The limited radius of investigation of petrophysical measurements made in boreholes and the relatively large distances between wells result in an incomplete sensing of the subsurface through well logging. CORRELATOR is a program for estimating geological properties between logged boreholes. An initial and fundamental step is the lithostratigraphic correlation of logs in different wells. The method employed by the program closely emulates the process of visual inspection used by experienced subsurface geologists in manual correlation. Mathematically, the determination of lithostratigraphical equivalence is based on the simultaneous assessment of similarity in shale content, similarity in the patterns of vertical variation in a petrophysical property that is measured with high vertical resolution, and spatial consistency of stratigraphic relationships as determined by an expert system. Multiple additional options for processing log readings allow maximization in the extraction of information from pairs of logs per well and great flexibility in the final display of results in the form of cross sections and dip diagrams. ?? 2004 Elsevier Ltd. All rights reserved.