CONTINUOUS GRAVITY RECORDING WITH SCINTREX CG-3M METERS - A PROMISINGTOOL FOR MONITORING ACTIVE ZONES

We acquired continuous series of microgravity measurements using sever al Scintrex CG-3M gravity meters for several weeks in 1997. The meters with 1 mu Gal resolution were installed side by side in a stable refe rence station at the ORSTOM research centre to perform identical data acquisition. We present and compare the instrumental responses obtaine d for the various gravity meters (measurement series of gravity held, standard deviation, internal temperature, tilts) and analyse their cor relation with simultaneous recordings of meteorological parameters. Th e data have been processed in order to (1) establish the mid- to long- term relative stability and the accuracy of the instruments, (2) estim ate the contribution of instrumental effects to gravity data measureme nts and (3) quantify the amplitude of the time variations of the gravi ty field that might be detected with such instruments. This study emph asizes the sensitivity of some instrumental responses of the Scintrex CG-3M gravity meters (such as internal temperature or tilt) to local a tmospheric-pressure variations. This sensitivity can lead to non-negli gible perturbations of the gravity measurements through automatic corr ections applied in real-time mode by the integrated software. We show that most of these instrumental artefacts can be easily removed in dat a post-processing by using simultaneous atmospheric-pressure data. Aft er removal of an accurate Earth tide model, the instrumental drift and the instrumental effects, the temporal series are compared by computi ng differential signals. These residual signals obtained over a period of several weeks exhibit the following characteristics: (1) the gravi ty residuals have a maximum amplitude ranging from 5 to 10 mu Gal and from 10 to 15 mu Gal for filtered and unfiltered data, respectively; a nd (2) the standard error, tilts and internal temperature measurements of the various gravity meters are very consistent; their respective r esidual amplitudes are +/-2 mu Gal, +/-3 arcsec and +/-0.05 mK. In ord er to calibrate the gravity meters precisely in the measurement range used in this study, we have measured a calibration line established in the framework of the fourth intercomparison of absolute and relative gravity meters. This calibration was achieved with an accuracy of 5 mu Gal. This result is consistent with other held tests already performe d with such gravity meters. In addition, we also checked the accuracy of the tilt sensors by increasing the electronic read-out by a factor of 10. The tilt response of the whole gravity meter to a small induced inclinometric variation indicates that the precision of the tilt meas urements is about a few tenths of an are second. This study reveals th at temporal variations of the gravity field could potentially be detec ted in the held with an accuracy of about 5-15 mu Gal by permanent net works of Scintrex CG-3M gravity meters set up a few kilometres apart. This result is of particular interest in held surveys of temporal grav ity changes related to some environmental or geodynamical processes, w here the expected gravity variations are greater than a few tens of mu Gal. In particular, in volcanological applications, the continuous mo nitoring of active volcanoes with such permanent networks of gravity m eters co-located with subcentimetre-accuracy GPS receivers should be v ery helpful to understand internal magmatic processes better and to de tect possible gravity and inclinometric signals occurring during pre-e ruptive phases. In this field, continuous microgravity recordings asso ciated with classical reiteration networks will probably improve hazar d mitigation in the near future.