This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 90909, "Interactive Log Simulation and Inversion on the Web," by V. Polyakov, SPE, T. Habashy, R. Kocian, J. Pabon, and B. Anderson, SPE, Schlumberger-Doll Research, prepared for the 2004 SPE Annual Technical Conference and Exhibition, Houston, 26-29 September. Log simulation is critical for understanding and interpreting logging-tool responses. Simulation helps the log analyst understand near-wellbore measurements in complex environments, particularly in anomalous situations. Simulated logs also can be used in “what-if” scenarios or as part of an iterative scheme to determine the formation’s geometry and material properties. A forward-modeling and inversion environment with a multitier Web-based architecture was developed to provide universal access from the user’s desktop, through a common Web browser, to the simulator engine running on a high-performance compute server. Introduction The essence of log interpretation is the solution of the inverse problem: determining formation parameters from logging data. This task becomes difficult when the logging environment departs from simple, well-understood geometries. Many tools are designed to make deep-reading measurements and, therefore, are susceptible to formation heterogeneities. The most com-mon environmental effects are formation dip, invasion of borehole fluids, and the influence of adjacent formations. Frequently, a combination of these effects can obscure the true formation properties (e.g., formation resistivity and porosity), making it necessary to construct a complex 3D model of the formation to obtain an accurate solution. The only way to validate these complex models is by tool-response simulation, or inversion. Some logging tools (such as dual induction and dual laterolog) do not provide sufficient information to perform accurate inversion. Iterative forward modeling of what-if scenarios is used routinely in these cases. Iterative modeling simulates the exact tool response in a series of given formations. However, this process requires a high level of user interaction. The initial formation model must be constructed from log, core, and geological information, then updated manually. A solution is reached when a computed log matches the field log. Automated inversion is more efficient because no interaction is required after the simulation is launched. During the past decade, the introduction of array measurements with greater information content has made it possible to use inversion in log interpretation routinely. Some modern tools record so much data that interpreting their response without the use of inversion presents a considerable challenge. The interactive Web-based interface provides a convenient and universal user access to the forward-modeling and inversion software. Also, the Web-based-programming interface makes the simulator available as a library for other oil and gas applications.