Brain Log - Think it out. Write it down.

Pressure Fluctuations with Depth due to Surface Waves

In our AUV operations for seafloor mapping we have seen artifacts that result from the modulation of the AUV’s pressure sensor by waves passing over head.

In a recent survey, we were operating the AUV in 12 meters of water at a depth of 6 meters down over very flat seafloor (<10 cm relief). There were relatively large surface waves with a peak-to-peak amplitude of about 2 meters. These were surface shallow-water gravity waves whose wavenumber, k, can be calculated from their wave speed, c and their period, T by k = 2*pi / (c*T). Wave speed, c, in turn, for this kind of wave is calculated from c = sqrt(g*h) where g is gravity and h is the total water depth. Using these relations and by equating the pressure at depth due to surface gravity waves with the equation for hydrostatic pressure (above) I calculated the error one should see in bathymetry measurements made relative to the AUV’s depth when the pressure sensor is used as its depth reference. (See Water wave mechanics for engineers and scientists By Robert George Dean and Robert A. Dalrymple for more of the details.)

The plot above shows that for our scenario, we should see artifacts of about 80 cm. In fact, this was exactly what we saw!