On the sea state dependence of sea level measurements at Platform Harvest

Abstract

This paper investigates correlations between sea level measurements and H 1/3 at Texaco's Platform Harvest. In order to do so, a relationship is shown between the sample standard deviations of the 6‐min NOAA acoustic system measurements and H 1/3 as estimated from CU Paros depth sensors. A transfer function for the NOAA acoustic system is developed by comparison between spectra of 2‐s tsunami mode data with spectra from 1.1‐s pressure data. Part of this transfer function is caused by the decay in pressure effects due to waves between the surface and the orifice of the protective well. The rest is due to damping characteristics of the protective well itself. The transfer function helps show the limitations of using the sample standard deviations of the NOAA acoustic system as a measure of H 1/3 . Differences between sea level measurements show a distinct component related to H 1/3 . The effects related to H 1/3 low‐frequency differences in the measurements, and wave effects are separated using an iterative procedure. If H 1/3 ‐related effects are removed, then the sea level measurements agree to better than 1 cm rms for all three systems. The cause of the low‐frequency differences is not understood. An estimate of the H 1/3 dependence of each individual system is found by removing a 1‐year tidal prediction based on the NOAA acoustic system from sea level from each of the systems. It is assumed that real H 1/3 dependencies in sea level are small. The resulting residuals are iteratively separated as before. These results indicate that the greatest H 1/3 dependence is found in the NOAA Digibub system and the least in the NOAA acoustic system. It should be noted that the H 1/3 dependency found here occurs because of the harsh open ocean conditions at Platform Harvest and would not be expected to be observed under normal conditions in sheltered waters such as harbors, rivers, or lagoons.