A GRAVITY SURVEY NEARBY CORNELL UNIVERSITY LOOKING FOR STRUCTURES POTENTIALLY INTERFERING WITH A PROPOSED GEOTHERMAL CAMPUS HEATING PROJECT

Several exploration geophysical techniques are being deployed in support of Cornell University’s proposed Earth Source Heat (deep direct use geothermal heating) project. The authors recently finished collecting a local gravity survey on Cornell’s Ithaca campus and surrounding areas of the community, in order to identify and locate any lateral mass discontinuities in Ithaca’s subsurface. Gravity readings were collected on a notional 500 meter spacing grid using a Scintrex CG5 gravity meter. The CG5 collects data at 6 Hz. We collected data for 60 seconds at each of approximately 400 sites — yielding about 360 contributory readings to each reported average; minus readings rejected by the meter as being outliers. The median value of our standard deviations was 23 μGal, about a factor of 4 worse than the lab attainable precision for the CG5. Data is currently being processed and interpreted results will be presented at the meeting.

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.

Reference:

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.