Alterant geophysical tomography

We describe and evaluate a new geophysical technique used to remotely map fractures between boreholes: alterant geophysical tomography (AGT). The method requires that the attenuation properties of rock fractures be altered by forcing into the rock a fluid with different electrical properties than those of the native fluids in the rock. Measurements of electromagnetic attenuation factor are performed before and after the tracer is used. Measuring changes in attenuation properties offers significant advantages over measuring absolute attentuation properties. Results of an experiment in which this technique was employed are discussed. 4 references, 4 figures.