每次sysdba或者sysoper权限登录都会写入一个audit trail文件或者在现有的trail文件后面添加登录信息，而这个并不受audit_trail参数限制，audit_trail是显示是否启用审计和审计内容存储的具体位置
有关audit_sys_operations参数：
AUDIT_SYS_OPERATIONS enables or disables the auditing of top-level operations,which are sql statements directly issued by users when connecting with SYSDBA or SYSOPER privileges. (sql statements run from within PL/sql procedures or functions are not considered top-level.) The audit records are written to the operating system‘s audit trail. The audit records will be written in XML format if the AUDIT_TRAIL initialization parameter is set to xml or xml,extended.
On UNIX platforms,if the AUDIT_SYSLOG_LEVEL parameter has also been set,then it overrides the AUDIT_TRAIL parameter and SYS audit records are written to the system audit log using the SYSLOG utility.
该参数默认为false，但是sysdba和sysoper登录还是会记录audit到相应的audit trail文件的，只是不记录之后sysdba操作的信息，而如果设置true，还会记录之后sysdba或者sysoper操作记录到audit trail中。
可以关闭审计，但是sysdba或者sysoper登录记录audit trail却无法禁掉。

整理自：https://blog.csdn.net/zhaoyangjian724/article/details/50637409