A GRAVITY SURVEY NEARBY CORNELL UNIVERSITY LOOKING FOR STRUCTURES POTENTIALLY INTERFERING WITH A PROPOSED GEOTHERMAL CAMPUS HEATING PROJECT
Confusingly, a region of the planned survey located on recent sediments in areas close to downtown Ithaca produced standard deviations about a factor of 10 higher than the rest of the survey. In order to assess the the problem, we recorded the full 6 Hz sampled time series downtown in the early morning – a quiet period for traffic noise. A similar time series was recorded at our base station during business hours.
Compared to our base station in the hills, the downtown time series displayed unmistakable signs of aliasing frequencies higher than the 3 Hz Nyquist frequency into the recordings — even with the CG5's "seismic" filter active. This caused us to abandon plans to measure on the flat sediments near downtown Ithaca. Interestingly, Cladouhos et al. (1989) had identified strong Mercali intensities in the same region from the 1988 Saguenay (Quebec) Earthquake. We suggest that such basin amplification (resonant chamber?) seismic effects might well contribute to noisy gravity readings with Scintrex CG5 gravity meters in other, geologically or mechanically similar regions.
Cladouhos, T., Bekele, E., & Cahill, T. (1989). The November 25, 1988. Saguenay earthquake: Isoseismal survey in Ithaca region, central New York. Seismological Research Letters, 60 (3), 131-133.