An improved well logging technique is provided for more accurately deriving and correlating a plurality of measurements made during a single traversal of a logging instrument through subsurface formations. In one exemplary embodiment, methods and apparatus are provided for deriving a more accurate and precise measurement of depth at which real-time logging measurements are made, and in particular for correcting anomalies occurring in the depth indication from cable stretch, yo-yo of the sonde in the borehole and the like. The more accurate and precise depth measurement is then utilized for generating well logging measurements on a depth-dependent basis, deriving at least some of such measurements in digital form and alternately transmitting to the surface digital and analog representations of such measurements. Furthermore, methods and apparatus are provided for deriving measurements of subsurface earth formation from a plurality of logging instruments combined in a single tool, wherein such measurements are made during a single pass through a borehole with the resultant measurement data correlatively merged, recorded and displayed.
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