From Oracle 10g and onwards, the default value for LOG_ARCHIVE_FORMAT is %t_%S_%r.dbf
However, this parameter will only make a difference if you are NOT using a "Fast Recovery Area" (in Version 10 Oracle called the same thing a "Flash Recovery Area")
If you have defined a FRA by using the parameters db_recovery_file_dest and db_recovery_file_dest_size, the archivelogs will be automatically named according to OMF (Oracle Managed Files) standard and the value of LOG_ARCHIVE_FORMAT will be ignored.
An example of an OMF managed file in the FRA:
/fra/PRODDB01/archivelog/2014_06_30/o1mv_1_1_9vxzch5_.arc
From the documentation:
"The fast recovery area is an Oracle Database managed space that can be used to hold RMAN disk backups, control file autobackups and archived redo log files. The files placed in this location are maintained by Oracle Database and the generated file names are maintained in Oracle Managed Files (OMF) format."
At first glance, letting the LOG_ARCHIVE_FORMAT default to such a poor naming standard may seem like a bad idea, but I guess it illustrates the point of defining an FRA vs. using conventional archive log destinations.
