may not be mapped directly for some time to come. A much better spatial resolution is
achievable by satellite altimetry to the ocean surface but the possibility of detecting
tectonically-generated, temporal variations in the gravity field needs to be demonstrated.
Terrestrial methods
With the introduction offield-worthy absolute gravimeters (Faller et. al., 1983) terrestrial
gravity measurements have recently achieved the accuracy necessary for the study of
temporal variations over a variety of spatial scales. Formerly, the monitoring of temporal
gravity variations was carried out using relative gravimeters and was restricted by cost and
accuracy to relatively small areas (50 km X 100 km)(e.g., Dragert et aI., 1981; Becker et aI.,
1985) or along profiles connected by roads (e.g., Makinen et aI., 1986). Precise gravity
networks maintained by relative gravimeters become labour intensive and costly when
accuracy is maintained as the number of stations and the distances between them increase.
The International Absolute Gravity Basestation Network (IAGBN) was proposed with the
objective of determining the lower frequency motions and deformations of points fixed on
the Earth's crust and determining the gravitational potential and its temporal variations on
and outside the Earth's surface (Boedecker and Fritzer, 1986). Stations (36) were selected
to achieve an even distribution around the globe with a slight decrease in the extreme polar
regions (Figure 2). Care was taken to avoid plate boundary zones where large gravity
variations may mask deeper seated processes. It is obviously difficult, even for a relatively
small number of stations, to achieve the even global distribution of stations desired. There are
vast oceanic areas that remain unsampled. Although significant progress has been made in the
measurement of absolute gravity on the ocean bottom (Zumberge et aI., 1993), it is not likely
that the high accuracy (1-2 flGal) required to measure temporal variations on the ocean
bottom will be achieved over the next decade.
In spite of the spatial sampling problem faced by terrestrial gravity measurements, it has
been shown that changes in the location of the geocenter could be determined to ± 2 cm in
each co-ordinate from secular changes in gravity observed in a network similar to the
IAGBN; measurements over a period greater than 10 years are required, if random errors of
± 10 flGal are experienced (Mather et aI., 1977). As will be shown below, absolute
gravimeters with accuracies and stabilities significantly better than 10 flGal are now available
but a program of more regular observations at IAGBN stations is required.
The measurement of temporal variations of gravity by point observations on the Earth's
surface must take into account the problem of temporal and spatial aliasing. Assuming that
semi-annual observations are carried out at each point in a gravity network, all phenomena
with a period shorter than one year will be aliased into the variations of longer period.
Similarly, phenomena having spatial wavelengths shorter than twice the aperture or station
spacing of a network will be aliased into longer wavelength variations (Figure 1). The
gravitational effect of tides and synoptic atmospheric mass movement are generally reduced
to an insignificant level by modelling. It is hoped that seasonal groundwater effects can be
much reduced by comparing the fluctuations of semi-annual gravity observations with other
groundwater indicators. A dense, uniform network of stations over the Earth's surface
sampled semi-annually would eventually yield direct measurements of a large number of
phenomena. This is not possible in practice. Therefore, networks with different station
spacing are required for different applications. The IAGBN with a station spacing of about
6,000 km, for example, could resolve phenomena of truly global scale, such as global sea level
effects and large effects in the mantle, the core or the atmosphere (Boedecker and Kopaev,
1995). Shorter-scale phenomena, such as, postglacial rebound would be aliased into the
measurement of these truly global-scale effects. Networks or profiles with station spacing of
23
achievable by satellite altimetry to the ocean surface but the possibility of detecting
tectonically-generated, temporal variations in the gravity field needs to be demonstrated.
Terrestrial methods
With the introduction offield-worthy absolute gravimeters (Faller et. al., 1983) terrestrial
gravity measurements have recently achieved the accuracy necessary for the study of
temporal variations over a variety of spatial scales. Formerly, the monitoring of temporal
gravity variations was carried out using relative gravimeters and was restricted by cost and
accuracy to relatively small areas (50 km X 100 km)(e.g., Dragert et aI., 1981; Becker et aI.,
1985) or along profiles connected by roads (e.g., Makinen et aI., 1986). Precise gravity
networks maintained by relative gravimeters become labour intensive and costly when
accuracy is maintained as the number of stations and the distances between them increase.
The International Absolute Gravity Basestation Network (IAGBN) was proposed with the
objective of determining the lower frequency motions and deformations of points fixed on
the Earth's crust and determining the gravitational potential and its temporal variations on
and outside the Earth's surface (Boedecker and Fritzer, 1986). Stations (36) were selected
to achieve an even distribution around the globe with a slight decrease in the extreme polar
regions (Figure 2). Care was taken to avoid plate boundary zones where large gravity
variations may mask deeper seated processes. It is obviously difficult, even for a relatively
small number of stations, to achieve the even global distribution of stations desired. There are
vast oceanic areas that remain unsampled. Although significant progress has been made in the
measurement of absolute gravity on the ocean bottom (Zumberge et aI., 1993), it is not likely
that the high accuracy (1-2 flGal) required to measure temporal variations on the ocean
bottom will be achieved over the next decade.
In spite of the spatial sampling problem faced by terrestrial gravity measurements, it has
been shown that changes in the location of the geocenter could be determined to ± 2 cm in
each co-ordinate from secular changes in gravity observed in a network similar to the
IAGBN; measurements over a period greater than 10 years are required, if random errors of
± 10 flGal are experienced (Mather et aI., 1977). As will be shown below, absolute
gravimeters with accuracies and stabilities significantly better than 10 flGal are now available
but a program of more regular observations at IAGBN stations is required.
The measurement of temporal variations of gravity by point observations on the Earth's
surface must take into account the problem of temporal and spatial aliasing. Assuming that
semi-annual observations are carried out at each point in a gravity network, all phenomena
with a period shorter than one year will be aliased into the variations of longer period.
Similarly, phenomena having spatial wavelengths shorter than twice the aperture or station
spacing of a network will be aliased into longer wavelength variations (Figure 1). The
gravitational effect of tides and synoptic atmospheric mass movement are generally reduced
to an insignificant level by modelling. It is hoped that seasonal groundwater effects can be
much reduced by comparing the fluctuations of semi-annual gravity observations with other
groundwater indicators. A dense, uniform network of stations over the Earth's surface
sampled semi-annually would eventually yield direct measurements of a large number of
phenomena. This is not possible in practice. Therefore, networks with different station
spacing are required for different applications. The IAGBN with a station spacing of about
6,000 km, for example, could resolve phenomena of truly global scale, such as global sea level
effects and large effects in the mantle, the core or the atmosphere (Boedecker and Kopaev,
1995). Shorter-scale phenomena, such as, postglacial rebound would be aliased into the
measurement of these truly global-scale effects. Networks or profiles with station spacing of
23