THE ROLE AND CAPABILITY OF ABSOLUTE GRA VITY
MEASUREMENTS IN DETERMINING THE TEMPORAL
VARIATIONS IN THE EARTH'S GRA VITY FIELD
A. Lambert, T.S. James, J.O. Liard and N. Courtier
Geological Survey of Canada
INTRODUCTION
3 Observatory Cr., Ottawa, Ontario
KIA OY3 Canada
The temporal variations of the gravity field are the result of a superposition of the effects of
a large number of processes in the atmosphere, the hydrosphere, the cryosphere and the solid
Earth. A number of geodynamic processes, capable of being detected and studied as a result
of the changes in gravity they produce, can be specified in terms of their principal frequencies
and spatial wavelengths (Figure 1). The processes influencing gravity are a mixture of
primary and secondary effects. For example, polar motion is thought to be a response to
mass movements in the atmosphere, the hydrosphere and the Earth's core (e.g., Eubanks,
1993). Those processes having periods greater than 100 years generally appear as secular
variations with rates of 1 IlGal/yr or less (1 11 Gal = 10 nmfs2). Typical peak-to-peak gravity
variations of several 11 Gal are associated with seasonal groundwater movement, atmospheric
processes and polar motion. The body tides have an associated peak-to-peak gravity
variation of around 100 IlGal. Although monitoring of gravity with relatively drift-free
instrumentation is still in its infancy, spectral analysis of existing data indicates that the nontidal, gravity spectrum is "red", i.e., gravity variations are generally larger at the lowfrequency end of the spectrum. This is consistent with the fact that very-long-period
"geological" processes are associated with large displacements and movements of mass
resulting in gravity anomalies of the order of mGals.
The temporal variations of the gravity field measured on the Earth's surface are the result
of the vertical movement of the observation point through the local gravity field as well as the
direct and indirect (e.g., polar motion) effects of mass redistribution at the point. Repeated
surface gravity observations, therefore, may ultimately be be used as a proxy for vertical
movement observations but not until both the vertical movements and the temporal gravity
variations can be accurately predicted from a model for the process cre:ating them. Surface
gravity observations at present serve primarily as model constraints which, when corrected
for the vertical movement of the observation point, relate to the mass redistribution at depth
associated with the causative process.
This paper examines the role of absolute gravity measurements in the context of satellite
gravimetry and competing terrestrial methods.
SATELLITE AND TERRESTRIAL MEASUREMENT OF TEMPORAL VARIATIONS
Satellite and terrestrial measurements have been combined in a complementary way to provide
an optimum representation of the spatial variations of the Earth's gravity field (Rapp, 1994).
20
MEASUREMENTS IN DETERMINING THE TEMPORAL
VARIATIONS IN THE EARTH'S GRA VITY FIELD
A. Lambert, T.S. James, J.O. Liard and N. Courtier
Geological Survey of Canada
INTRODUCTION
3 Observatory Cr., Ottawa, Ontario
KIA OY3 Canada
The temporal variations of the gravity field are the result of a superposition of the effects of
a large number of processes in the atmosphere, the hydrosphere, the cryosphere and the solid
Earth. A number of geodynamic processes, capable of being detected and studied as a result
of the changes in gravity they produce, can be specified in terms of their principal frequencies
and spatial wavelengths (Figure 1). The processes influencing gravity are a mixture of
primary and secondary effects. For example, polar motion is thought to be a response to
mass movements in the atmosphere, the hydrosphere and the Earth's core (e.g., Eubanks,
1993). Those processes having periods greater than 100 years generally appear as secular
variations with rates of 1 IlGal/yr or less (1 11 Gal = 10 nmfs2). Typical peak-to-peak gravity
variations of several 11 Gal are associated with seasonal groundwater movement, atmospheric
processes and polar motion. The body tides have an associated peak-to-peak gravity
variation of around 100 IlGal. Although monitoring of gravity with relatively drift-free
instrumentation is still in its infancy, spectral analysis of existing data indicates that the nontidal, gravity spectrum is "red", i.e., gravity variations are generally larger at the lowfrequency end of the spectrum. This is consistent with the fact that very-long-period
"geological" processes are associated with large displacements and movements of mass
resulting in gravity anomalies of the order of mGals.
The temporal variations of the gravity field measured on the Earth's surface are the result
of the vertical movement of the observation point through the local gravity field as well as the
direct and indirect (e.g., polar motion) effects of mass redistribution at the point. Repeated
surface gravity observations, therefore, may ultimately be be used as a proxy for vertical
movement observations but not until both the vertical movements and the temporal gravity
variations can be accurately predicted from a model for the process cre:ating them. Surface
gravity observations at present serve primarily as model constraints which, when corrected
for the vertical movement of the observation point, relate to the mass redistribution at depth
associated with the causative process.
This paper examines the role of absolute gravity measurements in the context of satellite
gravimetry and competing terrestrial methods.
SATELLITE AND TERRESTRIAL MEASUREMENT OF TEMPORAL VARIATIONS
Satellite and terrestrial measurements have been combined in a complementary way to provide
an optimum representation of the spatial variations of the Earth's gravity field (Rapp, 1994).
20