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

Sea Level Measurement and InterpretationA modern version of the NGWLMS is called a SeaRanger which is claimed to have a number of advantagesover the earlier technology including self calibration(IOC, 2004)3.4.2 Acoustic Gauges without Sounding TubesSeveral acoustic instruments have been producedthat are operated without a sounding tube, normallylocated inside an existing stilling well or inside aplastic tube some 25 cm in diameter. Some of themmay operate in the open air, but are not normallyemployed for high-quality sea level measurements(see Table 3.1 in section 3.6). These acoustic instrumentsoperate at a frequency of 40–50 kHz and havea relatively narrow beam width of 5°. Their measurementrange is approximately 15 m and an overallaccuracy of 0.05% is claimed by the manufacturers(see websites below).Contradictory experiences can be found with this typeof acoustic sensor, from some problems in achievingthe stated accuracy under all environmental conditions(e.g. see presentation by Ruth Farre, in IOC, 2003), tothe high-quality and continuous operation of 15 tidegauges in the REDMAR network (Spain), most of theminstalled in 1992 and still in operation (e.g. see presentationby Begoña Pérez in IOC, 2003).A crucial aspect of this type of sensor is the dependenceof the velocity of sound on the environmentalconditions, such as the air temperature. On the otherhand, tubes tend to increase the temperature-gradientbetween the instrument and the sea surfaceunless special precautions are taken to ensure thatthe air is well mixed in the tube. A complementaryand necessary method is to compensate for soundvelocity variations using a reflector mounted at asuitable distance below the transmitter, as is the casefor the SRD gauges employed in the REDMAR network.A careful design of the installation, avoidingdifferent ambient conditions along the tube and followingthe maker’s requirements about the minimumdistance to the water surface, become crucial for thefinal accuracy of the data.The performance of one of these sensors (SRD) overan existing stilling well inside a hut or small buildingin Santander (Spain), has been incredibly good (nearlyperfect and continuous during 15 years). The conditionsof this installation are probably perfect, perhapsbecause the temperature inside the building is ratherhomogeneous. Data from this acoustic sensor have infact helped to correct malfunctions of the float gaugethat operates inside the same stilling well.Studies of mean sea levels from 12 years of data inSpain, comparing this type of acoustic sensor (SRD)with the traditional float gauges, has shown theirhigh quality and has even helped to identify referencejumps in the older float gauges. This is, again,a contradictory experience to the one in South Africa(see article by Farre in Appendix V of this volume).Nevertheless, it seems that radar gauges will replacethis type of acoustic sensor everywhere, in the nearfuture.3.5 Radar GaugesRadar tide gauges have become available during thelast few years from several manufacturers. Although thistechnology is relatively new, radar gauges are being purchasedand installed by a number of agencies as replacementsfor older instruments or for completely newnetworks. The reason is that they are as easy to operateand maintain as acoustic sensors, without their maindisadvantage: their high dependence on the air temperature.Radar gauges have a relatively low cost and theengineering work necessary to install them is relativelysimple compared to other systems. The instruments aresupplied with the necessary hardware and software toconvert the radar measurements into a sea-level height.In addition, the output signals are often compatible withexisting data loggers or can be interfaced to a communicationnetwork. Like many modern systems they can beset up using a portable computer.The active part of the gauge is located above thewater surface and measures the distance from thispoint to the air–sea interface. A diagram is given inFigure 3.6. The gauge has to be mounted in such away that there are no restrictions or reflectors in thepath of the radar beam, between the gauge mountingand the sea surface. It has to be positioned abovethe highest expected sea level and preferably abovethe highest expected wave height, so as to preventphysical damage.It has many advantages over traditional systems inthat it makes a direct measurement of sea level.The effects of density and temperature variations,even in the atmosphere, are unimportant. The mainconstraint is that the power consumption may berelatively large in radar systems if used on a continuousbasis in a rapid sampling mode. Averages aretypically taken over periods of minutes. This maylimit its use in some applications (e.g. tsunami warning)where observations are required on a continuoushigh-frequency (e.g. 15-second) basis. In such areas,pressure gauges may be more appropriate, althoughwork and research is still being done concerning thisparticular application.Radar gauges fall into two categories. Those thattransmit a continuous frequency and use the phaseshift between transmitted and received signal to determinesea level height (frequency-modulated continuousIOC <strong>Manual</strong>s and Guides No 14 vol IV19