Effects of temporal and vertical variability of echo amplitude on ADCP selection and performance

Abstract

Measurement of ocean currents using acoustic Doppler current profilers (ADCPs) depends on the presence of scatterers throughout the water column to create a backscattered signal whose Doppler shift can be measured. An insufficiency of scatterers results in missing or unreliable data, or reduced profiling range. ADCP echo amplitude data collected from rig-based downward looking ADCP systems over the past decade at numerous sites in the Gulf of Mexico and elsewhere, and using a variety of ADCP types and frequencies, consistently display a characteristic pattern of diel (24-hour) echo amplitude variability. This is attributed to vertical migration of biological organisms - namely the tendency of some species to reside closer to the surface during the night and further from the surface during daylight hours. In the uppermost region of the water column, nighttime profiles show higher echo amplitudes than daytime profiles, indicating a greater presence of scatterers in the surface layer at night. Daytime profiles show stronger echo amplitude signals in a deeper layer, indicating greater biological presence at these depths during the day. The transition between the nighttime and daytime scatterer positions is linked to sunrise and sunset, and is remarkably rapid, occurring over hundreds of metres of the upper ocean within an hour or so. Furthermore, both daytime and nighttime signals show a strong peak in echo amplitude in the 365 m to 425 m (1200 ft to 1400 ft) range, indicating a strong and consistent scattering layer there. A minimum in echo amplitude occurs at shallower depths, in the 215 m to 365 m (700 ft to 1200 ft) range. The minimum in echo amplitude can result in signal drop-outs and poor quality ADCP current data, limiting the effective range of a surface-mounted downward looking ADCP. Narrowband ADCPs are generally more tolerant of lower scattering intensity than are the broadband type, and rarely lose data in the minimum layer. The echo amplitude patterns presented in this paper are evident in data from narrowband and broadband instruments operating at 38 kHz and 75 kHz, deployed from drill rigs, drill ships, and deepwater moorings in the Gulf of Mexico for more than 10 years. Due to the universality of these scattering profile features, consideration should be given to selection of the most appropriate ADCP operating mode where maximum range is desired, especially for mission-critical measurement programs in the oil industry.