Login report of Sysadmin
You should write a logon trigger that will log only sysadmins.
Here you can find more about logon triggers: 
Logon Triggers

And here is a practical example of its use:
SQL Server – Login Auditing using Logon Triggers

P.S. To find out if the login is a sysadmin you can use this code: 

select is_srvrolemember ('sysadmin',original_login())

UPDATE

If the logon trigger does not use EXECUTE AS clause (and in case of OP this is true), it's executed under the current login. So to check out if the login is sysadmin is sufficient execute

select IS_SRVROLEMEMBER('sysadmin');


without using original_login(). This will work even in case when the AD group is sysadmin, and Windows login is not sysadmin directly but access the server through membership in this AD group