A GENERALIZED-APPROACH FOR THE INTERPRETATION OF GEOPHYSICAL WELL LOGS IN GROUNDWATER STUDIES - THEORY AND APPLICATION

Quantitative analysis of geophysical logs in ground-water studies ofte n involves at least as broad a range of applications and variation in lithology as is typically encountered in petroleum exploration, making such logs difficult to calibrate and complicating inversion problem f ormulation, At the same time, data inversion and analysis depend on in version model formulation and refinement, so that log interpretation c annot be deferred to a geophysical log specialist unless active involv ement with interpretation can be maintained by such an expert over the lifetime of the project. We propose a generalized log-interpretation procedure designed to guide hydrogeologists in the interpretation of g eophysical logs, and in the integration of log data into ground-water models that may be systematically refined and improved in an iterative way. The procedure is designed to maximize the effective use of three primary contributions from geophysical logs:(1) The continuous depth scale of the measurements along the well bore; (2) The in situ measure ment of lithologic properties and the correlation with hydraulic prope rties of the formations over a finite sample volume; and (3) Multiple independent measurements that can potentially be inverted for multiple physical or hydraulic properties of interest, The approach is formula ted in the context of geophysical inversion theory, and is designed to be interfaced with surface geophysical soundings and conventional hyd raulic testing, The step-by-step procedures given in our generalized i nterpretation and inversion technique are based on both qualitative an alysis designed to assist formulation of the interpretation model, and quantitative analysis used to assign numerical values to model parame ters, The approach bases a decision as to whether quantitative inversi on is statistically warranted by formulating an over-determined invers ion, If no such inversion is consistent with the inversion model, quan titative inversion is judged not possible with the given data set, Add itional statistical criteria such as the statistical significance of r egressions are used to guide the subsequent calibration of geophysical data in terms of hydraulic variables in those situations where quanti tative data inversion is considered appropriate.