Abstract

Animated Axial Surface Mapping is PC- and Mac-based software that was created to help geologists visualize complex two- and three-dimensional structures. It was created with easy-to-use multimedia authoring software that uses the stacks metaphor. A stack consists of a series of cards that are connected to each other by buttons. These buttons are hot-spots on the card that take the user to some other card or stack when clicked.

This enlarged version of AAPG's first DataShare Disk (Hook and others, 1994) brings to life the concepts and applications of Axial Surface Mapping, a new method of structural trend analysis. Through the dynamic use of color, sound, animation, and humor, this multimedia software takes the reader (or viewer) on an interactive tour of Rigid-Block Translation, Fault-Bend Folding, Axial Surface Projection Methods, and Axial Surface Mapping. Visualization is the key to understanding and exploring the complex two- and three-dimensional geometries of folds in the earth's crust. The interactive, animated line drawings in the accompanying software allow the user to see and hear why Rigid-Block Translation cannot produce folds in the rocks above nonplanar faults. Similar animations show how Fault-Bend Folding accommodates fault slip while conserving the line lengths, areas, and thicknesses of rocks as they fold above bends in faults.

Two methods for projecting axial surfaces (fold hinges) from depth to the mapping datum are described in the Projection Methods section. The Parallel Projection Method (PPM) is a geometric method that involves the projection of axial surfaces along their traces to mapping datum. The Vertical Projection Method (VPM) is a geometric and geologic procedure that involves the vertical projection of the intersection points of the axial surfaces with a mapped horizon to the datum. PPM only requires the explorationist to find changes in the dip of seismic reflections, thus locating the trace of the fold hinge. VPM requires the additional step of looping and tying a reflection around a seismic grid. The properties of these two methods are graphically illustrated by an interactive interpretation of a seismic section from Oklahoma. The ease with which VPM axial surface maps can be contoured is shown by an animated contour line working its way around the VPM axial surface map of a structure.

Animated axial surface maps with cross sectional models of growing fault-bend folds displayed on folding lines allow the viewer to "see" in four dimensions simultaneously: three-dimensional space plus time. Both the cross sectional views and the map view show the structure changing through time as fault slip increases progressively. The idea of a folding line is also graphically explained by an animated map. Models of three-dimensional structures are used to create theoretical map patterns under varying, but controlled structural conditions. Axial surface maps from real data can then be compared to the theoretical patterns. Seismic data and axial surface maps from a producing gas field in offshore California and an exploration play in Pennsylvania show the applicability of this method of structural analysis to real world problems.