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January 1995

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by Dwain K. Butler, U.S. Army Engineer Waterways Experiment Station, 3909 Halls Ferry Rd., Vicksburg, MS 39180-6199 Abstract: Gravity gradient profiles across subsurface structures that are approximately 2-D contain diagnostic information regarding depth, size, and structure (geometry). Gradient space plots, i.e., plots of horizontal gradient versus vertical gradient, present the complete magnitude and phase information in the gradient pro- files simultaneously. Considerable previous work demonstrates the possibility for complete structural interpretation of a truncated plate model from the gradient space plot. The qualitative and quantitative diagnostic information contained in gradient space plots is general, however. Examination of the characteristics of gradient space plots reveals that 2-D structures are readily classified as extended or localized. For example, the truncated plate model is an extended model, while the faulted plate model is a localized model. Comparison of measured or calculated gradient space plots to a model gradient space plot catalog allows a rapid, qualitative determination of structure or geometry. "Corners" of a polygonal cross-section model are then determined as profile points corresponding to maxima on the vertical gradient profile. A generalized approach to structural interpretation from gravity data consists of (1) determining vertical and horizontal gradient profiles perpendicular to the strike of a 2-D gravity anomaly, (2) determining the structural geometry from the gradient space plot, and (3) locating profile positions of structural corners from the vertical gradient profile. This generalized inversion procedure requires no quantitative information or as- sumption regarding density contrasts. Iterative for- ward modeling then predicts the density contrasts. Application of this generalized gravity gradient inver- sion procedure to high quality gravity data results in an effective density prediction consistent with measured near-surface densities and the known increase in den- sity with depth in deep sedimentary basins. See Extended description for more information.

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K26-01422

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