Bottom mount

The bottom frame is typically used in fixed deployment situations. The advantage of mounting the instrument at the bottom is the reduced chance of measurement errors due to instrument motion and movement from interaction with the surface (waves, wind), or motion transferred from the mooring line during in-line mounting. When using a bottom mount, the emitted beams are not in the risk of encountering any obstructions related to the deployment setup.

If the deployment depth is shallow, the mounting frame may be exposed to strong forces due to the orbital currents that waves generate. The design of the frame should therefore be compact and heavy. This and other considerations regarding bottom frames are listed in the equipment chapter. Note that if your plan is to make measurements near the surface from a bottom mounted instrument, the tilt must be as small as possible. If the bottom is not level, it might the use of a gimbal could be beneficial, or use a subsurface buoy as mentioned in the previous section. 

If measuring waves is the focus of a deployment, bottom mounted ADCPs work well in shallow water. However, in depths greater than a certain limit, bottom mounted instruments are not able to provide the directional resolution necessary for most research and commercial requirements. This limit is determined by the signals associated with orbital velocities and pressure attenuation, and the fact that they attenuate exponentially with depth. Read more about this in the Principles of Operation - Waves.