The measurement of surface gravity

Abstract

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.

Volcanic signatures in time gravity variations during the volcanic unrest on El Hierro (Canary Islands)

“…Following a common procedure for processing time gravity series [Hinderer et al, 2007;Crossley et al, 2013], the residual gravity, g res , can be expressed as…”

GRACE, time-varying gravity, Earth system dynamics and climate change

“…V (r, θ, λ) = a e l=0 m=0 r P lm (cos θ)(C lm cos mλ + S lm sin mλ) (1) where G is the gravitational constant, M the mass of the Earth and a e denotes its mean equatorial radius. P lm are the Legendre polynomials of degree l and order m, and C lm and S lm are the spherical harmonic coefficients.…”

Nonlinear Drift of the Spring Gravimeter Caused by Air Pressure from the Kunming GS15 Gravimeters

“…Meanwhile, using model (1) in the elimination process of the above preprocessing to simulate gravity residual time series which includes: formation process of the spring relaxation, change of the linear shift before and after the machine failure and the nonlinear drift of the spring gravimeter caused by drift or other causes before and after the failure, and correct them [10,11]. Figure 1 is the pre-processing with Kunming GS15 gravimeter as asn example.…”

“…Comparing the above pressure-corrected gravity residual time series with the gravity change caused by this area water-loaded seasonal factor which is simulated by satellite gravity (Grace) and global land water model (Gldas) [11,12], the result shows that the above two all reach the smallest gravity in fall in 2012, 2013, while reaching the biggest in summer, this is the seasonal changing characteristics. The inconsistency of magnitude of the seasonal load influence may be the result of simulation model which simulated the largest space scale, it is not completely consistent with the gravity change of Kunming station Local area.…”