Use :log_depth from options in indent, or log_depth (for TriG).

lib/rdf/turtle/writer.rb

@@ -380,7 +380,7 @@ def preprocess_statement(statement)
     # @param [Integer] modifier Increase depth by specified amount
     # @return [String] A number of spaces, depending on current depth
     def indent(modifier = 0)
-      " " * (log_depth * 2 + modifier)
+      " " * (@options.fetch(:log_depth, log_depth) * 2 + modifier)
     end
 
     # Reset internal helper instance variables