This was an external load operation to move seismographic oil exploration equipment. The pilot flew to the loading zone with a 150 ft cable and an aluminum bucket. After arriving, he lowered the bucket and hovered while the bucket was loaded. The pilot then lifted the loaded bucket to about 40 ft AGL. At about that time, he transmitted that he had a problem and had to sit the helicopter down. The bucket came down hard about 10 ft from its pickup point and the helicopter moved to a clear area. Subsequently, the helicopter hit the top of a tree, then crashed into the ground about 90 ft from the tree. The aluminum bucket had been dragged about 200 ft before it came to rest at the bottom of the tree. The cable was draped across the tree and the right skid but was found released. An examination revealed no precrash malfunction of the flight controls and no mechanical failure of the engine was verified.