The grid of the horizontal gradient of gravity anomalies of Canada shows variations in the gravity field caused by lateral variations in the density of the Earth's crust and upper mantle that reflect variations in composition and thickness. Systematic gravity mapping began in Canada in 1944 and is ongoing. All data are tied to the International Gravity Standardization Network 1971. Local gravity anomalies result from the juxtaposition of relatively high- and low-density rock types. The horizontal gradient achieves local maxima over or near contacts between rock units with different density.

This grid presents the horizontal gradient magnitude of Bouguer gravity anomalies on land and free-air gravity anomalies offshore. The data were compiled from the holdings of the Canadian Geodetic Information System maintained by the Geodetic Survey Division, Geomatics Canada. They were collected to map the variation in gravitational attraction over the Canadian landmass and offshore areas. Variations in the force of gravity are due to variations in the mass of underlying materials. These data are useful for geological interpretation and have applications in oil, gas, and mineral exploration. The gravity field is also used to define the geoid, which is the ideal shape of the Earth, or mean sea level if the Earth were completely covered with water.

The data used to compile this grid consist of approximately 678 000 gravity observations, including 165 000 on land, acquired between 1944 and 2003. The data spacing ranges from less than 1 km to over 20 km, with an average spacing between 5 and 10 km. All measurements were reduced to the IGSN71 datum. Theoretical gravity values were calculated from the Geodetic Reference System 1980 (GRS80) gravity formula. The observation's coordinates are referred to NAD83. Bouguer anomalies were calculated using a vertical gravity gradient of 0.3086 mGal·m^-1 and a crustal density of 2 670 kg·m^-3. Areas on land are represented by Bouguer anomalies and areas offshore, by free-air anomalies. The data were gridded to a 2 km interval, with a blanking radius of 20 km. The absolute value of the horizontal gradient magnitude was calculated from a plane generated from a least squares fit of a 5 x 5 array of grid cells centred on the calculation point. The filtered data enhance short wavelength anomalies that correspond to near-surface density contrasts. The magnitude of the horizontal gradient reaches a maximum over contacts between bodies with contrasting densities. Cordell (1979) showed that the magnitude reaches a local maximum directly over vertical contacts and is offset in the down dip direction over dipping contacts.

Cordell, L., 1979. Gravimetric expression of graben faulting in Santa Fe Country and the Espanola Basin, New Mexico. New Mexico Geol. Soc. Guidebook, 30th Field Conference, p. 59-64.

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