Trend surfaces are represented generally by power-series polynomials, fitted by the least-squares method to areal geological data. The technique allows the regional component, represented by the trend surface, to be separated from the local components or residuals. Trend surfaces may be used to predict depths to stratigraphic horizons and to extrapolate data from known to unknown areas. Mapping of residuals can delimit structural irregularities that may have functioned as oil traps.

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