NGS, October 2009

land-based gravity land-based gravity measurements using absolute measurements using absolute

and superconducting gravimetersand superconducting gravimeters

M. Van Camp1 & O. Francis2

1Royal Observatory of Belgium2U. Luxembourg

Average (Jülich not included) : 1.21.4 nm/s²/yrSubsidence of 0.60.7 mm/yr (1 nm/s² 0.5 mm) (1 )

Slow oscillations?Cause: Hydrology?(see poster)

Example of repeated AG measurements

403 nm/s²/yr

The Membach Geodynamic Station

AG: since 1996:1 measurement /1 month

SG: continuously since 1995

PSDs of AG and SG (Membach)

Fractional Brownian noise: = -1.25

Probably white AG instrumental noise : 100 nms-2 Hz-0.5

@ 5 s period : resolution = (1.0E4/(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

AG & SG provide same information

10 days 1 day100 days

Van Camp et al., JGR, 2004

Toward FOGM?

(see poster)

AG meets SG at 1 day

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 );

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).

Conclusions