Depth is one of the most fundamental measurements one wishes to make in an aquatic environment. Measurements of water depth from a ship are typically made using acoustics, specifically an echo sounder, which measures the time it takes for a pulse of sound to travel from the ship, reflect off of the seafloor, and return to the ship. Knowing the speed that sound travels through water allows one to convert the measured travel time to depth. The accuracy of this acoustic technique is limited to the accuracy of the knowledge of the local sound speed.
An instrument situated on the seafloor that wants to measure its own depth may not be able to use an echo sounder due to power restrictions, or it may need to measure the depth with more precision than can be obtained with acoustics. In this scenario the most accurate method of measuring depth is by measuring the pressure exerted by the column of water above the observer. Knowing the density of the seawater allows one to convert the measured pressure into depth.
While simple in principle, measuring depth of the seafloor with high precision and high accuracy presents significant challenges. This is especially true for long-term measurements. Quad Geometrics offers several products and services specifically tailored to address these applications.
Filloux, J. H., 1980: Pressure Fluctuations on the Open Ocean Floor Over a Broad Frequency Range: New Program and Early Results. J. Phys. Oceanogr., 10, 1959–1971.
Snodgrass, Frank, Wendell Brown, Walter Munk, 1975: MODE:IGPP Measurements of Bottom Pressure and Temperature. J. Phys. Oceanogr., 5, 63–74.
Fox, Christopher, 1999: In situ ground deformation measurements from the summit of Axial Volcano during the 1998 volcanic episode. Geophys. Res. Lett.,26, 3437-3440.