ADCP Acoustic Doppler Current Profiler Morven Muilwijk 2014 University of Bergen, Bjerknes Centre for Climate Research

What is an ADCP? Acoustic Doppler Current Profiler An acoustic Doppler current profiler (ADCP or ADP) is a hydroacoustic current meter similar to a sonar, attempting to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column. The name ADCP is now used by several companies but was originally introduced by the company Teledyne who started production around the 1980’s. What makes the ACP different from all other current meters is that is gives a 3D picture of the whole water column with speed and direction at certain depth intervals called bins. The predecessor of ADCPs was the Doppler speed log, an instrument that measures the speed of ships through the water or over the sea bottom Morven Muilwijk 2014

The Doppler Shift The ADCP measures water currents with sound, using a principle of sound waves called the Doppler effect. A sound wave has a higher frequency, or pitch, when it moves to you than when it moves away. You hear the Doppler effect in action when a car speeds past with a characteristic building of sound that fades when the car passes. Morven Muilwijk 2014

The Doppler Shift The ADCP works by transmitting "pings" of sound at a constant frequency into the water. As the sound waves travel, they ricochet off particles suspended in the moving water, and reflect back to the instrument. sound waves bounced back from a particle moving away from the profiler have lowered frequency when they return. Particles moving toward the instrument send back higher frequency waves. sound scatterers are small particles or plankton that reflect the sound back to the ADCP. Scatterers are everywhere in the ocean. They float in the water and on average they move at the same horizontal velocity as the water Morven Muilwijk 2014

The Doppler Shift Speed of sound = frequency × wavelength The difference in frequency between the waves the profiler sends out and the waves it receives is called the Doppler shift. The instrument uses this shift to calculate how fast the particle and the water around it are moving. Sound waves that hit particles far from the profiler take longer to come back than waves that strike close by. By measuring the time it takes for the waves to bounce back and the Doppler shift, the profiler can measure current speed at many different depths with each series of pings. An ADCP computes sound speed based on an assumed salinity and transducer depth and on the tem-perature measured at the transducer. The ADCP uses this sound velocity to convert velocity data into engineering units and to compute distances along the beams. The working frequencies range from 38 kHz to several megahertz. Speed of sound = frequency × wavelength Morven Muilwijk 2014

The Doppler Shift Most ADCP has 3 or 4 acoustic transducers that emit and receive acoustical pulses from 4 different directions. Current direction is computed by using trigonometric relations to convert the return signal from the 4 transducers to ‘earth’ coordinates (north-south, east-west and up-down). When an ADCP uses multiple beams pointed in different directions, it senses different velocity components. For example, if the ADCP points one beam east and another north, it will measure east and north current components. If the ADCP beams point in other directions, trigonometric relations can convert current speed into north and east components. A key point is that one beam is required for each current component. Morven Muilwijk 2014

Current profiling and 3D velocity measurements 4 Measurements at once: 1. Speed and direction 2. Bottom tracking 3. Height or range 4. Distribution of material (intensity) As a result, the ADCP makes four different measurements at once. 1. Speed and direction of water currents are determined from changes in frequency (or Doppler shifts) of the echoes compared with the transmitted signal. For each “ping”, these velocity data are obtained at many levels through the measurement range--a “current profile”. 2. Similarly, echoes scattered by the seabed reveal the speed-over-ground in applications where the ADCP is moving. This bottom tracking information is used to remove the boat motion from the measured currents and to map the path followed by the boat. 3. Like an echo sounder, the ADCP measures height or range to the scattering object (in the water column or the surface or seabed) from the elapsed time (after transmit) before the echo is heard. 4. The distribution of suspended material through the measurement range (e.g, a mid-water plume) is described by the strength or intensity of the echoes. These data permit a 2-d image of the contents of the water column. Morven Muilwijk 2014

Different types, different purposes An ADCP computes sound speed based on an assumed salinity and transducer depth and on the tem-perature measured at the transducer. The ADCP uses this sound velocity to convert velocity data into engineering units and to compute distances along the beams. Morven Muilwijk 2014

Different types, different purposes An ADCP computes sound speed based on an assumed salinity and transducer depth and on the tem-perature measured at the transducer. The ADCP uses this sound velocity to convert velocity data into engineering units and to compute distances along the beams. Morven Muilwijk 2014

Applications of the ADCP Oceanography Vessel Traffic Safety in the Port & Harbor Tidal currents observations Efficient Seismic Survey Operations Measuring currents is a fundamental practice of physical oceanographers. By determining how ocean waters move, scientists can determine how organisms, nutrients and other biological and chemical constituents are transported throughout the ocean. Morven Muilwijk 2014

Applications of the ADCP Oil industry ADCPs for renewable energy projects Oceanography Oil industry Vessel Traffic Safety in the Port & Harbor Tidal currents observations Efficient Seismic Survey Operations ADCPs for renewable energy projects Morven Muilwijk 2014

New York power plant In the East River in New York City, Verdant Power is installing a field of turbines to capture the energy of tidal currents reaching 4 knots off Roosevelt Island. As well as revealing the impressive speeds of these currents, the ADCP boat sections showed an asymmetric distribution of the currents across the river section, which will be key information for deploying the field of turbines. Morven Muilwijk 2014

Different types of platforms Ships Submersibles Mounbted Moorings Rivers Floats Morven Muilwijk 2014

Examples of studies Measuring tidal currents in a sound Morven Muilwijk 2014

Examples of studies Measuring tidal currents in a sound Morven Muilwijk 2014

Advantages Easy to use and to make profiles No moving parts Small scales Absolute speed of water Measures columns up to 1000m In the past, measuring the current depth profile required the use of long strings of current meters. This is no longer needed. Measures small scale currents Unlike previous technology, ADCPs measure the absolute speed of the water, not just how fast one water mass is moving in relation to another. Measures a water column up to 1000m long Morven Muilwijk 2014

Disadvantages Barnacles and algea Clear waters Compromise between distance or precision Bubbles in turbulent water Battery power with frequent pings High frequency pings yield more precise data, but low frequency pings travel farther in the water. So scientists must make a compromise between the distance that the profiler can measure and the precision of the measurements. ADCPs set to "ping" rapidly also run out of batteries rapidly If the water is very clear, as in the tropics, the pings may not hit enough particles to produce reliable data Bubbles in turbulent water or schools of swimming marine life can cause the instrument to miscalculate the current Users must take precautions to keep barnacles and algae from growing on the transducers. Morven Muilwijk 2014

Good to know when working with ADCP’s Backscatter Ping Main Lobe Blank zone Ambiguity Backscatter: 1) The portion of a sound wave that is reflected by scatterers directly back toward the source. Ping: The entirety of the sound generated by an ADCP transducer for a single measurement cycle. A broadband ping contains a coded series of pulses and lags, while a narrowband ping contains a single pulse. Main Lobe: The main focus of energy emitted from a transducer. If the transducer were a flashlight, the main lobe would be the visible beam of light. Blank Zone: The area near the head of an ADCP in which no measurements are taken. Ambiguity: ADCPs determine the radial motion between a source and scatterer by measuring the phase change of the reflected signal. Because phase is periodic, this solution is multi-valued. For example, all three of the displacements shown below will return the same phase measurement, which results in ambiguity: Morven Muilwijk 2014

ADCP Acoustic Doppler Current Profiler Morven Muilwijk 2014 University of Bergen, Bjerknes Centre for Climate Research