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Sea Level Measurement and InterpretationAnalog dataFloat in stilling well Datalogger: Sutron 8210Stilling wellAnalogLP-filterEncoderA/DconverterDigital dataData rate1 HzLP-filterArithmeticmean over3 minutesDecimatingto 10minutevaluesISDN orGPRSData rate1/600 HzFigure 3. Schematic description of sampling and filtering in sea level gauge.Sampling and filtering in the sea level gaugeThe stilling well represents a mechanical low pass (LP) filter.The attenuation R depends on the relationship betweenthe area of the cross-section of the well and the area ofthe orifice: R = (area of well)/(area of orifice) (Forrester1983). This relationship also affects the response time,which is defined as the time it takes before the sea levelon the inside has changed to the mid-point of a suddenand permanent change on the outside. A high value of Rgives high attenuation and long response time. It is recommendedby Forrester (1983) to choose R = 100, whichwill pass waves with periods of, for example, 12 h (representativefor tide) and periods of 6 min (representative forharbour seiche), but will attenuate waves with a periodof 6 s (representative for surface swell). The –3 dB cutofffrequency will be roughly 1/40 Hz (taken from plot in Shihand Rodgers (1981). R = 100 signifies a response time of11 s (Forrester 1983). Most of our sea level gauges have Rclose to 100. A sampling frequency of 1 Hz inside a stillingwell of this type should be satisfactory.In the datalogger a 3-min arithmetic mean is calculatedevery 10 min. Studies at NHS have shown that this filterdoes not remove all frequencies above the Nyquist frequencyfor 10-minute sampling (f Ny = f s /2 = 1/(2x10x60)Hz = 0.0008333… Hz). There is now a test of calculatingthe 1-minute arithmetic mean every minute in the datalogger and transmitting 1-minute data to the office. In thiscase the filter is better suited for the sampling rate, and weshall get access to a higher data rate as well.Processing of data at the officeThe received dataare stored in a database. Automatic and manual qualitycontrol are applied to the data.One-hour values are used for the harmonic analysis. Toavoid aliasing, all frequencies above the Nyquist frequency(f Nyq ) should be removed before decimating from 10-minutevalues to 1-hour-values. The Nyquist frequency f Nyq ishalf of the sampling frequency f s = 1/(1 hour).A 4th-order Butterworth filter is used for this purpose(Hodnesdal 1983; see amplitude response in Figure 4).The cut off frequency is 1/(3 hours). The time-series isrun forwards and backwards through this filter to ensurezero phase response. The squared amplitude response is–29.4 dB (=0.034 Hz) at f Nyq . This is an acceptable antialiasingfilter because almost all energy above the Nyquistfrequency is removed. The problem is, however, that someof the over-harmonic constituents will be partly filtered aswell. Frequencies from the fifth-diurnal (period of circa 4.8h) and higher will be attenuated by the filter. For stationswith significant shallow water constituents, a filter with ahigher cut-off and faster roll-off should be used.For further information, go to: http://vannstand.statkart.no/Magnitude SquaredFigure 4. Amplitude response of 4th-order Butterworthfilter.ReferencesForrester, W.D. 1983. Canadian Tidal <strong>Manual</strong>. Departmentof Fisheries and Oceans, Canadian Hydrographic Service,Ottawa. 138pp.Hodnesdal, H. 2003. Use of Butterworth filter for LPfilteringof water level data in NHS. Technical Report.Norwegian Hydrographic Service, Stavanger, Norway.Shih, H.H. and Rogers, D. 1981. Error analysis for tidemeasurement systems utilizing stilling wells. TechnicalReport. US Department Commerce, National Oceanicand Atmospheric Administration (NOAA), USA.60IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and InterpretationThe ESEAS-RI Sea Level Pilot Station in Vilagarcía de Arousa (Spain)B. Martín, B. Pérez, E. Alvarez FanjulPuertos del Estado, Madrid, Spain, E-mail: bego@puertos.esIntroductionDuring the ESEAS-RI project (European Sea Level Service –Research Infrastructure), a test station for sea level sensorswas established at Vilagarcía de Arousa, on the northwestcoast of Spain. One of the objectives was to experimentwith different kinds of radar sensor, an emergingtechnology for measuring sea level, and compare theirperformance with other traditional and well proven tidegauges. The main advantage of radar appeared to be itsdemanded accuracy, low maintenance and lack of influenceof air temperature, humidity or density of the water.The experiment was carried out when the existing networkswere requiring renewal, owing to the age of theequipment, and radar appeared as an even better optionthan acoustic sensors, and possible new applications ofsea level data had to be taken into account.Experiment and description of the sensorsThe period of operation of the different tide gauges isshown in Figure 2, and varies mainly due to the differentdates of incorporation into the experiment and toproblems encountered during the first year, sometimeseven due to lack of experience with the equipment.Most of the equipment installed is very well known anddescribed in the literature and in previous IOC manuals,such as the pressure (both the single pressure sensorand the bubbler sensor from POL) and the acousticgauges (both the Aquatrak from NOAA and the SRDfrom the REDMAR network). As was mentioned, themain contribution of the experiment was the testing ofseveral new radar sensors and the simultaneous installationof so many tide gauges for the first time.Figure 1. Location of Vilagarcía de Arousa (left), photo of the test station (centre) and of the REDMAR station (right,an SRD acoustic sensor on another dock).The main modern technologies have been tested inVilagarcía: pressure, acoustic and radar, a total ofeight different sensors. Different institutions and privatecompanies provided sensors for the test; the NationalOceanographic and Atmospheric Administration (NOAA,USA) lent an Aquatrak acoustic sensor, and the ProudmanOceanographic Laboratory (POL, UK) lent a bubbler pressuresensor. Apart from this, two FMCW radar sensorswere provided by the companies Miros and ENRAF(Radac). The contribution of Puertos del Estado was theacoustic sensor of the REDMAR network (SRD), and twopulse radar stations: Seba and Geónica. Finally, a SeaBirdpressure sensor was also installed during the experimentby the maintenance company SIDMAR Bernhard Pack.Focusing on the radar technology, two different typesof radar were installed (sensors were always located ata certain height above the sea surface):Pulse radar: operates on a similar principle to that ofthe acoustic sensor, by measuring the travel time ofmicrowave pulses between the sensor and the watersurface and the return echo to the sensor. The mainadvantage over the acoustic gauges is that the velocityof propagation (light velocity, c, in air) of the wave doesnot depend on environmental conditions, in particularon temperature gradients along the path of the pulse.This makes the installation requirements less strict andno protective tube or pre-measurement calibration isneeded. Geónica and Seba radars are pulse radars andin fact use the same Vegapuls transducer.IOC <strong>Manual</strong>s and Guides No 14 vol IV61
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