The Research On Water Pressure Fluctuation And New Method Of Absolute Elevation Survey At A Fix Location Underwater

Elevation survey and control of underwater structure is common task in near shore engineering. Conventional method is that using level together with leveling rod, or utilizing Global Positioning System combined with leveling rod to pass the absolute elevation reading from on land control points to points underwater. Under the constrain upon back sight distance by the resolution of instrument, platforms have to be setup above water surface to bear the instrument so that elevation can be passed over. In the case of far location of construction site from the shore line, the quantity of survey work and the costs of temporary platform both rise up to an unreasonable degree, and the level plus leveling rod method is no longer applicable. In the case of large water depth, flexivity of the leveling rod results in significant tolerance in elevation of underwater points being surveyed. This affects the surveying results acquired with both GPS or level, and in this case conventional method become unacceptable. Therefore new approach has to be developed. The needs become even more exigent when considering the adoption of elevation control approaches in underwater construction equipments in which case the conventional methods are totally not applicable in some certain circumstances.Water pressure relies linearly on water depth, and this simple function relationship can well be used to pass elevation from on shore control points to points underwater. But this cannot be realized until method is developed for canceling wave influence resulting in pressure fluctuation. Therefore pressure fluctuation must be studied so as to find approach of computing swing magnitude and frequency of the fluctuation.This paper, setting out from traditional wave theory, probed into algebra relationship between magnitude and frequency of water pressure fluctuation at a fixed point and that of the surface wave, present the expression of water depth at the fixed point in terms of fluctuation magnitude at the fixed point and the reference point, verified the correctness of the expression with numerical method by implementing code for electronic computing;discussed the suitability of adoption of the above mentioned method to real engineering circumstances based on statistical characteristic values of random wave;applied the spectral analysis method to isolating constituent fluctuation contents and to calculating swing magnitude of energetically significant fluctuation content, present the method of limiting the tolerance of swing magnitude calculation resulting from variance of power spectral density estimation after looked closely into Discrete Fourier transform algorithm during the process of which the margin of the tolerance has been computed;put forward the idea about works to be done for the purpose of perfecting the aforesaid new method of underwater elevation survey and control.