Some basic auditing questions

We have a small datamart using Business Objects as an OLAP tool by which

all users log in as the same Oracle user (BOBJ), but I've been asked to provide
time logged in for each user which I think I can get from osuser in V$session.

In trying to get underway with understanding auditing, I noticed that our
AUDIT_TRAIL parameter is set to true, yet I see no records at all in sys.aud$.
I do however see little scripts in $ORACLE_HOME/rdbms/audit that seem to do
nothing but record a connection by svrmgrl every now and then (although I haven't
looked at all of them) - but my understanding from my documentation for range of values
which is pasted right here...

Range of values: NONE (FALSE), DB (TRUE), OS

is that TRUE would turn on DB auditing, and sys.aud$ would fill up with various information. Now I realize from the documentation that certain options have to be set to decide what to audit, but why are they (they
- meaning probably some default) going to a file system and not the
sys.aud$ table?

If anyone has a method of tracking osusers in minutes per day I would appreciate any advice.

• Dc.