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{{cite|The effect of seismic events is to induce frequency-modulated side bands, in the 0.2-1 Hz region, around the 'Earth line', which indicate the presence of rotational components associated with seismic events.}}
 
{{cite|The effect of seismic events is to induce frequency-modulated side bands, in the 0.2-1 Hz region, around the 'Earth line', which indicate the presence of rotational components associated with seismic events.}}
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===University of Canterbury===
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In a PhD thesis by B Tom King it is admitted that the nature of the Earth induced line is unknown and unverified.
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{{cite|While lasingin single longitudinal mode and unlocked under the Earth's rotation a Lissajous figure is generated on an oscilloscope with the Sagnac signal from G0 on the x-axis and a sinusoid (from a signal generator) on the y-axis. The figure can be made stationary by varying the frequency of the generated signal. Once achieved this becomes an extremely sensitive detector of phase shifts in the G0 output. A variety of tests can then be performed. Artificial seismic events are easily detected as a jitter in the Lissajous figure. Also the introduction of a magnetic field near the gain medium has the effect of shifting the beat frequency and hence causes the Lissajous figure to rotate at a constant rate. '''However, the question remains as to whether or not this signal is necessarily the Earth induced line. It is difficult to impose a rotation on the laser as it is rigidly mounted to a solid concrete wall which in turn is moulded around the solid volcanic rock that forms the interior cavern wall. It is conceivable that the signal can be traced back to some oscillation in the gain medium. This problem is yet to be clarified.'''}}
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The author goes on to deduce that because the earth induced line agrees with an equation (which was possibly made after the fact to match observation), that it is assumed that the rate is indeed from the earth.
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{{cite|However, it is assumed that the signal is indeed the Earth induced ''rate'' mostly because of the excellent agreement between the measured output frequency (see Figs. 6. 7 and 6.8) and the expected frequency of 287. 75 ± 0.46 Hz.}}
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On p.152 it is shown that the rotation rate of the earth involves the input of 86164s into the equation, which is the Sierael Day. The equation is using a predefined period to assess a daily variation in the (seismic) noise and create an output.
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[[File:Siderial day rotation rate.png|500px]]
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The 'Sidreal Day' also happens to be the time it takes for the sun, stars, and celestial bodies to return to their spots above the earth. This is opposed to the Solar Day, which is supposedly the true rotation of the earth.
  
 
==Seismic Wave Propagation==
 
==Seismic Wave Propagation==

Revision as of 20:41, 20 February 2019

The Ring Laser Gyroscope is a type of gyroscope that is based on laser beams which move around a cavity. The principle of operation of a RLG is based on the Sagnac Effect, which was famed for showing that light changes velocity on a rotating platform. The changing velocity of light as seen in the RLG and Sagnac Experiments is said to be contrary to the consistency of light as proposed by Special Relativity (See the Michelson-Morley Experiment and Sagnac Experiment).

In the Ring Laser Gyroscope two counter-propagating laser beams are shown to have different frequencies with the difference dependent on rotation rate. Measurements of this difference provides the rotation angle or rotation rate about the RLG's sensitive axis.

The Earth Line

Strapdown Inertial Navigation Technology
By David Titterton, John L. Weston, John Weston

From Chapter 15.6 on p.497

  “ The Fourier transform of the output from the laser cavity gives a spectral line associated with the rotation of the Earth often termed 'the Earth line'. Analysis of this spectral line is used to monitor the impact of seismic waves and other events. This ability to make high-precision measurements of rotation rates has led to applications in geodesy. ”

p.498

  “ The effect of seismic events is to induce frequency-modulated side bands, in the 0.2-1 Hz region, around the 'Earth line', which indicate the presence of rotational components associated with seismic events. ”

University of Canterbury

In a PhD thesis by B Tom King it is admitted that the nature of the Earth induced line is unknown and unverified.

  “ While lasingin single longitudinal mode and unlocked under the Earth's rotation a Lissajous figure is generated on an oscilloscope with the Sagnac signal from G0 on the x-axis and a sinusoid (from a signal generator) on the y-axis. The figure can be made stationary by varying the frequency of the generated signal. Once achieved this becomes an extremely sensitive detector of phase shifts in the G0 output. A variety of tests can then be performed. Artificial seismic events are easily detected as a jitter in the Lissajous figure. Also the introduction of a magnetic field near the gain medium has the effect of shifting the beat frequency and hence causes the Lissajous figure to rotate at a constant rate. However, the question remains as to whether or not this signal is necessarily the Earth induced line. It is difficult to impose a rotation on the laser as it is rigidly mounted to a solid concrete wall which in turn is moulded around the solid volcanic rock that forms the interior cavern wall. It is conceivable that the signal can be traced back to some oscillation in the gain medium. This problem is yet to be clarified. ”

The author goes on to deduce that because the earth induced line agrees with an equation (which was possibly made after the fact to match observation), that it is assumed that the rate is indeed from the earth.

  “ However, it is assumed that the signal is indeed the Earth induced rate mostly because of the excellent agreement between the measured output frequency (see Figs. 6. 7 and 6.8) and the expected frequency of 287. 75 ± 0.46 Hz. ”

On p.152 it is shown that the rotation rate of the earth involves the input of 86164s into the equation, which is the Sierael Day. The equation is using a predefined period to assess a daily variation in the (seismic) noise and create an output.

Siderial day rotation rate.png

The 'Sidreal Day' also happens to be the time it takes for the sun, stars, and celestial bodies to return to their spots above the earth. This is opposed to the Solar Day, which is supposedly the true rotation of the earth.

Seismic Wave Propagation

Background Noise

From http://microglacoste.com/gPhoneNoise/gPhoneSeismicNoise.pdf we read:

  “ It is interesting to speculate on the precise origin of the background seismic noise. Haubrich et al ii for example, open their article with the following description of the seismic noise background and the large interest it has generated over the years as well as the intractability of its investigation:

The low‐level background unrest of the earth, called microseisms or earth noise, has puzzled seismologists and other scientists for nearly a century. The problem of its nature and causes has proved particularly unyielding, not, however, for lack of investigation. A bibliography covering work up to 1955 [Gutenberg and Andrews, 1956] iii lists over 600 articles on the subject; one covering the years from 1955 to 1964 [Hjortenberg, 1967] iv lists 566. Unfortunately, much of this work has advanced the subject but slightly. ”

Airborne Transmission

https://en.wikipedia.org/wiki/Seismic_wave

  “ Primary waves (P-waves) are compressional waves that are longitudinal in nature. P waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name "Primary". These waves can travel through any type of material, including fluids, and can travel nearly 1.7 times faster than the S waves. In air, they take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite. ”

https://amp.livescience.com/24209-earthquakes-infrasound.html

  “ As earthquake waves ripple through the Earth, the crust buckles, rumbles and roars — both audibly and at infrasonic frequencies, below the range of human hearing. A new study finds the Earth's surface acts like a speaker for low-frequency vibrations, transmitting an earthquake's infrasonic tumult into the air. ”