The grid of the first vertical derivative 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. Shorter wavelength anomalies, representing nearer surface density contrasts, are enhanced by the derivative filter.

This grid presents the first vertical derivative 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 grid was converted to the frequency domain using a fast Fourier transform. The vertical derivative transform function was applied to the frequency domain data. The data were upward continued by 2 km and then returned to the spatial domain using an inverse fast Fourier transform. The derivative filter enhances the short wavelength component at the expense of longer wavelength anomalies and helps resolve closely spaced, even superposed, anomalies.

The gravity anomalies correspond to variations in lateral density and mass in the upper mantle and the crust. Most high-frequency anomalies are caused by near-surface contacts of rocks that have significantly different densities. These anomalies are enhanced by the vertical derivative filter. For example, in the Superior Province of the Canadian Shield, granitic plutons are less dense than the volcanic rocks they intrude and are local gravity lows. The boundary between the Trans-Hudson Orogen and the Superior Province is marked by an extensive, curvilinear gravity high that corresponds to the denser material of associated magmatic arcs. The Kapuskasing Uplift in the Superior Province is represented by a vertical derivative high due to highly metamorphosed, high density rock uplifted to the surface, as well as denser material underplating the uplifted crust. Longer wavelength anomalies are generally associated with variations in crustal thickness and tend to be attenuated by the vertical derivative filter.

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