Tectonic or anthropogenic deformation causing seafloor height change can be detected by observing ambient seawater pressure, whose value is governed primarily by depth. Modern pressure sensors based on quartz strain gauge technology can detect the pressure shift associated with subsidence or uplift of the seafloor at the one cm level.
However, all of these gauges exhibit measurable drift over periods exceeding several weeks or months and this drift can be misinterpreted as real seafloor height change. Making matters worse is that the error introduced by the gauge drift will have an unpredictable sign and magnitude and will vary from gauge to gauge. These factors make it difficult or impossible to perform long-term measurements of seafloor height variation without frequent recalibration of the pressure gauges.
Calibrating a pressure gauge involves applying a known pressure to the gauge in question and comparing the gauge’s output to the true pressure value. The difference between the gauge’s output and the known true pressure value is the gauge error and this error can then be subtracted from future measurements to produce a corrected reading.
Generation of this calibration pressure can be done in a number of ways. The most accurate method (the one used in calibration labs) is to use what’s known as a dead weight tester, which consists of a precision mass pressing on a piston cylinder and generating a calibrated pressure in the piston fluid. Proper use of a dead weight tester has historically required a skilled operator to produce an accurate result.
The fact that this calibration procedure requires bulky, specialized equipment combined with the fact that precision pressure gauges require frequent calibration to maintain their accuracy have made long-term measurements of seafloor depth extremely problematic.