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Elevations in 7,500 exploratory wells from 100,000 mi2 (250,000 km2) of the Interior Plains in southern and central Alberta were used to construct structure-contour maps of four stratigraphic surfaces: top of the Devonian carbonate rocks, sub-Cretaceous unconformity, base of the Fish Scales Sandstone, and top of the First White Speckled Shale. Each map, which shows structures of various scales as undulations, may be represented by the sum of a series of sinusoid surfaces of various frequencies, amplitudes, phases, and orientations--the larger undulations are represented mainly by the lower spatial frequencies, and the smaller undulations by the higher frequencies. Spatial filtering permits the preparation of subsidiary maps that show only those undul tions contained in the original maps that fall within a predetermined range of scales. This process requires a digital computer and involves (1) determining the spatial frequencies in the input map and in the desired undulations, (2) designing a spatial filter in the frequency domain to suppress frequencies other than those in the desired undulations, (3) transforming this filter into the distance domain, and (4) convolving it with the digitized input map.
In this study one filter was used to produce four maps showing undulations with widths of 20-40 mi (32-64 km). The dominant trends, northeast-southwest and northwest-southeast, parallel those in the underlying Precambrian basement. Relief, up to 600 ft (185 m), is greatest on the unconformity and least on the Cretaceous surfaces. Many of the undulations probably represent tabular structures bounded by steeply dipping faults, one of which may be a northwest-trending wrench fault with a minimum displacement of 10 mi (16 km). Some undulations are coincident with known petroleum occurrences.
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