Hydrographic Survey, Echo Sounding - Essay Example

Medway Ports, Kent, UK, Hydrographic Surveying: Conversion of Single-beam Echo Sounders to Multiple-beams Ones: Implications and Strategies Introduction Hydrography is a science that produces charts and maps with points that have the following characteristics - they show latitudinal and longitudinal position, they show depth of water at that position with chronological compatibility to tidal variations and they also show distance from particular land features like coastline indentures (Mason, 2007). In the United Kingdom, hydrography has a long history since the British were always a sea-faring nation with easy access to the sea and with little alternative for trade and commerce with other nations than by the sea. King Charles 2 appointed Captain Grenville Collins as the 'Hydrographer to the King' as far back in time as 1683 (Mason, 2007). Since then the science has evolved from a crude form of conjecture to a fairly accurate predictive one. This paper prepares a report form of investigation in which the implications of converting hydrographic surveying based on single-beam echo sounders to multiple-beam ones shall be studied and considered in detail to assess feasibility and functional efficacy with particular reference to Medway Ports, UK. To initiate this noble venture it is first necessary to study a little of Medway Port, Kent, UK, especially in detail to its known navigation features. Medway Port, Kent, UK The Medway Ports authorities define the Medway River as one of the principal trade arteries of the United Kingdom with particular utility to the South East of England through which this river flows before ending into the sea strategically near to the English Channel and the North Sea (Overview, Medway Ports, 2007). This proximal position to two main European trade routes, the port's efficient functionality and its easy accessibility to its hinterland by road, rail and water has eventually evolved it into a port of choice for many export and import operators. Cargoes such a fresh produce, forest products, new vehicles and steel and containers pass through it to and fro British soil (Overview, Medway Ports, 2007). Actually, the Medway Ports authority is a statutory one and is responsible for a 27 mile stretch of water that is a combination of the Medway and Swale rivers. The authority is responsible for maintaining the rivers and for providing and coordinating pilotage and vessel traffic services to ensure safety of all river users including pleasure boat ones (Overview, Medway Ports, 2007). The Ports are mainly a combination of the Sheerness and Chatham docks (Peel Ports Group, Medway Ports, 2007). Britain's first LNG handling terminal was constructed at the Isle of Grain within the Ports complex (Peel Ports Group, Medway Ports, 2007). Pilotage services provided by the Ports authorities are compulsory for vessels of 50 metres and above in length. The Medway Conservancy Board has taken over hydrographic surveying of the Medway Ports system from the Royal Navy Hydrographic Inshore Squadron since the 1960s. Presently, it has tow crafts at its service - the 18.9 metre Medway Surveyor that surveys the main reaches of the Medway and the deeper offshore waters and the 6 metre Medway Recorder which surveys the berths and shallower waters. Both vessels have permanently installed echo sounders that can also work as side scan sonar recorders (Hydrographic Service, Medway Navigation Service, 2007). It is assumed that the echo-sounders are all single beam ones. Hydrographic Surveying Hydrographic surveying is undertaken in two types of operational locations - oceanic or relatively deeper offshore waters and relatively shallower inshore waters. As has already been evidenced with Medway Ports, the former locations are usually surveyed by larger survey ships like the Medway Surveyor and the latter locations by smaller ships like the Medway Recorder. Such ships, if in combination as in the case of the Medway Ports, undertake survey work in tandem to establish the following navigation-related features of the coastline and its immediately offshore waters. They establish clearly evident datum positions based on previous survey work or on present survey data. They fix the coastline and considerable features relevant to mariners coming inshore off the sea or going off towards it. They fix the positions of islands, shoals and other dangerous obstructive structures. They make recordings of depths of water at epochal periods to supply tidal databases and changing current and water level recorders. (Mason, 2007) It is notable that, even for small survey ships like the Medway Recorder, if the water is still too shallow, small auxiliary vessels are called into service for sounding operations (Mason, 2007). To allow collection of the datasets that shall assist in pinpointing the above positional factors core hydrographic survey work entails fixing geographic variables like latitude and longitude that are easily available nowadays from GPS (Global Positioning Systems) associated with satellites and other variables that are collected as follows. 1. There may be necessity to measure the Earth's magnetic field in coastal waters for contingency situations like entry of foreign submarines that can effectively change magnetic fields and allow detection. 2. Sea temperature at various depths is often crucial to producing sensitive hydrographical data that may subsequently be utilised to produce navigation-related information. 3. Core sample collection from undersea floors can provide sensitive information on natural coastal structures relevant to navigation. 4. Radars and sonars are utilised extensively nowadays to provide data on water depths and undersea structures. (Mason, 2007) It is notable in this context that hydrograpic surveying, though observed here from the viewpoint of providing data for navigability of a port, can very well have other implications related to modern day coastal management. Conservation of coastal flora and fauna, environmental controls like monitoring of effluents let out into the sea or river area, tourism potentials and recreational activities can all benefit from such data revealed by hydrographic surveying (Abbott, 2001). The net bathymetric data yielded by hydrographic surveys using the above elements are primarily Port Harmonic Constants, Shallow Water Constants and Seasonal Changes in Mean Levels, the most important aids to navigation and other possible river, sea and ocean uses. Aside from the above elements atmospheric pressure, changeable according to wind strengths are other meteorological factors, also influences bathymetric decisions since elements like storm surges and others are, in turn, influenced by these and may affect depth and other purely bathemetric data lines (Poltips 3, Undated) yielded by hydrographic surveys. Hydrographic Surveys: Uncertainties Bathymetric data elicited through electronic systems in hydrographic surveys are known to possess certain uncertainties. Many of these uncertainties have been outlined in Hare, Godin and Mayer 1995 'Uncertainty Model' (Brennan et al, Undated). It has since been estimated that the highest percentage of total error in calculations for data yielded by near coastal (as for Medway Ports here) hydrographic surveys is constituted of water level uncertainties (Brennan et al, Undated). Water Level Corrections: Traditional water level correction methods used discrete zones within the survey area and developed constant phase and amplitude differentials from the Mean Lower Low Water (MLLW) to all assessed water levels within that particular zone. Thus, net gradients become artificially evident at many zonal boundaries (Brennan et al, Undated) as the differentials varied zonally. In 1995, a new correction method - the Tidal Constituent and Residual Interpolation (TCARI) method was developed (Brennan et al, Undated). TCARI has the ability to interpolate the amplitude and phase of various tidal harmonic constituents, the residual water level and other datum for any point with the survey zone. Its speciality is that there are no apparent gradients between and among various regions within the survey zone as no discrete zonal assignments are used. It, instead, produces an apparent lack of spatial discontinuity that can allow it to account for high water and low water intervals, changes in tide types - diurnal, semidiurnal or mixed between stations and it is designed to accommodate spatiotemporal variations in other non-tidal components (Brennan et al, Undated). The TCARI-produced apparently continuous water level topography, when aided by high resolution bathymetry, can provide real time water level corrections creating possibilities for producing dynamic 'Electronic Navigation Charts' (ENC), such as for Medway Ports, that can allow vessels to chart along a continuous course from point to point (Brennan et al, Undated). TCARI Error Types: The first error type is associated with the Harmonic Constants. Each harmonic constant has two components - the amplitude and the epoch or phase. For a particular geographic location it may need up to forty tidal constituents to develop the separate harmonic constants for each constituent and these constants are particular to each location within the survey area. The probability of error is much increased by the diverse and wide-ranged data series'. The next error type evolves from the water level measurements that are the mean and standard deviations of standard sample measurements taken at regular intervals over demarked periods of time. The third source of error is the Laplace Equation interpolation that creates spatial weighing functions for each of the station values - harmonic constants for each tidal constituents, residuals and datum - within the survey region. There is also complicacy of the errors of modelling and Laplace algorithm. The last source of error is the datum. It is assumed that the time variance during the short period of surveying is too negligible yet error probabilities have to be considered and provided for. (Brennan et al, Undated) The Harmonic Constants: Tidal constituents like the pattern (cycle) of pull or push effects of the sun's gravity is well known for planet earth. Once constituents such as these are known through tidal analysis their locationally unique amplitude and phase (the harmonic constants at that place) at a particular location is assessed through hydrographic surveys. These harmonic constants together with the speed of the tidal constituents are used in combination to predict the componential rise or fall in water that tidal constituent is responsible for (Poltips 3, Undated). The total effect of all the tidal constituents at a particular geographical location allows accurate prediction of the tidal rise or ebb. Echo Sounding Single Beam Sonar: Single beam sonar (Sound navigation and ranging) equipment consist primarily of a sensor device that has a transducer fitted to a signal processor and a display device. The signal producer emits a single acoustic energy beam that slices through the water till it is reflected back from the sea-floor. Sensor devices in the transducer picks up the reflected acoustic energy beam and, by measuring strength and time of return, determines the distance of the sea-floor from the sonar-carrying vessel (Remote Sensing for Coastal Management, Single Sonar, Undated). Since there is only a single beam the survey area is rather loosely assessed for depth and the data collected is raster type. Multi-Beam Sonar: Multi-beam sonar uses a multiple of acoustic energy beams in a descending fan form to scan the sea-floor for depth. The acoustic beams are reflected from over a larger sea-floor area than single-beam sonar devices and an overlap of the multiple-beam fans ensure thorough coverage of the sea-floor. Unlike with single beam sonar where the intrinsic properties of the beams determine depth multi-beam sonar beams ar4e affected by the roll, pitch and heave of the vessel. Thus, modern multi-beam devices use continuous vessel altitude measurements to correct these vessel movement characteristics (Remote Sensing for Coastal Management, Multibeam Sonar, Undated) Conclusion The reason why the paper has used most of its strength in explaining the most recent techniques in hydrographic surveying instead of emphasising on sonar single and multi-beam devices is simple. It is now obvious that for the most modern technique of TCARI for water level corrections multi-beam sonar surveys are essential. This is so because continuous depth data is essential so that interpolation of all the bathymetric data collected for the various tidal constituents is possible. Single-beam sonar would yield large gaps in sea-floor bathymetric data and this would produce large anomalies for such data collated for the various constituents. The anomalies would be so large that it would be difficult to produce a single differential that would cover the entire port area with an effective electronic navigation chart (ENC) that vessels entering the port can map across. Multi-beam sonar, on the other hand, can produce continuous bathymetric data that can be used effectively in TCARI to produce a reliable ENC. Also, multi-beam sonar is capable of other discrete operations like sensing diffracted beams from uneven sea-floors to produce ENCs of sea-floor structures that explain variations in depth due to the unevenness thereunder. The paper believes that it has amply explained the utility of using multi-beam sonar in hydrographic surveying to produce reliable ENCs for ports like Medway, Kent, UK where the river meets sea and produces more depth anomalies than is evident in ports that have no complications of river drafts. References Abbott, Dr. Victor, The Surveyor and the Coastal Zone, The Hydrographic Journal, Issue 99, 2001. Extracted on 20th December, 2007, from: http://www.hydrographicsociety.org/Articles/journal/2001/99-3.htm Brennan, Lt. Richard T. et al, The Design of an Uncertainty Model for the Tidal Constituent and Residual Interpolation (TCARI) Method for Tidal Correction of Bathymetric Data, Undated. Extracted on 20th December, 2007, from: http://www.thsoa.org/hy05/09_2a.pdf Hydrographic Service, Medway Navigation Service, 2007. Extracted on 20th December, 2007, from: http://www.medwaynavigation.co.uk/services.html Mason, Geoffery B., Hydrographic Survey Work in the Royal Navy up to the 1980s, 2007. Extracted on 15th December, 2007, from: http://www.naval-history.net/xGM-Tech-HydrographicSurvey.htm Overview, Peel Ports Group, Medway Ports, 2007. Extracted on 20th December, 2007, from: http://www.medwayports.com/index2.htm Poltips 3, User Guide, Proudman Oceanographic Laboratory, Bidston Observatory, Merseyside, Undated. Extracted on 20th December, 2007, from: http://www.pol.ac.uk/appl/downloads/PT3guide.pdf Remote Sensing for Coastal Management, Single Sonar, Multibeam Sonar, Undated. Extracted on 20th December, 2007, from: http://www.csc.noaa.gov/crs/rs_apps/sensors/single_beam.htm Read More