Inversion of pressure observations: an integral formulation

It is possible to express the inverse problem for flow in an integral form. The integral formulation affords some insight concerning the recovery of s ubsurface flow properties. In particular, we find that rapid or high freque ncy variations in head are primarily sensitive to the specific storage. Con versely, slow or low frequency variations are most strongly influenced by h ydraulic conductivity. The nonlinearity of the inverse problem is due to th e presence of product terms in which the head field, or its gradient, multi plies a term dependent on flow properties. Thus, in their discrete form, th e equations describing the inverse problem are algebraic. The integral form ulation provides an alternative approach to linearizing the inverse problem , the Born approximation. We apply the methodology to data from a set of cr osswell pressure tests conducted at the Raymond field site. We are able to image two fracture zones within the granitic subsurface. The location of th ese zones agrees with results from crosswell seismic tomography and borehol e logging. Published by Elsevier Science B.V.