Acoustic Backscatter
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Acoustic backscatter tells us how much of a transmitted acoustic signal is bounced
back to a transducer. Acoustic pings can reflect off of a variety of things ranging
from zooplankton, bubbles, suspended sediment, turbulent stratified water, the ocean
surface to fish. Any change in acoustic medium has the potential to scatter
some of the signal back to its source, but of course the instrument
sensitivity does have limitations. Due to the variety of sources of
backscatter, one should use caution in drawing conclusions about what has
reflected the signal.
In addition to measuring the velocity field, acoustic doppler current profilers (ADCPs) measure the echo intensity. It is not calibrated in any way to compensate for the spreading loss of the signal as it gets farther from the transducer face, absorption of sound energy by seawater, or the calibration of the transceiver. However, it is entirely possible to perform such a calibration (Kent L. Deines) to get a more accurate picture of what is occurring in the water column at different times. We've implemented this version of the sonar equation in our software, and examples of the results are shown below. |
| Notice that the range dependence which is so apparent in the echo intensity dissappears almost completely when the actual backscatter is computed. Please note that the callibration is not so precise that the numbers always work out the same, but regardless of the coefficients used, structure is the same, and it is not range dependant. |
Harvey Seim <harvey_seim@unc.edu>