Oracle 11g: How to trace in alert log file

To trace sonn messahe in the alert log file :

exec dbms_system.ksdwrt(2, 'This is a message sent at ' || TO_CHAR(SYSDATE, 'dd/mm/yyyy HH24"H"MI'));

note : For more information about dbms_system package see http://psoug.org/reference/dbms_system.html