What's the first step in analyzing a dip log--from either a conventional dipmeter tool or a borehole image tool?

The answer is, "it depends": was the data set picked by hand or by machine? If the data set was picked by machine using a correlation algorithm that uses a search angle, the first step is to check the data set for missing and/or false data.

For a case study, see "Re-interpretation of the north flank of the Qarun Field (Western Desert, Egypt), based on SCAT analysis and reprocessing of the A-17 dipmeter data", (here and here), a paper presented at the November 2007 AAPG meeting in Athens. The paper describes how CG used SCAT and Geodes to identify and correct serious problems with a set of machine-picked dip data from a key well in the steep flank of Qarun Field. The original data set, which was correlated with a search angle that turned out to be much too low, suffered from both missing data and false data.

So the first step in analyzing a dip log is to determine whether or not the data set in question was machine-picked. If it was machine-picked, the next step is to check the data set for missing and/or false data and to correct any problems, if possible, by filtering the machine-picked data and/or by reprocessing the conductivity curves.