Guernsey Police says the incident happened on Sunday 23 August, but has not been able to give a time.
Anyone who witnessed the reported attack and has information about it can contact PC 121 Sharman at the Police Station on 725111.
Alternatively, people can report what they saw to Crimestoppers anonymously on 0800 555 111.
Pictured top: Police are looking for leads in their investigation of the attack, which is reported to have happened at L’Ancresse Common last Sunday.