Nelson-Nordgren, Astatic Spring, Broadband Force Balance, Vertical (FBV) Seismometers

Using the magnitude squared coherence function, gamma^{2}, to help dig out Spherical Earth modes

These Spherical Earth Modes were observed from two broad band Yuma seismometers, in the basement of a residential home in Princeton Junction, NJ. The high gain output, 28,300 V/m/s, was used with a period extending inverse filter, in WinSDR, which increases the response down to approximately 0.002Hz.
The signals used were from the M8.2 Chilean earthquake of 4/1/2014, starting at 4/2/2014 05:00:00 UTC and continuing for 91 hours. The Fourier spectrum of each signal was point-by-point averaged, de-trended, and scaled to produce a single spectrum. This spectrum was then compared to gamma^{2} in three frequency bands: 2.0mHz-3.5mHz, 3.5mHz-5.0mHz, and 5.0mHz- 6.0mHz. The following graphs for each band show the overlay of coherence and Fourier spectrum and the points singled out for frequency measurement. The interpretation is somewhat seat-of-the-pants at times, but the values seem to agree with published results. The frequency values and digits of precision are reported by Sigview and there was no attempt to put error bars on the results.

## ----Measured Modes---- |
||
---|---|---|

Mode | mHz | |

0S12 ----- | 2.0203 | |

0S13 ----- | 2.0935 | |

? ---------- | 2.1759 | |

0S14 ----- | 2.2888 | |

0S15 ----- | 2.359 | |

0S16 ----- | 2.42 | |

? ---------- | 2.4963 | |

0S17 ----- | 2.5818 | |

0S18 ----- | 2.6703 | |

0S19 ----- | 2.774 | |

0S20 ----- | 2.8748 | |

0S21 ----- | 2.9785 | |

0S22 ----- | 3.0975 | |

0S23 ----- | 3.1769 | |

0S24 ----- | 3.2684 | |

0S25 ----- | 3.36 | |

0S26 ----- | 3.4546 | |

0S27 ----- | 3.5492 | |

0S28 ----- | 3.6285 | |

0S29 ----- | 3.7415 | |

0S30 ----- | 3.8147 | |

0S31 ----- | 3.9063 | |

0S32 ----- | 3.9825 | |

0S33 ----- | 4.0833 | |

0S34 ----- | 4.1656 | |

------------ | ------ | |

0S40 ----- | 4.715 | |

0S41 ----- | 4.7852 | |

0S42 ----- | 4.8889 | |

0S43 ----- | 4.9927 | |

0S44 ----- | 5.0964 | |

0S45 ----- | 5.1636 | |

0S46 ----- | 5.2704 | |

0S47 ----- | 5.3223 | |

0S48 ----- | 5.4169 | |

0S49 ----- | 5.481 | |

? ---------- | 5.5389 | |

0S50 ----- | 5.6091 | |

0S51 ----- | 5.7007 | |

0S52 ----- | 5.7831 | |

0S53 ----- | 5.8899 |

Coherence and Spectrum plots for 2.0mHz-3.5mHz.

Coherence and Spectrum plots for 3.5mHz-5.0mHz.

Coherence and Spectrum plots for 5.0mHz-6.0mHz.