A portable field ultrasonic sensor for crop canopy characterization

An ultrasonic pulse wave transceiver was devised and tested in com, soybean, rice, and sorghum fields for sensitivity to canopy structural differences. An ultrasonic pulse was emitted 30–50 cm above the canopy surface, and its echo was measured. Received echo patterns were logged into a microcomputer memory and analyzed. Light extinction and leaf area by strata measurements were made right after the ultrasonic measurements. Profiles of echo peaks for 20, 40, and 60 kHz “sound” frequencies with height in the canopies were compared with profiles of plant parts (green leaves, stems, dry leaves, tassels, ears, and pods), dry weight, and relative transmitted light. The ultrasonic pulse was especially sensitive to horizontal components in the plant canopies, indicating the feasibility of surveying the vertical distribution of crop leaves in a canopy nondestructively. The 20 kHz frequency reached the lower, inner positions of the canopies better than the 40 kHz frequency.