Jacques Liard

New constraints on Laurentide postglacial rebound from absolute gravity measurements

Repeated absolute gravity measurements have been made over a period of several years at six sites along a 3000 km-long, mid-continental, North American profile from the coast of Hudson Bay southward to Iowa. With the exception of the southern-most site, the observed rates of change of gravity are significantly higher than rates predicted by current models, such as ICE-3G and a laterally homogeneous, standard Earth. The observed gravity change rates suggest significant modifications, such as a 2 to 3-fold increase in lower mantle viscosity or a 50% increase in Laurentide ice sheet thickness west of Lake Superior.

Canadian Absolute Gravity Program: Applications in geodesy and geodynamics

The vast distances, poor accessibility and need for uniform accuracy in the Canadian Gravity Standardization Network (CGSN) have required that stations be connected by numerous interlocking gravity ties involving expensive air transportation of relative gravimeters. It is expected that the introduction of regular high-accuracy absolute gravity measurements into the Canadian network will ensure that the accuracy of the network is sufficient to meet modern geodetic requirements and will render a selected number of stations suitable for geodynamic studies. As the first step in providing the required absolute measurements, the Geological Survey of Canada has acquired an absolute gravimeter of the direct free-fall type developed by the Joint Institute for Laboratory Astrophysics (JILA), National Institute of Standards and Technology and University of Colorado, Boulder [Faller et al, 1979, 1983].

The Role and Capability of Absolute Gravity Measurements in Determining the Temporal Variations in the Earth’s Gravity Field

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 µGal/yr or less (lµGal = 10 nm/s2). Typical peak-to-peak gravity variations of several µGal 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 µGal. Although monitoring of gravity with relatively drift-free instrumentation is still in its infancy, spectral analysis of existing data indicates that the non-tidal, gravity spectrum is “red”, i.e., gravity variations are generally larger at the low-frequency 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.

Absolute gravity observations on BIPM site A3 during the 1989 and 1994 International Comparisons of Absolute Gravimeters

Observations made with the JILA2 absolute gravimeter operated by the Geological Survey of Canada, during the 1989 and 1994 International Comparisons of Absolute Gravimeters, show that orientation-dependent systematic errors are introduced at the BIPM A3 site. With improved electronics, we can better identify instrument-induced vibrations which have a deleterious effect on the observations. Observations made in 1989 are compared with the measurements made in 1994. Furthermore, instrument orientation in 1994 is shown to have a damaging effect on the observations. This may suggest better designs for absolute gravity piers located on unconsolidated material.

Laboratory Method of Calibrating Lacoste and Romberg Model-D Gravity Meters.

A method for the detection and measurement of periodic errors, also known as “circular errors”, in the LaCoste & Romberg model D gravity meters has been tested in the laboratories of the Geophysical Division of the Geological Survey of Canada (GSC). This method involves a computer controlled apparatus which performs the measurements automatically as well as the preliminary analysis of the observations. The ground work to establish methodologies and analysis techniques was done on the gravity meter D-28 of the GSC.