PRECIPITATION EFFECTS ON GRAVITY MEASUREMENTS AT THE CANADIAN ABSOLUTE GRAVITY SITE

The effects of ground water and precipitation are investigated with th e aim of reducing an uncertainty of several microgals which remains in both superconducting and absolute gravity measurements at the Canadia n Absolute Gravity Site in the days-to-seasons frequency band after co rrections have been made for tides, polar motion, local atmospheric pr essure and presumed gravimeter drift (in the case of the superconducti ng gravimeter). The study is based on superconducting gravimeter data (GWR12) generated during the period 1990-1993. Daily values of cumulat ive precipitation, maximum potential evapotranspiration, and snowmelt potential are estimated from precipitation data, snow-cover data, temp erature and energy considerations and then regressed on year-long resi dual gravity data, This accounts for about 90% of the variance in each year-long record of gravity residuals. From the annual regression coe fficients, a single set of four coefficients (trend is also fitted) ca n be selected which, with a single reservation, yields a satisfactory fit to the full 3 1/2 years of residual data. The single reservation c oncerns the estimate of snowmelt effect for one of the four spring per iods and needs further attention. Higher-order regression analysis of precipitation on gravity and precipitation on well-level reveals inter esting dynamic effects due to ground water movement. (C) 1998 Elsevier Science B.V. All rights reserved.