This review covers basic theory and techniques behind the use of ground-based gravimetry at the Earth's surface. The orientation is toward modern instrumentation, data processing and interpretation for observing surface, land-based, time-variable changes to the geopotential. The instrumentation side is covered in some detail, with specifications and performance of the most widely used models of the three main types: the absolute gravimeters (FG5, A10 from Micro-g LaCoste), superconducting gravimeters (OSG, iGrav from GWR instruments), and the new generation of spring instruments (Micro-g LaCoste gPhone, Scintrex CG5 and Burris ZLS). A wide range of applications is covered, with selected examples from tides and ocean loading, atmospheric effects on gravity, local and global hydrology, seismology and normal modes, long period and tectonics, volcanology, exploration gravimetry, and some examples of gravimetry connected to fundamental physics. We show that there are only a modest number of very large signals, i.e. hundreds of µGal (10(-8) m s(-2)), that are easy to see with all gravimeters (e.g. tides, volcanic eruptions, large earthquakes, seasonal hydrology). The majority of signals of interest are in the range 0.1-5.0 µGal and occur at a wide range of time scales (minutes to years) and spatial extent (a few meters to global). Here the competing effects require a careful combination of different gravimeter types and measurement strategies to efficiently characterize and distinguish the signals. Gravimeters are sophisticated instruments, with substantial up-front costs, and they place demands on the operators to maximize the results. Nevertheless their performance characteristics such as drift and precision have improved dramatically in recent years, and their data recording ability and ruggedness have seen similar advances. Many subtle signals are now routinely connected with known geophysical effects such as coseismic earthquake displacements, post-glacial rebound, local hydrological mass balances, and detection of non-steric sea level changes.

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We report on the results of a comparative analysis carried out on more than ten months of co-located records collected at the J9 gravity station in Strasbourg (France) with a new generation spring gravimeter, the Micro-g LaCoste, Inc. gPhone-054 and the GWR-C026 superconducting gravimeter (SG-C026). The gPhone is essentially a LaCoste & Romberg, model G meter, but with an improved thermal system (a double oven) for increased temperature stability, which should result in an unprecedented improvement in noise and drift of this mechanical gravimeter. We test the performances of the gPhone-054 in terms of resolution, accuracy, noise level and long-term stability (drift) with respect to the SG-C026. Our comparative analysis is performed in a wide spectral domain, ranging from the body tides to the seismic band. This study confirms that the SG has better performances over the whole analysed spectral band. The gPhone-054 instrumental drift observed during this study still remains a critical point preventing the study of long-term gravity changes. In fact, compared with the SG, the drift is large and even non-linear. We observed a drift rate evolution characterized by a decrease from 50 µGal/day to 15 µGal/day, after about 1 month of operation. This makes it hard to distinguish real-time gravity changes from the instrumental drift. We tried to improve the drift modelling by using frequent absolute gravity (AG) measurements, but unfortunately during that time no significant gravity changes have been detected which would have helped us to discriminate short-term drift excursions from real gravity changes. In terms of noise levels, the gPhone-054 turns out to be about 10 times noisier than the SG-C026 at seismic frequencies, while in the tidal band, it is twice as noisy. In between, at periods ranging from 1 h to 6 h, the gPhone-054 is about 3 times noisier than the SG-C026 but performs slightly better than the Scintrex CG5 in terms of noise level and precision.

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Since 1973, the Gravimetric Observatory of Strasbourg (France) is located in an old fort named J9 and has been the place for various gravity experiments. We present a comparison of the noise levels of various instruments that are or were continuously recording at J9, including the LaCoste&Romberg Earth-Tide Meter ET-5 (1973???1985), the GWR Superconducting Gravimeter TT-T005 (1987???1996), the Superconducting Gravimeter C026 (since 1996), the STS-2 seismometer (since 2010) and the LaCoste&Romberg ET-11 (continuously since October 2010). Besides these instruments, the J9 Observatory has hosted temporary gravity experiments with the Micro-g LaCoste Inc. gPhone-054 (May???December 2008 and May???September 2009) and the Micro-g LaCoste Inc. Graviton-EG1194 (June???October 2011). We include also in the comparison the absolute gravimeter Micro-g FG5 #206 which is regularly performing absolute gravity measurements at J9 since 1997 and a spring gravimeter Scintrex CG5 which recorded at J9 between March 2009 and February 2010. We present the performances of these various instruments in terms of noise levels using a standardized procedure based on the computation of the residual power spectral densities over a quiet time period. The different responses to atmospheric pressure changes of all the instruments are also investigated. A final part is devoted to the instrumental self-noise of the SG C026, STS-2 and L&R ET-11 using the three channel correlation analysis method applied to 1-Hz data

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