Gravimetry for monitoring vertical crustal movements: Potential and problems

Through recent developments in absolute and relative techniques, gravimetry has reached an accuracy of a few hundredths to a few tenths of μm s^{?2 *}in small or large-scale networks, respectively. Consequently, gravimetric techniques can now be employed as one efficient tool for detecting vertical crustal movements. Gravity data are necessary for converting the results of geometric levelling to heights defined in the gravity field, and—by repeated surveys—to control time variations of the height-reference surface. More important is the use of repeated gravity surveys for strengthening and partly replacing levelling, being a time-consuming procedure with unfavourable error propagation over larger distances. The successful application of observed gravity variations with time in the detection of vertical crustal movements depends on the reliability of the conversion factor between gravity and height changes, and on the accuracy of the gravity measurements. The conversion factor should be determined through simultaneous levelling and gravimetry, in representative parts of the survey region. Strategies for establishing gravimetric control are changing now with the availability of transportable absolute gravity meters, and the possibility of accurately calibrating relative instruments and observing small gravity differences with feed-back-systems. Consequently, more attention has to be given now to disturbing effects of environmental character, such as microseismics, atmospheric pressure and groundwater table variations, and to periodic effects such as gravimetric earth tides and polar motion.