Zeroth-order inversion of transient pressure observations

A high frequency, asymptotic solution for transient pressure, appropriate for a medium containing smoothly-varying heterogeneity, provides a basis for efficient inverse modeling. The semi-analytic solution is trajectory based, akin to ray methods used in modeling wave propagation, and may be constructed by post-processing the output of a numerical simulator. For high frequencies, the amplitude sensitivities, the relationship between changes in flow properties and changes in pressure amplitude, are dominated by the phase term which may be computed directly from the output of the simulator. Thus, transient pressure waveforms may be inverted with little more computation than is required to invert arrival times. An application to synthetic pressure values indicates that the technique can be used to improve the fit to waveforms. An application to transient pressure data from the Migration experiment in Switzerland reveals a narrow, high conductivity pathway within a 0.5 m thick zone of fracturing.