landbased gravity measurements using absolute and superconducting gravimeters

land-based gravity measurements using absolute and superconducting gravimeters M. Van Camp 1 & O. Francis 2 1 Royal Observatory of Belgium 2 U. Luxembourg NGS, October 2009

Example of repeated AG measurements 40 3 nm/s²/yr Slow oscillations? Cause: Hydrology? (see poster) Average (Jülich not included) : 1. 2 1. 4 nm/s²/yr Subsidence of 0. 6 0. 7 mm/yr

The Membach Geodynamic Station SG: continuously since 1995 AG: since 1996: 1 measurement /1 month

PSDs of AG and SG (Membach) 100 days 10 days Toward FOGM? (see poster) 1 day Fractional Brownian noise: k = -1. 25 AG & SG provide same information AG meets SG at 1 day Probably white AG instrumental noise : 100 nms-2 Hz-0. 5 @ 5 s period : resolution = (1. 0 E 4/(2*5))0. 5 =30 nm/s² White SG instrumental noise : 2. 2 nms-2 Hz-0. 5 e. g. : @ 1 h period : resolution = (5/(2*3600)) 0. 5 = 0. 03 nm/s² @ 100 s period : resolution = (5/(2*100))0. 5 = 0. 2 nm/s² Van Camp et al. , JGR, 2004

Conclusions Based on collocated SG/AG measurements : AG “set-up” noise ~16 nm/s²; At periods longer than ~1 -2 months : both the AG and SG tell the same story; In Belgium and Germany : no significant gravity rates of change > 3 nm/s²/yr 1. 5 mm/yr (2 s); Hydrological effects should not prevent one to measure slow tectonic processes (unless climate changes? ) (Van Camp et al. , JGR, in review); Worst case: 1 nm/s²/yr needs 16 years Conservative result based on 18 short SG time series (spanning 5 -13 years); This may be revised when similar analyze performed on longer time series; Measuring slow processes using AGs should not be hopeless (the AG profile in Belgium and Germany seems to confirm this).