Multi-frequency Phased Array Transducer for ADCP Applications

Abstract

Acoustic Doppler Current Profilers (ADCP) and Doppler Velocity Logs (DVL) have been used over the past 40 years on moving vessels in river and ocean environments to remotely measure, along the path of the vessel, the vessel relative velocity to 1) the water versus depth (current profiles) and 2) the bottom (bottom tracking) to produce earth referenced velocity profiles by subtracting the two measurements to remove the vessel speed from the current profiles. These ADCPs typically employ four narrow, downward slanting single-frequency acoustic beams pointing in orthogonal directions (referred to as Janus configuration beams). These beam sets are produced by either 4 separate inclined circular piston transducers, or a single flat two-dimensional phased array transducer which utilizes the full aperture for all 4 beams, and thus has a factor of two smaller diameter. Typical operation is at a single frequency within the range of 38 to 1200 kHz. Lower frequency ADCP transducers achieve longer profiling, but with lower depth, velocity and time resolution. Higher frequency ADCP transducers achieve shorter range profiling, but with higher depth, velocity and time resolution. Thus, a performance compromise decision must be made for each vessel installation. In this paper, a new ADCP with a multi-frequency phased array transducer using two frequencies of 38 and 300 kHz is presented, which enables simultaneous measurement high resolution current profiles in the more dynamic upper ocean or shallow water (<300 m), and, by correlating 38 and 300 kHz measurements, improved velocity resolution measurements in the deep ocean of up to 1500 m. Results are presented as a function of range and velocity measurements for the multi-frequency phased array transducers operated at two different frequencies.