2008-10-07_GENERAL DOCUMENTS - M2005071 (2)

Home Browse Search

summary dated August 8, 2008 he states "In the recent testing at the Red Shale Quarry site, the peak ground motion caused by the D9 Dozer, at a distance of 20 feet was 0.08 in/s. This value is less than the peak predicted value of 0.011 in/s". Since when is 0.08 less than 0.011? Due to the decreased sensitivity of the recording instruments used in the latest study they could not detect vibrations at distances greater than a couple hundred feet. As a result, Mr. Revey implies in his conclusions that the vibrations were so small they were immeasurable! I operate a seismograph station on my property that is part of the Mesa State seismic network and is registered in the international directory of seismic stations (station CECO). Although I am 0.67 of a mile (3540 feet) from the Benson quarry site, ripping of bedrock at the quarry creates such strong vibrations that my instrument goes off scale and the signals are clipped! These vibrations are far from immeasurable! Spectral analysis of my data shows that during ripping the vibrations have a peak frequency of 6.5 Hz (see Figure 1). When the rock crusher is operating without any ripping it generates a strong narrow resonance at a frequency of 4 Hz (see Figure 2) which is exactly the frequency that I predicted in my letter in February, 2006 for the fundamental mode of a standing wave between the crusher and the cliff face. This prediction was challenged by Mr. Revey saying "this calculations has no meaningful purpose and is not technically supportable". His critique of my calculation, given in his letter dated 4/19/2006, indicated that he had no understanding of how to compute the fundamental frequency of standing waves and their harmonics. This is something that can be verified in any book on basic physics. Part of Mr. Revey's criticism was based upon his belief that no low frequency ground motions were generated by heavy equipment. He claims on page 2 of his letter that the minimum recorded frequency at the Austin, Colorado site was 14 Hz and that the average frequency was over 35 Hz. The heavy equipment used for mining at the Benson quarry generates most vibration energy at frequencies below 10 Hz as seen in Figure 1. The peak vibration energy occurs at 6.5 Hz, and the amplitude is down by an order of magnitude by 10 Hz. My recording system has a high-cut filter starting at 10 Hz that rolls off at 20 db/octave. Removal of this filter slope still indicates the spectrum is declining out to the 25 Hz Nyquist frequency corresponding to the sampling rate of 50 samples/second. For anyone experienced in seismic monitoring to think that heavy equipment does not generate low frequency vibrations is very disconcerting. If Mr. Revey is correct about the minimum recorded frequency, it raises a concern that a low-cut filter may have been set in the recording instruments. If this was the case then the data would be useless since the major portion of ground motions would have been filtered out before recording. One of the standard and critical quality controls in seismic monitoring is to supply the impulse response of the instrumentation so that the frequency response of geophones and instrument recording filters can be verified. This was not provided with either study, and was apparently not required by the DMLR staff overseeing these studies, so the source of this discrepancy cannot be determined. The maximum clipped amplitude that I recorded during ripping is 0.67 x 10- 3 in/s, so the actual peak particle velocities exceed this value by some unknown amount.