Unlike ASP.NET, unhandled web services exception can not be caught in global.asax(Application_Error event). Here is the workaround for that
1.Create a SoapExtension class
2.Add the SoapExtensionTypes element in your web.config file and configure your SoapExtension class.
3.In the overrided ProcessMessage method, check whether SoapMessage.Exception has any value
4.Log the exception details only if the SoapMessageStage is SoapMessageStage.AfterSerialize.
5.Make sure ASP.NET account has enough permission to write the log
More about SoapExtension class from MSDN
http://msdn2.microsoft.com/en-us/library/system.web.services.protocols.soapextension(VS.71).aspx