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National Centers for Environmental Information (NCEI)

ECHO-SOUNDING CORRECTION TABLES FOR COMPUTER USE ("Matthews' Tables" - Carter's 3rd Edition)

Data Announcement 84-MGG-18

     An echo sounding is measurement of the two-way travel time of an acoustic signal between a shipborne transducer and a submerged reflecting surface, usually the ocean bottom. To convert an echo sounding to depth, the travel-time measurement is commonly halved and multiplied by an assumed speed of sound through seawater, either 4,800 ft/sec, 800 fathoms/sec or 1,500 m/sec. The resulting value must be corrected for variations in the speed of sound because of variations in the temperature, salinity, and pressure of sea water. Traditionally, corrections to echo soundings have been interpolated for various areas of the world's oceans from tables published by D.J. Matthews in 1939.* Though the Matthews tables are long outdated, their continued use allowed new depth values to be merged with the large existing data base of depths derived similarly. In 1980, the United Kingdom's Hydrographic Department published a new edition of the correction tables, based on the work of D.J.T. Carter of the United Kingdom's Institute of Oceanographic Sciences.** The Carter tables were adopted in place of Matthews tables by the Twelfth International Hydrographic Conference in Monaco in 1982.

     The Institute of Oceanographic Sciences has developed two FORTRAN subroutines and a data base to make the Carter tables available in more convenient form. The subroutines, originally designed for use on the RRS DISCOVERY'S minicomputer, compute the square-degree area containing the observation to be corrected, retrieve the correction coefficients for that location and covert the echo sounding from "uncorrected meters" (computed using a sound speed of 1,500 m/sec) to "true depth."

     The National Geophysical Data Center (NGDC) (now the National Centers for Environmental Information (NCEI)) has available for purchase these subroutines, four files of data, and documentation on magnetic tape. In addition, NGDC (now NCEI) has translated the subroutines into BASIC and is making them available together with the data base on floppy disc for use with popular micro- computer systems, such as those from Apple, Inc., and International Business Machines.
* Matthews, D.J. (1939). "Tables of the Velocity of Sound in Pure Water and Sea Water for Use in Echo-Sounding and Sound- Ranging," (2nd Ed.), Hydrographic Department, Ministry of Defense, United Kingdom.

** Carter, D.J.T. (1980). "Echo-Sounding Correction Tables," (3rd Ed.), Hydrographic Department, Ministry of Defense, United Kingdom.