Bouguer gravity

Bouguer gravity is a representation of the Earth's gravity field obtained by eliminating variations of the gravity field related to: differences in elevation of the sites at which gravity is measured, differences in latitudinal position, influences of topography, fluctuations in gravity at a site caused by Earth tides (similar to the ocean tides, but in this case the solid Earth undergoes small distortions, which influence the gravity field). After corrections have been applied for all of these influences, the theoretical value of the Earth's gravity field at a measurement point is subtracted from the corrected measured (or observed) values and the resultant difference is referred to as Bouguer gravity. Differences in a map of Bouguer gravity, theoretically, are related to differences in rock density. Where the density is larger, the Bouguer gravity field is stronger, which is often expressed as being more positive. Gravity maps provide patterns of gravity anomalies that may be related to specific rock units or structures (e.g. faults). Such patterns are extremely useful for tracing said unit or structure under glacial overburden, water, desert sands. Certain mineral resources may be associated with minerals that have higher than average density (e.g. copper, zinc, iron, lead) or lower than average (e.g. salt, potash, which produce prominent local perturbations (anomalies) in the gravity field.

A Bouguer gravity anomaly map (free-air anomalies offshore) of the Slave Province is shaded by simulated light inclined at 500, emanating from a source at 1100 declination. Station spacing on land generally ranges from about 10 to 15 km, with a probable mean value lying near the middle of this range. Offshore, spacing is about 6 km. The gravity data set used to generate the map Bouguer gravity anomalies on land, and Free-air gravity anomalies offshore are available for download from the Natural Resources Canada Geoscience Data Repsitory.

Outline of Slave Province defined by the solid black line.

The Bouguer gravity field is expressed in the unit of milligal, abbreviated to mGal. An acceleration of 1 cm/s2 is referred to as 1 Gal (1 cm/s2 is a sub-multiple of the fundamental SI unit for acceleration which is 1 m/s2: SI is the abbreviation for the Système international d'unités (International System of Units)). The Gal is a unit that is permitted for use with SI. This unit is too large for magnitudes of changes in gravitational attraction commonly encountered in geophysical work, and so a quantity one thousand times smaller, the milligal (mGal) is commonly employed. Gravitational attraction at the surface of the Earth is roughly 980 Gals. Gravity meters generally measure this attraction to a precision of 0.01 mGal. Hence it is possible to measure approximately 1/100,000,000 of the Earth's total attraction.

Rock Density

Locations of rock samples measured for density data are superposed on the above Bouguer gravity anomaly map. The size of the dot representing the sample location is proportional to the density value. The physical properties of rocks control the measured geophysical fields, and knowledge of such constitutes a critical aid to calibrating geophysical anomalies with geology, and for modelling studies. The density data set depicted on the above map is based on measurements of density carried out on samples collected at regional gravity stations by the Dominion Observatory (later the Earth Physics Branch of the Department of Energy, Mines and Resources) during the national gravity mapping program. Because rocks in the area are generally crystalline and presumably have very low porosities, the reported densities may be considered as representative of the respective rock units at depth below the water table.

Rock density is a measure of the mass of a rock relative to the mass of an equivalent volume of water. One cubic centimeter of water weighs 1 gram, whereas 1 cubic centimeter of rock typically weighs from about 2 to 3 times more. Density is expressed as the ratio of the mass (weight) of a material to its volume. Water has a density of 1, since water with a mass of 1 gram has a volume of 1 cm3. A rapid method for determining the density of a rock is to weigh it in air, and then weigh it when it is submerged in water (in which it weighs less). The density is determined by dividing the weight in air by the difference between the weight in air and weight in water, and is expressed in units of g/cm3 or in the SI system as kg/m3.

The density data set are available for download [XLS, 137.2 kb] and contain the sample number, latitude, longitude, density (g/cm3), rock-type, easting and northing. The easting and northing values are for a NAD83 Lambert conformal conic projection with standard parallels of 62° and 67°, a latitude of origin of 64.5°, a central meridian of 112°, and a false easting and northing equal to zero.

Location of reflection seismic data obtained as part of the LITHOPROBE program is indicated on the above map by a white line passing through Yellowknife. LITHOPROBE is Canada's national Earth science research project to investigate the 3-dimensional structure and evolution of Canada's landmass and continental margins.