Sound Velocity Program

The Sound Velocity Program is a systematic and precise method of managing NOS sound velocity data and providing necessary products for accurate hydrographic and bathymetric surveys. Its use assures that all sound velocity cast data is processed in a uniform manner by all ships and field units, independent of the individual hydrographer and the instrumentation used to acquire the data.

During 1988, HSTP developed an easy to use, modular, PC-based computer program to process and manage sound velocity profile data. The purpose of the computer program, named VELOCITY, was to standardize and improve the way that NOS performed several different tasks involving sound velocity data.

The program was originally designed to accept input from a variety of measurement sources (both manual and electronic) with data quality assurance checks. As presently designed, each data set is stored in a specific format as an ASCII file with a unique filename. The file contains header information including the ship's name, ship position, date, time, and instrument type. The header information also includes an audit trail of the name of the individual who processed or modified the data and the date of the processing.

A statistical edit routine flags improbable data. National Oceanographic Data Center (NODC) historical data from the relevant geographical area are employed as an operational tool to review the edited data. Sound velocity profiles may be extended in three ways by: use of historical data, manual selection, or a "most probable slope" algorithm. Sound velocity correctors, required for accurate hydrographic surveying, are computed and displayed. Sound velocity profiles may be compared in pairs to investigate and establish the allowable geographic and temporal boundaries of velocity zones for both single beam and multibeam survey systems. Summary reports of all calculations and all graphic displays may be obtained as hard copy. Finally, user data file management is available with backup of data files to external disks.

Over the years, the program has been modified to reflect changing modes of surveying operations and different instrumentation. In 1988, NOS was engaged in deep water surveying in conjunction with its Exclusive Economic Zone (EEZ) project. The VELOCITY program was designed to read a NOAA SEAS III Expendable Bathythermograph (XBT) binary data disk and extract the necessary sound velocity cast information. Also, there was an option to transfer raw Applied Microsystems Limited (AML) velocimeter data electronically to the computer hard disk. Data could also be entered from the Odom Digibar velocimeter, Conductivity, Temperature and Depth (CTD) sensor, and Nansen bottle. Presently, the NOS focus is on shallow water surveying and the cast instrument used mostly is the SEACAT SBE 19 CTD profiler from Sea-Bird Electronics, Inc. As a result, features have been added to the VELOCITY program to allow for retrieval and processing of CTD data from the SEACAT.

NOAA divers investigate probable hazards to surface navigation using portable, precision, absolute pressure gauges. Additional software has been incorporated into the VELOCITY computer program, to compute the least depth of such hazards from the gauge pressure recorded by the diver and from a SEACAT CTD cast taken in the vicinity. Additional enhancements to the program include DQA techniques for the SEACAT, the ODOM digibar, and the diver gauge.

Click here to link to a description of the PC-based computer program, VELOCITY