Geophysical well logs represent measurements of a variety of properties of the rocks and fl uids encountered by a well bore and are used by petroleum industry analysts to guide decisions regarding further well development and investigation. Nuclear logs of natural gamma rays, neutron moderation, electron density, and photoelectric absorption are extremely common and are sensitive measures of rock types and mineral compositions. The Oz Machine is a Java applet providing online, interactive

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... ction in geological interpretation of these nuclear well logs. It employs a simple Markov chain simulation to generate a synthetic sequence of lithologies (rock types) and then generates a suite of corresponding well logs based on a mineralogical recipe for each lithology and the typical log responses for each mineral. The resulting synthetic logs are displayed, and the student paints a geological interpretation of the logs into the depth track, selecting from a palette of lithologies presented next to the log display. The realism of the simulated log suite is enhanced by inclusion of random variation in the mineralogical composition of each lithology and application of a smoothing fi lter to emulate tool resolution effects. Despite the simplicity of the underlying simulation, the generated lithological and log sequences are surprisingly realistic, providing an essentially endless supply of "mirror-world" exercises in geological log interpretation.