Sound Velocity Program

Software Coding

The original version of VELOCITY was coded in Microsoft QuickBasic, Version 4.5 and ran as a DOS executable program. The program was converted to Visual Basic, Version 6 by the HSTP staff, so that it is compatible with the WINDOWS operating systems of NOAA's ships and field units. This change facilitated the use of different printers and a variety of external storage media. Presently NOS requires that for multibeam hydrographic surveys, casts must be taken every four hours. The current program features an "automatic" mode. Several of the VELOCITY program tasks, previously done in sequence, are now done in batch mode with a minimum of operator interaction. The automatic option saves the operator time in the field.

Program Description

VELOCITY performs a variety of tasks, all selected from a main menu. Figure 1 shows the top level of the main menu and the FILE options submenu.

Figure 1

1. File submenu

SEACAT PreCast Setup - Checks the status of the instrument and verifies that calibration files are on the hard disk. The command to initialize logging of data is issued.

SEACAT PostCast Data Retrieval - Downloads the raw binary data recorded in the instrument, applies the individual instrument's calibration data and notes the date of the most recent calibration of the CTD. (Note: see "CD: S/N 1251, 12/27/96" in figures 6, 7, 8.)

New - Starts the creation of a new sound velocity vs depth data file. The operator may select "Custom" in which case all procedures are performed separately in sequence and the operator can review the results at each stage. This is the mode of running the current program. With the next WINDOWS version, the "Automatic" mode will be available in which seven basic procedures are run in batch to save time in surveys where casts are taken every four hours.

Open - Allows the user to view an existing file and modify any manually entered data.

Print - Prints a selected data set summary or graphic.

Backup - Permits backup of the necessary files associated with each cast.

Figure 2

2. Custom-Options submenu (Figure 2)

These options may be performed individually, in sequence, by the operator. However, if the operator specified the automatic option on the File submenu, then the seven procedures for SEACAT data are performed in batch mode.

SEACAT Process Retrieved Data - Computes sound velocity from Wilson's Equation and creates a bin-averaged file containing pressure, salinity, temperature, density, and sound velocity.

SEACAT DQA - Performs a data quality assurance check of the SEACAT instrument from manual input of two measurements taken from a bucket surface water sample. The operator is requested to enter either temperature/density or temperature/sound velocity. The measurements are made with a calibrated thermometer and hydrometer (or Digibar velocimeter). The corresponding near-surface measurements from a specified SEACAT cast are compared to the manual inputs to verify that the data are "in the ballpark". In addition, this procedure performs an monthly DQA check of the Digibar instrument, if desired.

Create SEACAT SV vs Depth - The program takes as input the SEACAT bin-averaged file and creates an ASCII sound velocity vs depth file with the header information as described above. Date, time, and serial number are extracted from the retrieved calibration data.

Create Manual Input SV vs Depth - An ASCII sound velocity vs depth file is created from operator input. Input data is checked for out-of-bounds values.

Figure 3

Display/Edit - displays a sound velocity profile on the computer screen (see Figure 3). An automatic edit routine flags "bad" data, i.e., any data points that lie outside prescribed boundaries based on local means and standard deviations. In the figure, the AML data contain one bad point at depth 244 meters. No points are discarded. All points are retained with their corresponding edit flags as part of the data set. However, only good points are used in subsequent calculations.

Figure 4

Figure 4 shows a screen display of part of the edited data set. The sound velocity vs depth data are shown in a scrollable grid. The edit flag of -1 indicates a bad point.


Review/Extend - Extends an edited sound velocity file in one of three ways: (a) invoking an extrapolation algorithm which uses historical data as reference; (b) allowing the operator to select graphically an additional point; (c) invoking a "most probable slope algorithm.

Figure 5

Figure 5 displays the above edited profile in the context of the mean historical profile. The historical data have been supplied by the National Oceanic Data Center (NODC) and consist of annual summaries of sound velocity and salinity at standard NODC depths. Also included are the standard deviations and number of observations for the composite measurement at each depth. The historical bands in the figure represent the mean historical value +/- two standard deviations for the relevant geographical area.

Extrapolation using historical data as reference is best suited for deep water. The cast is extended down to the depth of the historical data unless the maximum cast depth is less than 300 meters, in which case the profile is extended by 30 percent. In the figure, points have been added at the standard NODC depths by means of an algorithm that determines the relative position of the measured profile between the historical bands and employs that relationship in the extrapolation. All extrapolated points are flagged appropriately and appended to the edited data file.

The other two options are more applicable for shallow water. The "most probable slope" method is intended for use in coastal embayments, estuaries, and rivers which might not be well represented in the historical data base. The operator extension method is intended when the hydrographer has good knowledge of the expected sound velocity profile. In these last two options, a single point is added at a depth 30 percent beyond the maximum edited profile depth. Figure 6 shows an edited SEACAT profile extended by means of the most probable slope method. Figure 7 shows the screen display of the flagged data for the bottom part of the profile.

Figure 6

Figure 7

Compute Correctors - Generates a sound velocity corrector table for each cast in a hydrographic survey. The corrections are to be applied to single beam echo soundings in order to correct for the variations in the velocity of sound in the water column.

The algorithm for this procedure divides the water column into layers determined by the profile depths. The sound velocity curve is assumed to be linear within a layer. The program computes the velocity correctors at each profile depth by integrating from the draft to the profile depth using the trapezoidal rule. Then, in accordance with the layering method described in the NOS Hydrographic Manual, velocity corrections at selected increments are determined. The program then uses a second order curve fit to extend the correctors down to the bottom of the extended profile.

Figure 8

Figure 8 lists the correctors vs the applicable depth interval below the surface for a draft of 2.15 meters for the above SEACAT profile. A graphic display of this table is also provided. The correction table is stored in a disk file and may be transferred to the NOS Hydrographic Processing System (HPS).

Generate NODC File - Creates a file of pressure, salinity, and temperature from the same SEACAT bin-averaged file used above to create a VELOCITY program sound velocity data file. In contrast to the sound velocity data file which contains depth in meters, the NODC file contains pressure in decibars.. The NODC file is created in the appropriate CTD exchange format required for submission to NODC. The resulting file is archived to a user-selected disk. At the end of the survey season, these files are transmitted to the NODC where they are incorporated into the relevant database.

3. Zone submenu

This option has been used to establish geographic and temporal boundaries of sound velocity zones for either single or multibeam systems. A sound velocity zone is defined as a region in space and time where the use of a given sound velocity profile (which may differ from an actual precise profile at that point in space and time) does not produce depth errors in excess of those allowed for sound velocity. This procedure may be run at present for any of the following systems: Single Beam, HydroChart II, Reson 9001, Reson 9003, and Reson 8101.

Two edited sound velocity profiles are compared to determine if they are sufficiently alike. The two files are input to the zoning procedure. The water column is divided into layers with cast depths as boundaries. A ray path subroutine is invoked in which the sound wave is assumed to travel a circular path in each layer. Beginning at the surface with the initial angle of the ray at the transducer, the program computes the travel time, cross track distance, and angle at the end of each layer for both profiles. Using a method of successive approximations, the program then generates a travel time table (Figure 9) showing differences in depth and cross track for the two profiles at travel time increments of 0.01 seconds. This table guides ship personnel in determining if a new sound velocity zone has been entered.

Figure 9

Currently, SEACAT casts are taken at least every four hours on NOS multibeam surveys. Sequential casts are compared using the zoning algorithm. If the percent difference in depth exceeds 0.25 then the casts are considered to be in different zones. In this case, the next cast will be taken in two hours instead of four.

This zoning algorithm is also being used as a Data Quality Assurance (DQA) technique for the SEACAT instrument. Two casts taken simultaneously, alongside each other, with two SEACAT instruments, are compared using the program. If they differ sufficiently, then a message is issued that one or both of the instruments used is functioning improperly. The operator is instructed to investigate further by performing simultaneous casts of each of the instruments with a third instrument and then rerunning the zoning procedure with the two new pairs of casts to determine which one of the instruments is not functioning properly.

4. Diver submenu This submenu has two items. The first is used to compute the least depth of a probable hazard to surface navigation investigated by NOAA divers. The diver uses a portable, precision absolute pressure gauge to record the pressure on-deck and at the designated submerged object. These two values are input to the program. Using the calibration data for the particular gauge and the data from a SEACAT cast taken in the vicinity, the program computes the least depth of the object in question. Figure 10 shows the printed report form generated by this part of the program.

Figure 10

The second item in the Diver submenu is a daily DQA technique for the gauge. This procedure takes as input the values of two on-deck observations: The on-deck pressure reading of the gauge and the ship's micro barometer reading. The program uses the gauge's calibration data and its range and stated accuracy to determine if it is operating within acceptable bounds. The results of this procedure not only indicate an out-of-bound measurement but, also, any tendency to "drift" (in which case the gauge might need to be recalibrated.)

5. Calibrate submenu
This option is used to enter calibration data for the SEACAT instrument.

6. Help submenu This option provides the user with on-line help appropriate to the task being performed.

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