acoustics. It can provide real-time measurements with high resolution and is becoming widely adopted across several sectors, including seismology.
DAS is an optoelectronic device comprised of a laser light source and an optical detector. It measures something called Rayleigh backscattering, which is produced by imperfections in the optical fiber.
DAS samples scattered light to detect small changes in the strain of the material around the fiber. Because it can do so with a wide frequency range and high spatial resolution, a DAS unit can record Seismic measurements at thousands of points, producing a very dense seismic array.
Fiber optic cables are durable and can withstand extreme environmental conditions, such as high temperature and pressure, that usually destroy conventional seismic sensors. Therefore, DAS is well suited to measuring seismic activity in locations of extreme conditions – from the Arctic to geothermal and volcanic regions. Because of their high sensitivity, they have been used in microseismic monitoring. Microseismic monitoring refers to the passive detection of small-scale earthquakes that arise from human activities.
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References and Further Reading
Fernández-Ruiz, M., Soto, M., Williams, E., Martin-Lopez, S., Zhan, Z., Gonzalez-Herraez, M. and Martins, H., (2020) Distributed acoustic sensing for seismic activity monitoring. APL Photonics, 5(3), p.030901. https://aip.scitation.org/doi/full/10.1063/1.5139602
British Geological Survey. (2022) How are earthquakes detected, located and measured?. [online] Available at: https://www.bgs.ac.uk/discovering-geology/earth-hazards/earthquakes/how-are-earthquakes-detected/#:~:text=Seismometers%20allow%20us%20to%20detect,big%20a%20particular%20earthquake%20is.
Santos, J., Catapang, A. and Reyta, E., (2019) Understanding the Fundamentals of Earthquake Signal Sensing Networks | Analog Devices. [online] Analog.com. Available at: https://www.analog.com/en/analog-dialogue/articles/understanding-the-fundamentals-of-earthquake-signal-sensing-networks.html
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