12/11/2013& egm502 seafloor mapping

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1 egm502 seafloor mapping lecture 13 multi-beam echo-sounders The majority of the current charts of the ocean floors have been produced from single beam echo-sounder data. Even though these data have been acquired on a continuous basis for at least 50 years, large tracts of the seafloor remain unmapped. One of the reasons for this is that survey lines for conventional bathymetric surveys are often spaced 100's to 1000's of kilometres apart. This leaves us in the bizarre situation that more is known about the surface of many of the planets in the solar system than is know about Earth. reading lurton, 2010, Section 8.3 Geological Survey of Norway, 2004 The MBES is an extension of the SBES. Instead of transmitting and receiving a single vertical beam, the MBES transmits and receives a fan of beams with small beam angles (typically 1 or 2 ) across the ship axis. Typically achieving 200 or more depth measurements, imaging a wide swath up to 150 (7.5 times water depth). Now the primary tool for seafloor mapping. Two types of swath systems are commonly employed for bathymetric mapping multi-beam echo-sounders (beamforming) and interferometric sonars (also termed bathymetric sidescan sonar). Each of these systems has its own advantages and disadvantages. To put it simply, MBES systems offer higher-resolution bathymetric data with low-order backscatter data as a by-product, whereas interferometric systems offer lower-order bathymetric data and true side-scan. The more sophisticated MBES can simultaneously acquire depth and backscatter data for each point insonified on the seafloor 1&

2 Beamforming Artificially steers the array directivity pattern. Beamforming (sometimes refereed to as spatial filtering) is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. In simple terms..for a sonar to transmit a sharp pulse of underwater sound towards the seafloor, simply transmitting that sharp pulse from every projector in an array simultaneously fails because the near-field is insonified before the far-field. The beamforming technique solves this by sending the pulse from each projector at slightly different times (the central projector last), so that every pulse hits the seafloor at exactly the same time, producing the effect of a single strong pulse from a single powerful projector. Interferometric sonar (Gostnell, 2010) IFMS systems are not beam-forming, but measure depths at precise locations on the seafloor via the use of exactly spaced phase differencing transducer elements which measure the phase offsets of acoustic returns. The phase offset is used to calculate the angle ( ) from which the return was received. The angle, in combination with range based on two way travel time, is used to calculate the position of the seafloor and objects upon it relative to the instrument. This provides accurate bathymetric data co-located with SSS imagery which can be used to create either side-scan sonar (SSS) imagery, bathymetry, or imagery with associated depths. MBES have varied applications. 3 main categories: Deep water systems Typically 12 khz for deep ocean and 30 khz for continental slopes Designed for regional mapping Large arrays, limited t large deep-sea vessels Shallow-water systems Typically khz Designed for mapping continental shelves Best suited to hydrography High-resolution systems Typically khz Designed for high-resolution imaging Hydrography, shipwreck location, inspection of underwater structures Small sizes means suitable for deployment on ROVs and AUVs 2&

3 ADUS, 2005 ADUS Transmission and reception arrays Narrow horizontal sector, narrow beam angles insonify thin strip acrosstrack Wide vertical sector, broad angular sector, large across-track sector Most commonly employed reception arrays: Horizontal linear V-shaped U-shaped ADUS Simplest configuration, limits useful swath width to Unstable hull mount. & Two linear arrays, inclined (aka dualhead). Allows large swaths with moderate beam-steering. & Circular section array, enables simple beamforming. & 3&

4 Resolution considerations Across Track Along Track Across track resolution controlled by: No. of beams, angular sector, beamwidth and beam spacing. Along track resolution controlled by: Vessel speed and pulse rate. Beamwidth and data resolution Same seafloor imaged with 2 different systems Angular Sector (α) (α)# d MBES are often described by their achievable angular sector (α), e.g. 120 o α" w If α = 120, then w = 2d tan(120/2) w = 3.46d The swath width of the 120 o system therefore approximates 3.5 times the water depth 4&

5 HYDROGRAPHIC SURVEYING (IHO, 2005, 2011) Survey Specifications Name Craig Dyer Degree BSc Marine Science I graduated with a 2.1 in 2011 and started with Fugro EMU in Southampton. I am now a Senior Hydrographic Surveyor, responsible for the collection and consolidation of high-resolution bathymetric data. To accommodate in a systematic manner different accuracy requirements for areas to be surveyed, four orders of (hydrographic) survey are defined by IHO in publication S-44 5th Edition One of the many surveys we are completing is the UK Civil Hydrography Programme, helping the UKHO and MCA update nautical charts for safe navigation. 5&

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