Manual on sea level measurement and ... - unesdoc - Unesco
Manual on sea level measurement and ... - unesdoc - Unesco Manual on sea level measurement and ... - unesdoc - Unesco
Sea Level Measurement and Interpretationshould fix the datum to approximately centimetre accuracy.However it is tedious and can only be carried outinfrequently in remote areas.3.3.4 Multiple Pressure Transducer Systems(‘B’ gauges)A method was developed at POL in the early 1990sfor precise datum control of sea level records frompressure tide gauges. An additional pressure pointwas located at approximately mean sea level andfixed relative to the contact point of the gauge. Thejuncture at which the tide fell below this secondsensor could be used to fix the datum of the recordfrom the principal sensor. The technique was foundto be extremely reliable and accurate and now formsthe basis of gauges, called ‘B’ gauges, in POL’s SouthAtlantic and Antarctic networks (Spencer et al.,1993). The principle of the technique was describedin detail in Volume 2 of the
Sea Level Measurement and Interpretationamore. They are self-contained instruments poweredby batteries. They have little application to the longtermmeasurement of coastal sea level but have beenused extensively to obtain initial tidal knowledge ofan area where a coastal gauge is planned. Their mainproblem in the GLOSS context is the lack of a datum.They have principally proved their value offshore andin the deep ocean (Spencer and Vassie, 1997).3.4 Acoustic Tide GaugesA number of acoustic tide gauges have been developedwhich depend on measuring the travel timeof acoustic pulses reflected vertically from the seasurface. This type of measurement can theoreticallybe made in the open with the acoustic transducermounted vertically above the sea surface, but in certainconditions the reflected signals may be lost. Toensure continuous and reliable operation the sensoris located inside a tube that provides some degreeof surface stilling and protects the equipment; somesensors even constrain the acoustic pulses within anarrow vertical tube, which is contained inside theprevious one. This outer tube does not completelyfilter out wave action but, by averaging a number ofmeasurements, the desired filtering is achieved.bThe velocity of sound in air varies significantly withtemperature and humidity (about 0.17%/°C) andsome form of compensation is necessary to obtainsufficient accuracy. The simplest method is to measurethe air temperature continuously at a point inthe air column and use this to calculate the soundvelocity. To account for temperature gradients in theair column, temperature sensors may be required ata number of different levels.A more accurate method of compensation is by useof an acoustic reflector at a fixed level in the air column.By relating the reflection from the sea surfaceto that from the fixed reflector, direct compensationfor variation in sound velocity between the acoustictransducer and the fixed reflector can be achieved.However this still does not account for any variationin sound velocity between the fixed reflector andthe sea surface. To achieve full compensation wouldrequire, in principle, a number of fixed reflectorscovering the full tidal range, but none of the knownacoustic sensors has this possibility.Figure 3.4 (a,b) Schematics of operation of a ‘B’ gauge.3.4.1 Acoustic Gauges with Sounding TubesThe National Oceanic and Atmospheric Administration(NOAA), National Ocean Service (NOS) in the USA,initiated over a decade ago a multi-year implementationof a Next-Generation Water Level MeasurementSystem (NGWLMS), both within the US nationaltide gauge network and at selected sites aroundIOC
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Sea Level Measurement <strong>and</strong> Interpretati<strong>on</strong>should fix the datum to approximately centimetre accuracy.However it is tedious <strong>and</strong> can <strong>on</strong>ly be carried outinfrequently in remote areas.3.3.4 Multiple Pressure Transducer Systems(‘B’ gauges)A method was developed at POL in the early 1990sfor precise datum c<strong>on</strong>trol of <strong>sea</strong> <strong>level</strong> records frompressure tide gauges. An additi<strong>on</strong>al pressure pointwas located at approximately mean <strong>sea</strong> <strong>level</strong> <strong>and</strong>fixed relative to the c<strong>on</strong>tact point of the gauge. Thejuncture at which the tide fell below this sec<strong>on</strong>dsensor could be used to fix the datum of the recordfrom the principal sensor. The technique was foundto be extremely reliable <strong>and</strong> accurate <strong>and</strong> now formsthe basis of gauges, called ‘B’ gauges, in POL’s SouthAtlantic <strong>and</strong> Antarctic networks (Spencer et al.,1993). The principle of the technique was describedin detail in Volume 2 of the <str<strong>on</strong>g>Manual</str<strong>on</strong>g> (IOC, 1994) <strong>and</strong>in the scientific literature (Woodworth et al., 1996).At the time of writing, it is not possible to purcha<strong>sea</strong> ‘B’ gauge although expressi<strong>on</strong>s of interest in theirmanufacture have been obtained from major suppliers.A schematic ‘B’ gauge setup is shown in Figure 3.4,with an absolute pressure sensor in the water (‘C’)<strong>and</strong> another in the atmosphere (‘A’). Paroscientificdigiquartz sensors are employed throughout, althoughless expensive sensors should work reas<strong>on</strong>ably well<strong>and</strong> are being investigated. The difference C–A gives<strong>sea</strong> <strong>level</strong>, after correcti<strong>on</strong>s for <strong>sea</strong>water density <strong>and</strong>accelerati<strong>on</strong> due to gravity are applied. A third sensoris placed at ‘Datum B’ which is near mean <strong>sea</strong> <strong>level</strong>.The height of ‘Datum B’ has to be known accuratelyrelative to the c<strong>on</strong>tact point of the installati<strong>on</strong> <strong>and</strong> tothe local l<strong>and</strong> <strong>level</strong>ling network. The difference B–A isagain a <strong>sea</strong> <strong>level</strong> height, but <strong>on</strong>ly when the <strong>sea</strong> <strong>level</strong>is above ‘Datum B’. The top part of this record can befitted to the equivalent part of the record from theprincipal sensor to transpose the known datum to thefull <strong>sea</strong> <strong>level</strong> record. It is important that all sensors aredriven from the same c<strong>on</strong>trol <strong>and</strong> logging system tomaintain synchr<strong>on</strong>y. Sampling the data at 15-minuteintervals or less is preferred for the identificati<strong>on</strong> ofthe inflexi<strong>on</strong> points, i.e. the time at which the <strong>sea</strong> <strong>level</strong>falls (or rises) below (or above) ‘Datum-B’.The essential feature is that, while any pressure measuredby a sensor at B will c<strong>on</strong>tain an offset, <strong>and</strong> perhapsa drift, the vertical height of its effective pressurepoint can be positi<strong>on</strong>ed at ‘Datum B’ very accurately.So, although it is not known what it is measuring towithin perhaps a few hectopascals (centimetres), it isknown where it is measuring with millimetric precisi<strong>on</strong>.The flat part of B–A <strong>and</strong> its inflexi<strong>on</strong> points provide anextremely precisely defined shape which is immune toany problems with datum offsets <strong>and</strong> low-frequencyinstrumental drifts. Experience with several instrumentsat different sites suggests that datums can be fixed towithin a few millimetres by this technique.To work properly, the method needs a sizable tidal range,so that B will spend half its time in water <strong>and</strong> half in air.It will not work in lakes or microtidal areas, but mostcoastal <strong>and</strong> many isl<strong>and</strong> sites have usable tidal ranges,even if <strong>on</strong>ly at spring tides. In the presence of waves,the flat porti<strong>on</strong> of the ‘B’ gauge is reduced in length<strong>and</strong> may be unu<strong>sea</strong>ble under large wave c<strong>on</strong>diti<strong>on</strong>s.However, there are is always a sufficient number of calmdays during which the technique can be applied.In practice, the two pressure sensors in the <strong>sea</strong> are colocatednear the base of the installati<strong>on</strong> with a rigidtube c<strong>on</strong>necting the ‘B’ gauge to its appropriate datumpoint. This avoids the ‘B’ sensor being subject to atmospherictemperature variati<strong>on</strong>s that are more severethan those of the <strong>sea</strong>. The barometric sensor may alsobe installed at the same positi<strong>on</strong> with a tube open toatmosphere. Alternatively it may be installed as partof the data logger in the tide gauge hut. The methoddoes not require the actual installed height of C or A tobe known. Where it is difficult to install a fixed gaugeC below the water, because of shallow gradients perhaps,then a pop-up or bottom-mounted gauge couldequally well be used.3.3.5 Pressure Transducers in Stilling WellsA variant of the ‘B’ gauge method described aboveis to install an absolute pressure sensor below lowwater in a stilling well that has been used hithertoin a float system. This transducer will be functi<strong>on</strong>allythe same as sensor ‘C’ <strong>and</strong> will be complementedby a transducer ‘A’ that records atmospheric pressure,as described above. Alternatively, a ‘differential’sensor could be used. Instead of a third sensoremployed in the ‘B’ gauge, datum c<strong>on</strong>trol for theC–A pressure-difference time-series is provided bymeans of regular, preferably daily, electr<strong>on</strong>ic datumprobe checks of the <strong>level</strong> in the well relative to thetide gauge CP <strong>and</strong> TGBM. Comparis<strong>on</strong> of the valuesof C–A, corrected for density <strong>and</strong> accelerati<strong>on</strong>due to gravity, with the well soundings, providesan <strong>on</strong>going datum for the time-series which canaccommodate transducer drift <strong>and</strong> variati<strong>on</strong>s in theproperties of the <strong>sea</strong> water.This method has many of the advantages of pressuresystems <strong>and</strong> of electr<strong>on</strong>ic datum probes, combinedwith the recognized disadvantages inherent in theuse of stilling wells (Lenn<strong>on</strong>, 1971). It may be apreferred opti<strong>on</strong> if <strong>measurement</strong>s are required froma well that has produced l<strong>on</strong>g-term <strong>measurement</strong>sfrom a float gauge.3.3.6 Bottom-mounted Pressure GaugesBottom pressure gauges rest <strong>on</strong> the <strong>sea</strong> bed <strong>and</strong>record pressure at intervals over periods of a year or16IOC <str<strong>on</strong>g>Manual</str<strong>on</strong>g>s <strong>and</strong> Guides No 14 vol IV