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Brew, Graham1
(1) Dynamic Graphics, Inc, Alameda, CA

ABSTRACT: Gravity Calculations from 3-D, Geologically-Driven Models: A New Approach

A new routine calculates the gravity anomaly of any 3-dimensional (3-D) geologic model with arbitrary density distribution. The calculation uses a common 3-D modeling software package that facilitates model-building using geologically intuitive steps including deposition, erosion, faulting etc. Hence, models are not required to be built from unnatural polyhedrons, a shortcoming of many previous modeling schemes. Furthermore, the gravity modeling is integrated with the earth models routinely built as part of the exploration / exploitation workflow.
Once the geological model is built, it is populated with density data. This is achieved using 3-D grids within stratigraphic layers consisting of either constants, gradients, or any arbitrary density distribution.
Using the density model, the gravity contribution at a chosen height is calculated. For each point at that height the contribution of each density element in the model is computed, and all the contributions are summed to yield the total anomaly. Modern computing power renders this elemental approach feasible within acceptable time-frames.
Once computed, the power of 3-D visualization can be used to inspect the suitability of the result. The resulting grid of calculated gravity anomalies can be compared to observations, draped over the model, used in grid arithmetic etc. This completely flexible, fully 3-D approach - within a geologically-driven earth model - yields new power and utility to traditional gravity modeling. We show examples from salt domes, intrusions, and other cases where gravity modeling can significantly assist the search for hydrocarbons.

