The first vertical derivative of magnetic anomalies grid of Canada shows small variations in the magnetic field. The derivative is calculated from the residual magnetic field and enhances the short wavelength component of the field. The magnetic character of a rock depends on its composition and its deformational and metamorphic history. To map these variations, the Geological Survey of Canada has been acquiring aeromagnetic data since 1947. Over the years, more than 500 surveys have been carried out, generally with a flight-line spacing of 800 m and an altitude of 305 m above the ground. These aeromagnetic surveys have been levelled to each other to correct for arbitrary datums, slow variations of the Earth's magnetic field over time, and differing survey specifications. The dominant structural trends of geological provinces, truncation of those trends at structural boundaries, and the characteristic patterns of suture zones can be recognized on the magnetic anomaly data. The magnetic signature of Precambrian basement rocks can be seen through the Phanerozoic sedimentary basin cover. Major dyke swarms can be traced over hundreds of kilometres from their radiating linear magnetic pattern. Oceanic crust has a characteristic striped magnetic pattern that is due to changes in the polarity of the Earth's magnetic field, which occur over intervals of millions of years.

This dataset presents an enhancement of small variations in the magnetic field over Canada, called the 'first vertical derivative magnetic anomalies'. These variations are due to variations in the magnetic properties of the Earth's crust. The data are derived from the holdings of the Canadian Aeromagnetic Data Base maintained by the Geological Survey of Canada (GSC) and have been collected as part of an ongoing program to map the intensity of the Earth's magnetic field over the Canadian landmass and adjacent offshore areas. Aeromagnetic maps are produced from these data at a variety of scales; they are useful for geological mapping and have applications in mineral, oil, and gas exploration.

A magnetic field is produced by the flow of an electrical current. Orbitals of electrons in atoms create magnetic dipole moments. Molecules may also be magnetic dipoles. These can be aligned by an external field by the process of magnetic induction. The ancillary field produced by aligned dipoles, M, augments the magnetizing field, H. The ancillary field M is proportional to and aligned with H for low, external magnetic fields according to the formula M = kH, where k is the magnetic susceptibility. Thus, the magnetic susceptibility of a body measures how magnetized that body can become in the presence of an external field. The unit of magnetic induction is the nanotesla (nT).

The Earth's magnetic field is largely produced by three main sources, the geomagnetic or core field, the induced field, and the remanent field. The core field is generated by the dynamo effect of electric currents flowing in the Earth's liquid core. It varies slowly with time, a change that is called 'secular variation'. The induced magnetic field is the product of the intensity of the geomagnetic field and the magnetic susceptibility of the underlying rocks. Magnetic susceptibility is a physical property of a material that reflects the material's magnetic mineral content and character. Magnetite is the principal mineral phase responsible for ferrimagnetic susceptibility, although pyrrhotite and some members of the titanohematite series may be locally important. Remanent magnetization is also a property of crustal rocks and produces a magnetic field even in the absence of an ambient field. Remanent magnetization records the direction of the geomagnetic field at the time the minerals were magnetized, for example by cooling to a temperature lower than the Curie temperature. The intensity of the remanent magnetization of a rock body depends on the proportion of ferrimagnetic minerals present, the strength of the geomagnetic field at the time of origin of the remanence, and the geological history of the rock (Sharma, 1978). Because of unstable remanence, which over time realigns with the induced field, and the general heterogeneity of the remanent magnetization, induced magnetization is usually dominant. When interpreting magnetic anomalies over land, the effect of remanent magnetization is usually ignored. For oceanic regions, remanent magnetization is the most important factor because, in geological terms, ocean crust is young (younger than 200 million years) and has a relatively simple cooling and deformation history.

The residual total magnetic field is computed from the observed total field by subtracting the International Geomagnetic Reference Field model of the core field, which includes the secular variation and therefore varies slightly from year to year. The result isolates the component of the total field that is dominated by the magnetic effects of the crustal rock units. The residual total magnetic field map is a useful geological mapping tool because it reflects a physical property of the underlying rocks.

Airborne magnetic surveys are conducted with constant flight-line orientations, usually perpendicular to the regional geological strike, and with constant line spacing. The GSC has been acquiring aeromagnetic data since 1947 and current holdings comprise over 11 million line kilometres of data. Most aeromagnetic data were acquired at an altitude of 305 m mean terrain clearance, although over mountainous areas some surveys were flown at a constant barometric altitude, i.e. at a constant level above the highest peak in the survey area. The standard flight-line spacing for regional surveys is 800 m. In areas of deep sedimentary basins, the flight-line spacing was generally increased to 1600 m. In some areas of Canada, particularly in the Western Canada Basin and the Arctic, data were provided by oil and gas exploration companies and other non-GSC sources. In these areas, the flight-line spacing may exceed 6 km. Detailed, high-resolution surveys are also acquired, with flight-line spacings between 150 m and 300 m. However, coincident regional data were generally used in the production of this grid.

Most offshore magnetic surveying is done aboard ships, with a magnetometer towed at a sufficient distance from the ship to make the ship's magnetic effect negligible. The ship-track spacing and orientation depend on the purpose of the survey and possibly also on other geophysical measurements that are usually done at the same time (e.g. gravity, depth sounding); spacing is typically in the order of 5 to 10 km over deep water and less over the continental shelves.

Aeromagnetic surveys over Canada must be levelled to a common datum and to each other to account for secular variations in the orientation and strength of the geomagnetic field, arbitrary magnetometer datums in older surveys, differences in flight-line spacing and orientation, and differences in flying height and data quality.

Most data acquired before the advent of digital data recording (late 1970s) were in the form of analogue profiles and compiled as 1:63 360 or 1:50 000 scale contour maps. These maps have been digitized along the flight lines at intersections with contour lines, gridded to an interval of 812.8 m, and levelled to adjacent surveys. This project began in the late 1970s and lasted about 10 years. The levelling of individual surveys was performed by first subtracting the International Geomagnetic Reference Field for the date and altitude of the survey for each grid. The difference at the boundary of adjacent surveys was removed using a low order polynomial. The remaining errors were locally smoothed out where required (Teskey et al.,1982). The unlevelled line data were archived on a survey by survey basis and this accumulated data resulted in the creation of the Canadian Aeromagnetic Data Base.

The levelling of Canadian aeromagnetic survey profile data was initiated in 1989 by the Ontario Geological Survey in co-operation with the GSC. The project involved making a single master aeromagnetic grid for the province of Ontario at a uniform grid spacing of 200 m (Reford et al., 1990). This required the regridding of the digitized line data to a finer grid cell size and the subsequent transfer of the levelling adjustment that had been applied to the regional grid in the first phase of the project. The existing 812.8 m cell size grid was regridded to match the unlevelled 200 m grid. The grid of the level adjustments was then subtracted from the original, unlevelled, total field grid. The line data for the digitized surveys were extracted by interpolation from the 200 m levelled grid. The levelling adjustments for the digitally acquired surveys were calculated from the adjustment grid and applied directly to the line data. Subsequently, a similar procedure was applied to aeromagnetic survey data from the Atlantic provinces, Manitoba, and Saskatchewan. The procedure was modified slightly for the processing of surveys from Quebec and the Northwest Territories. For these surveys, the levelling adjustment was calculated systematically from the adjustment grid. The adjustment was then applied to the line data, thus avoiding the regeneration of profile data from the levelled grid.

Constant barometric altitude surveys have been flown over the mountainous areas of Western Canada and northern Baffin Island. Linking of the drape-flown, levelled, aeromagnetic data to the constant-altitude, aeromagnetic survey data has been performed by computational draping of the constant-altitude surveys to an idealized 305 m altitude surface. The method used for draping is based on a Taylor series expansion of the magnetic field on the measurement surface (Pilkington and Roest, 1992). The computationally-draped data and the unlevelled, drape-flown data for Manitoba, Saskatchewan, British Columbia, and the Yukon Territory were levelled inhouse to the national datum and stored as profile data in the Canadian Aeromagnetic Data Base. The resulting levelled residual total field data were used to generate the grid.

The dataset presented here is the first vertical derivative of the residual total magnetic field intensity. It quantifies the rate of change in the magnetic field in the vertical direction and is equivalent to measuring the field with two vertically separated magnetometers and dividing the difference in field strength by the separation distance. A 200 m grid of the magnetic data was converted to the frequency domain using a fast Fourier transform. The vertical derivative transform function was applied to the frequency domain data enhancing higher frequencies. The data were then returned to the spatial domain using an inverse fast Fourier transform. The filter removes the long wavelength component and helps resolve closely spaced, even superposed anomalies. The derivative operation also enhances noise in the data which limits the utility of higher order derivatives. The vertical derivative is often used to trace contacts between large scale magnetic domains, as it has a value of zero over vertical contacts (Hood, 1965).

The vertical derivative of magnetic anomalies highlights the variation of magnetic properties in the rocks of the Earth's crust, and therefore provides an indication of the composition, and the deformational and metamorphic history of the underlying rocks. Compared to the total magnetic intensity, the vertical derivative reflects sources closer to the surface. Higher vertical derivatives are generally associated with highly magnetic rocks (for example, iron-rich volcanic rocks). Lower derivatives are generally associated with essentially nonmagnetic rocks (for example, some types of granites).

Hood, P.J., 1965: Gradient measurements in aeromagnetic surveying. Geophysics, v. 30, p. 891-902.

Pilkington, M. and Roest, W.R. 1992: Draping aeromagnetic data in areas of rugged topography; Journal of Applied Geophysics, v. 29, p. 135-142.

Reford, S.W., Gupta, V.K., Paterson, N.R., Kwan, K.C.H., and MacLeod, I.N. 1990: Ontario master aeromagnetic grid: a blueprint for detailed compilation of magnetic data on a regional scale; in 60th Annual International Meeting, Expanded Abstracts; Society of Exploration Geophysicists, Tulsa, Oklahoma, p. 617-619.

Sharma, P.V. 1978: Geophysical Methods in Geology; Elsevier North-Holland Inc., New York, 407 p.

Teskey, D.J., Dods, S.D., and Hood, P.J. 1982: Compilation techniques for the 1:1 million magnetic anomaly map series; in Current Research, Part A; Geological Survey of Canada, Paper 82-1A, p.351-358.