SPH Engineering ECT Echo Sounder

ECT 400S single-frequency echo sounder with an acoustic frequency of 450 kHz

It measures the distance till the first “obstacle” in the water - sediments, seaweeds, and even fish. Therefore, if the bottom of the survey area is covered with seaweed, this basic configuration may not work for such a case.
The measured depth will fluctuate erratically depending on what echo sounder received reflected sound - bottom or vegetation. That means that to get “clean” readings from EchoLogger ECT 400S the bottom should be without seaweeds, ensuring a precise depth measurement.
The benefit of EchoLogger ECT 400S is that it has a very short dead zone of 15cm - a minimum distance it can measure. That makes the system with ECT 400S suitable for very shallow surveys, starting from approximately 30cm deep.

ECT D24S dual-frequency echo sounder with an acoustic frequency of 450 and 200 kHz

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. with an acoustic frequency of 200 and 50 kHz

200 kHz sound impulses will reflect from the top surface of the bottom, but 50 kHz sound can penetrate through organic or light sediments, measuring the depth till the hard bottom.
That creates the possibility of mapping the depth and measuring the thickness and amount of sediments in the water body.
Note: the dead zone for 50 kHz is 1m, and the minimum practical survey depth will be around 1.2m.
EchoLogger ECT D052S is a dual-frequency echo sounder with tilt and temperature sensor.