Salinity: A measure of the quantity of dissolved salts in sea water. It is normally defind as the total amount of dissolved solids in sea water in parts per thousand (0/00) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized. These qualifications result from the chemical difficulty in drying the salts in sea water. In practice, salinity is not determined directly but is computed from chlorinity, electrical conductivity, refractive index, or some other property whose relationship to salinity is well established. Because of the Law of Constancy of Proportions, the amount of chlorinity in a sea water sample is used to establish the sample’s salinity. The relationship between chlorinity (Cl) and salinity (S) as set forth in Knudsen’s Tables is:
S=0.03 + 1.805 Cl
A joint committee of IAPSO, UNESCO, ICES, and SCOR proposed the universal adoption of the following equation for determining salinity from chlorinity:
It was adopted by IAPSO in 1963 and ICES in 1964.
Scale Marks: Equidistant, regular marks on a sonar display used to assist in the mensuration of targets and anomalies and to provide information on the range displacement of targets from the towfish path. Most sonar systems allow the user to choose how and where to display scale lines or scale marks. Common settings in side scan are 10, 15, 25 or 50 metres.
Scattering: The diffusion of the sonar signal in many directions through refraction, diffraction and reflection, primarily due to the material properties of the insonified areas. Scattering is one of the causes of attenuation in sonar, resulting in signal weakening. See also Backscatter.
Sea: The great body of salt water in general, as opposed to Land. One of the smaller divisions of the Oceans. The state of the surface of the Ocean with regard to wave or swell, as a calm sea. See Cross Sea, Head Sea, Beam Sea, Following Sea, Quartering Sea, Sugar Loaf Sea.
Sea Clutter: The images created in a sonar display by acoustic returns from a rough sea surface. When using side scan sonar, some energy is projected above the horizontal from the wide vertical beam. If the sea surface is rough and within the range setting of the system, formless patches may overlay the normal seabed returns. This display “clutter” may be more noticeable on one sonar channel depending on the direction of sea surface waves relative to the towpath.
Sea Level: The Height of the surface of the sea at any time. The surface of the sea used as a reference for elevation. The expression is often used as a short expression for Mean Sea Level.
Sea Level Datum: A determination of Mean Sea Level that has been adopted as a standard datum of Heights although it may differ from a later determination over a longer period of time.
Sea Water: The water of the seas, distinguished from fresh water by its appreciable salinity. The degree of the salinity greatly affects the water’s physical characteristics.
Seafloor: The bottom of the Ocean when there is a generally smooth gentle gradient. Also referred to as a sea bed, sea bottom.
Secular Change: An increase or decrease of intensity and/or change of direction of the total magnetic field over a period of many years.
Sediment: Particulate organic and inorganic matter which accumulates in a loose unconsolidated form. It may be chemically precipitated from solution, secreted by organisms, or transported by air, ice, wind, or water and deposited.
Sediment: Bottom: In general all sedimentary material regardless of origin found on or in the submarine bottom, including ballast or other material dumped into the sea by man. More specifically it is limited to unconsolidated mineral and organic material forming the sea bottom, not including coral reefs or bedrock.
Sedimentary: Formed by the deposition of sediment.
Shadow: A light area on a normal sonar record that is less insonified than the surrounding region; most often caused by signal blocking from an acoustically opaque object on or above the seafloor. Shadows in side scan data are an important aid to accurate record interpretation. Often, an acoustic shadow will divulge more about a reflector than the actual acoustic returns. Shadows are also used to calculate the height of an object standing proud of the seabed. The calculation uses an algebraic solution of similar triangles formed by the height of the towfish, the range to the target and the length of the shadow.
Side Scan: a category of sonar system used to map large areas of the sea floor for a wide variety of purposes. These include making nautical charts, detecting and identifying underwater objects and bathymetric features, conducting surveys for maritime archaeology. Side-scan sonar imagery is also used to detect debris and other obstructions on the seafloor that may be hazardous to shipping, rigs and platforms.
Slant Range: The straight-path time of arrival of a sonar signal along the hypotenuse of a triangle described by the towfish, the seafloor directly below it, and the seabed point of interest. In side scan sonar, because the imaging source point (transducer) is not on the seafloor but rather above it, slant range does not represent the true range between any two objects. Below the towfish, the data is compressed. Further away from the towfish, the data becomes less compressed with the least error at the outermost ranges. The near range compression can be corrected using algorithms within modern sonar systems.
Slant Range Correction: A computerized repositioning of sonar data on the display to counteract range data compression. See also Slant range.
Slip Ring: An electromechanical component, most often used on a winch, that allows full electrical continuity of a sonar cable during winch drum operation. When a winch is used for deployment, altitude control and recovery of a side scan towbody, it is important to keep the control display unit and towfish operating during these processes. A slip ring has an internal core usually attached to the turning drum, while the outside of the slip ring is attached to a non-turning part of the winch. The towcable conductors are attached to certain parts (rings) of the core while the deck cable conductors are attached to a part (brushes) of the non-turning portion. In this manner the brushes maintain electrical contact with the rings as they turn along with the drum. Poor quality or dirty slip rings will cause noise in sonar data during drum movement and a bad individual ring can cause a blank sonar channel. A quality, well maintained slip ring will have good continuity and be noise free.
Sonar Geometry: The spatial relationship between the sonar transducers and their environment, including the seafloor, targets and the sea surface. Because of acoustic paths in the ocean environment, sonograms may provide puzzling imagery at times. Accurate data interpretation sometimes requires an understanding of the sonar geometry. A good example of this is when sonar signals return to the transducer over several different paths. See also multipath.
Sonograph: A hard copy display of sonar data generated either in real time or from recorded data. Also known as sonograms, hard copy “records” of sonar data are generated either by a sonar printer, specialized graphics printer or with modern computerized sonar displays, by any drafting printer in colour or black and white. Early sonar printers used a wet paper technology creating dark and light zones on the paper through the migration of ions from a consumable print head or “blade.” These records were not dimensionally stable and shrank upon drying. Dry paper recorders were developed in the 1970s ultimately resulting in the use of thermal paper recorders in the 1990s. Unfortunately, many thermal recorders, although clean and easy to use, do not have the dynamic range of other dry paper machines and thermal paper is not an archive-able media. As a result sonar manufacturers and users must depend upon digital processing and mass storage of sonar data for archiving.
SOS: See speed of sound.
Sound Velocity: The rate of motion at which sound energy moves through a medium. The velocity of sound in sea water is a function of temperature, salinity, and the changes in pressure associated with changes in depth. An increase in any of these factors tends to increase the velocity. Also called speed (or velocity) of sound (SOS).
Specular Reflector: An object, to which incident sonar beams are largely normal to its surface, making it a strong reflector from a variety of angles. Objects in this category include cylindrical objects such as pipes and pilings and spherical objects such as subsurface floats. Specular reflectors may provide very strong sonar returns and will result in hyperbolic shaped lines in side scan data. The hyperbola is formed when a target is reflective enough to return even the low level energy in the side lobes of the sonar’s horizontal beam.
Speed: Rate of motion. The terms speed and velocity are often used interchangeably, but speed is a scalar, having magnitude only, while velocity is a vector quantity, having both magnitude and direction.
Speed Correction: The proportional matching of sonar chart length with the over-the-ground speed of the survey vessel. When towing side scan sonar at a constant speed over the bottom, if the image generation on the recorder or display unit is too slow, objects in the data will appear to be compressed in the transverse dimension. If the image generation is too fast, they will appear to be stretched. See Correction and Compression.
Speed of Sound (SOS): The speed at which acoustic pulse travels through water. See Sound velocity.
Spherical Coordinates: See Coordinates.
Spheroid: An ellipsoid; a figure resembling a sphere. Also called ellipsoid or ellipsoid of revolution, from the fact that it can be formed by revolving an ellipse about one of its axes. In geodesy this term is frequently used to mean reference spheroid. See also Spheroid: oblate and Spheroid: prolate.
Spheroid: Oblate: An ellipsoid of revolution, the minor axis of which is the axis of revolution. The Earth is approximately and oblate spheroid.
Spheroid: Prolate: An ellipsoid of revolution, the major axis of which is the axis of revolution.
Spheroid of Reference: See spheroid.
Stop Bit: In asynchronous transmission, the last bit, used to indicate the end of a character which serves to return the data line to its original state.
Sun: The luminous celestial body at the centre of the Solar System, around which the planets, planetoids, and comets revolve. It is an average star.
Sun: Apparent: The actual Sun as it appears in the sky. Also called true sun.
Sun: Mean: A fictitious Sun perceived to move eastward along the Celestial Equator at a rate that provides a uniform measure of Time equal to the average Apparent Time. It is used as a reference for reckoning Mean Time, Zone Time, and so on.
Survey: The orderly process of determining data relating to the physical or chemical characteristics of the Earth. The act or operation of making measurements for determining the relative position of points on, above or beneath the Earth surface. The result of such operations.
Survey: Aerial: A survey using aerial photographs as part of the surveying operation; also the taking of aerial photographs for surveying purposes.
Survey: Cadastral: A survey relating to land boundaries and subdivisions, made to create units suitable for transfer or to define the limitations of title. Also called land survey.
Survey: Coastal: A Hydrographic Survey of coastal area including coast-lining.
Survey: Exploratory: A survey executed for the purpose of obtaining general information concerning areas about which such information was not, previously, a matter of record.
Survey: Geodetic: A survey in which the figure and size of the Earth is considered. It is applicable for large areas and long lines and is used for the precise location of basic points suitable for controlling other surveys.
Survey: Geologic(al): A survey of investigation of the character and structure of the Earth, of the physical changes which the Earth’s Crust has undergone or is undergoing, and of the causes producing those changes.
Survey: Gravimetric: A survey made to determine the acceleration of gravity at various places on the Earth’s surface.
Survey: Ground: A survey made by ground methods, as distinguished from an aerial survey. A ground survey may or may not include the use of photographs.
Survey: Hydrographic: A survey having for its principal purpose the determination of data relating to bodies of water. A hydrographic survey may consist of the determination of one or several of the following classes of data; Depth of water; configuration and nature of the bottom; directions and force of currents; Heights and Times of Tides and water stages; and location of fixed objects for survey and navigation purposes.
Survey: Land: See Survey: Cadastral.
Survey: Large Scale: A hydrographic survey at a large scale. Large scale surveys are usually intended to furnish detailed information for dredging, or other types of harbour improvement.
Survey: Magnetic: A survey conducted to measure the strength and direction of the Earth’s magnetic field at specific points on or near the surface of the Earth.
Survey: Magnetometer: A survey wherein the Earth’s magnetic field is mapped by the use of a magnetometer.
Survey: Oceanographic: A study or examination of any physical, chemical, biological, geological of geophysical condition in the Ocean, or any part of it. An expedition to gather data, samples or information to conduct such studies or examination.
Survey: Photogrammetric: A survey using either terrestrial or aerial Photographs.
Survey: Preliminary: See Survey: Reconnaissance.
Survey: Reconnaissance: A hasty preliminary survey of a region made to provide some advance information regarding the area which may be useful, pending the execution of more complete surveys. Also called preliminary survey.
Survey: Running: A hydrographic survey in which the greater part of the work is done from the ship steaming along the coast. fixing chiefly by dead reckoning and astronomical observations, and at the same time observing angles and bearings to selected and prominent coastal features. A line of sounding is run on the course made good and any additional information of a hydrographic nature which can be obtained whilst under way, is collected.
Survey: Wire-Drag: A hydrographic survey made utilizing a wire drag. Mainly used prior to the availability of Multibeam Echo Sounder Systems, and used in areas of rocky bottom or where submerged obstacles such as wrecks are present, a wire-drag survey represents the most practical way of making sure that all obstructions or dangers have been found and least depths over them obtained. Also called wire drag sweep.
Swath Width: The lateral coverage of side scan sonar on the seabed. Because side scan sonar projects a beam out to the side of the towpath it creates a wide region of insonified seafloor. Both right and left sonar channels make up the swath. Swath width changes with range settings and is a factor in determining coverage and lane spacing.