Manual on sea level measurement and ... - unesdoc - Unesco

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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 <strong>Manual</strong> (IOC, 1994) andin the scientific literature (Woodworth et al., 1996).At the time of writing, it is not possible to purchasea ‘B’ gauge although expressions 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’)and another in the atmosphere (‘A’). Paroscientificdigiquartz sensors are employed throughout, althoughless expensive sensors should work reasonably welland are being investigated. The difference C–A givessea level, after corrections for seawater density andacceleration due to gravity are applied. A third sensoris placed at ‘Datum B’ which is near mean sea level.The height of ‘Datum B’ has to be known accuratelyrelative to the contact point of the installation and tothe local land levelling network. The difference B–A isagain a sea level height, but only when the sea levelis 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 sea level record. It is important that all sensors aredriven from the same control and logging system tomaintain synchrony. Sampling the data at 15-minuteintervals or less is preferred for the identification ofthe inflexion points, i.e. the time at which the sea levelfalls (or rises) below (or above) ‘Datum-B’.The essential feature is that, while any pressure measuredby a sensor at B will contain an offset, and perhapsa drift, the vertical height of its effective pressurepoint can be positioned 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 precision.The flat part of B–A and its inflexion points provide anextremely precisely defined shape which is immune toany problems with datum offsets and 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 and half in air.It will not work in lakes or microtidal areas, but mostcoastal and many island sites have usable tidal ranges,even if only at spring tides. In the presence of waves,the flat portion of the ‘B’ gauge is reduced in lengthand may be unuseable under large wave conditions.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 sea are colocatednear the base of the installation with a rigidtube connecting the ‘B’ gauge to its appropriate datumpoint. This avoids the ‘B’ sensor being subject to atmospherictemperature variations that are more severethan those of the sea. The barometric sensor may alsobe installed at the same position 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 functionallythe same as sensor ‘C’ and 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 control for theC–A pressure-difference time-series is provided bymeans of regular, preferably daily, electronic datumprobe checks of the level in the well relative to thetide gauge CP and TGBM. Comparison of the valuesof C–A, corrected for density and accelerationdue to gravity, with the well soundings, providesan ongoing datum for the time-series which canaccommodate transducer drift and variations in theproperties of the sea water.This method has many of the advantages of pressuresystems and of electronic datum probes, combinedwith the recognized disadvantages inherent in theuse of stilling wells (Lennon, 1971). It may be apreferred option if measurements are required froma well that has produced long-term measurementsfrom a float gauge.3.3.6 Bottom-mounted Pressure GaugesBottom pressure gauges rest on the sea bed andrecord pressure at intervals over periods of a year or16IOC <strong>Manual</strong>s and Guides No 14 vol IV
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 <strong>Manual</strong>s and Guides No 14 vol IV17
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