Investigation of fracture flow paths using alterant geophysical tomography

Abstract

The mapping of fractures is a problem of major importance to many geotechnical projects. It is a three dimensional problem requiring the use of measurement methods with sufficient resolution and probing range to map fractures at a distance from exposed surfaces. The National Research C6uncil's report on Rock Mechanics Research Requirements [1] identified a need for major improvements in the methods used for mapping remote fractures. This article discusses a new geophysical method (Alterant Geophysical Tomography, AGT) in which tracers are forced into the in situ rock fractures to change the transmission properties of electromagnetic waves propagated between boreholes or tunnels. The theoretical advantages of the method and the results of the first experiment where it was used are also presented. This technique is analogous to the medical diagnostics technique known as X-ray fluoroscopy wherein a tracer is used to attenuate X-rays. The movement of the tracer within the digestive tract or circulatory system of the human body is monitored by measuring the attenuation of X-rays. AGT involves the transmission of an electromagnetic energy field through fractured rock before, during, and/or after a tracer is forced into the fractures. By using alterant geophysical tomography, changes induced by these tracers are mapped in the plane defined by the two boreholes. This approach is intended for geologic environments where fracture flow dominates such that the tracer penetrates the fractures but not the rock matrix.