The log file containing the information referenced will usually be the one named with the hostname on which Tomcat is running. On a simple server (e.g. development environment) this is localhost.<datestamp>.log
There are two ways to specify how GET parameters are interpreted:
Set the URIEncoding attribute on the <Connector> element in server.xml to something specific (e.g. URIEncoding="UTF-8").
Set the useBodyEncodingForURI attribute on the <Connector> element in server.xml to true. This will cause the Connector to use the request body's encoding for GET parameters.