ECT D24S dual-frequency

Technically, a dual-frequency echo sounder is two devices with different acoustic frequencies, combined in one compact housing.

The higher frequency is 450 kHz, and it will give measurements till the first surface or obstacle. The lower frequency is 200 kHz, and sound with that frequency will pass through seaweeds/vegetation on the bottom, and the device will report clean readings of the bottom.

Note: the dead zone for 200 kHz is 0.5m, limiting the minimum practical survey depth for 
that device by approx 0.8m.

EchoLogger ECT D24S is a dual-frequency sounder with tilt and temperature sensor.