According to the operator, a preflight was completed before the helicopter was moved to the landing zone and no anomalies were noted. The helicopter was then equipped with an aerial platform and measuring equipment to perform the planned work in the vicinity of powerlines. Subsequently, the helicopter departed the landing zone. Due to the atmospheric condition at the aerial work site, the pilot discontinued the aerial work and returned to the landing zone. A "senior" pilot was consulted, and replaced the first pilot. The "senior" pilot decided to depart and assess the situation from the air. A short time later the flight returned to the landing zone. The first pilot performed a "short" postflight examination of the helicopter and noticed that a main rotor blade was substantially damaged. The operator reported no preimpact mechanical malfunctions or failures with the helicopter that would have precluded normal operation. In addition, neither pilots nor crew members noticed any abnormal flight characteristics of the helicopter prior to discovering the main rotor blade damage. The investigation was unable to determine which flight the main rotor blade incurred the substantial damage.