The pilot of the helicopter stated that he made two passes over the field to check for obstructions prior to beginning the aerial application flight, and remembered observing only one guy wire. While making a final application to the edges of the field, the pilot flew around the previously-observed guy wire, but the helicopter's skid contacted a second wire that he had not seen. The helicopter impacted the ground on its side, resulting in substantial damage. The pilot stated there were no mechanical malfunctions or anomalies of the helicopter that would have precluded normal operation.