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How does an inverted echo sounder measure changes in the temperature of a water column?

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An inverted echo sounder measures changes in the temperature of a water column by monitoring the travel time of acoustic pulses.1

The instrument, positioned on the seafloor, transmits a sound pulse that travels to the sea surface and reflects back to the receiver. Because the speed of sound in seawater is directly influenced by temperature—increasing by approximately 4.5 metres per second for every 1 °C rise in temperature—the total time taken for the signal to complete its round trip fluctuates in response to changes in the average temperature of the water column.12

When the average temperature of the water above the instrument changes, the velocity of the sound pulse is altered, causing a measurable shift in the time interval between the transmission and the reception of the acoustic signal.1