Predicting a Fault's Next Move     image of PDF button

An advanced tool aids in natural resource exploration as well as groundwater flow studies and seismic and volcanic hazard analyses

Stress is always just beneath the surface -- the surface of the Earth, that is. And where faults and fractures exist in the brittle rock layers of the Earth's crust, stress can have quite an impact, from triggering earthquakes to redirecting natural resources. Using 3DStress©, a unique computer program developed at Southwest Research Institute (SwRI), geologists and geophysicists can now quickly and accurately determine the tendencies of faults and fractures to slip or dilate.

Slip can result in an earthquake, whereas dilation widens existing fractures, creating channels through which water, hydrocarbons, or magma can travel. 3DStress is thus a valuable tool for oil and gas exploration, earthquake and volcanic hazard analyses, and groundwater flow and transport analyses.

The program applies computed stresses in three dimensions to fault traces on maps or three-dimensional surfaces of known or suspected faults. Fault data exported from 3DMove, developed by Midland Valley Exploration in Glasgow, United Kingdom, and ARC-INFO, developed by the Environmental Systems Research Institute in Redlands, California, can be loaded into 3DStress, as can user-created files.

The three-dimensional stress states are entered through a straightforward, graphical user interface. The results, which can be displayed in two or three dimensions, can be interactively adjusted by modifying the stresses to investigate slip tendency and direction on each individual surface.

The ratio of shear stress (parallel to a fault's surface) to normal stress (perpendicular to the surface) determines slip tendency. Fault dilation tendency is computed from the three-dimensional stress state and the stress normal to a fault or extension fracture surface. Previously, these complex calculations were done manually for one fault orientation at a time. With 3DStress, calculations are made for all fault and fracture orientations and simultaneously displayed using stereographic projections, fault maps, and three-dimensional models of faults.

"Oil and gas companies can use 3DStress before drilling to determine the risk of leakage from a hydrocarbon trap," says Dr. David A. Ferrill, structural geologist at SwRI. A leaky trap is a naturally occurring potential gas or oil reservoir that may contain intersecting faults that channel the hydrocarbon away from the trap, resulting in little or no hydrocarbon accumulation.

Volcanic hazard analyses rely to a great extent on probabilistic techniques. Ferrill explains that 3DStress provides a deterministic complement to these techniques by revealing the dilation tendencies of faults that could, under the right conditions, act as conduits for ascending magmas. The program can also be used to predict the effects of changing stress fields on existing faults in earthquake-prone areas.

"Groundwater flow in fractured aquifers is strongly controlled by fracture orientation and aperture, the product of dilation," Ferrill adds. "3DStress provides a rapid means for evaluating dilation tendency in naturally fractured aquifers and can thus be used to predict groundwater flow direction and contaminant migration."

3DStress was developed under sponsorship of the U.S. Nuclear Regulatory Commission. Codevelopers are Ferrill and D. Brent Henderson of the Center for Nuclear Waste Regulatory Analyses, SwRI, and Dr. Alan Morris, associate professor at the University of Texas at San Antonio. Commercial licensing is being handled by Midland Valley Services in Boerne, Texas.

Published in the Summer 1996 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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