mbvelocitytool
Section:  MB-System 5.0  (1)
Updated:  3 June 2013 
Index


 
NAME

mbvelocitytool - Interactive water sound velocity profile editor.

 
VERSION

Version 5.0

 
SYNOPSIS

mbvelocitytool [-Byr/mo/da/hr/mn/sc
-Eyr/mo/da/hr/mn/sc
-Fformat -Ifile -Ssvpfile -Wsvpfile -V -H]

 
DESCRIPTION

MBvelocitytool is an interactive water sound velocity profile
(SVP) editor used to examine multiple SVPs, to create
new SVPs, and to model the impact of SVP modification on swath bathymetry
data. SVPs created using MBvelocitytool can be used by the program
mbprocess to recalculate swath bathymetry from raw travel time
and angle data.

In general, MBvelocitytool is used to examine SVPs obtained from
swath data files (see mbsvplist manual page), XBTs, CTDs, or
databases, and to construct new profiles consistent with these various
sources of information. The SVPs are represented by a set of paired
depth and velocity values which are connected by linear interpolation.
Users may load a number of SVPs for display. Users may load or create
a single editable SVP and then interactively modify this profile.

When users load swath bathymetry data containing raw travel time and
angle data (many but not all swath data format include this information),
MBvelocitytool  recalculates the bathymetry by raytracing through
the current SVP model. The bathymetry of each ping is fit with a line,
and bathymetry residuals are calculated for each good beam relative
to the linear fit.  The average of the bathymetry residuals is displayed
along with "error bars" indicating the standard deviations of the
residuals. Anomalously shallow  bathymetry maps into negative residuals
and deep bathymetry into positive residuals; the residuals are displayed
so that shallow is up and deep is down. If the seafloor is reasonably
smooth so that a linear fit is appropriate, then the residuals will
accurately reflect any problems with the water velocity profile.  If
the water velocity profile is correct, then the residual plot will be
roughly flat.  If the water velocity profile is significantly in error,
then the outer beam depths may anomalously shallow (edge curl up) or
deep (edge curl down).  In practice, the editable velocity profile is
altered interactively until a reasonably residual pattern is achieved.

In order to calculate bathymetry values from travel time observations,
geometrical raypaths are traced through the SVP for each beam. Because
the sound velocity gradients are uniform between the depth-velocity
nodes (linear interpolation), the raypaths are be calculated
analytically as pieces of circular arcs. This raytracing algorithm
is the same used in the program mbprocess.

One important aspect of the raytracing is the handling of the initial
takeoff angles associated with each beam or sounding. In general, the
raytracing will begin at a point in the sound speed model that has a
sound velocity different than the surface sound velocity (SSV) used by
the mapping sonar for the original beamforming. The usual approach is to
use Snell's law to adjust the starting angle for this change in sound
velocity. This amounts to an assumption that the original SSV was correct
and that  the rays pass through an insignificantly thick layer in which the
sound speed equals the SSV before transitioning to the sound speed implied
by the SVP. This is the default setting for raytracing in MB-System.
Alternatively, one can proceed with raytracing using the original angle
but this is rarely useful or correct. Finally, if the SSV used by the sonar is
judged to have been incorrect, then the takeoff angle must be corrected for
the erroneous beamforming as well as for the difference between the SSV
and the initial raytracing sound velocity. This correction must take the
sonar geometry into account because the impact of changing the SSV on a beam
angle from a flat receive array is very different from a V-shaped or curved
array. All three of these angle correction modes are available in mbvelocitytool.

MBvelocitytool can be used in conjunction with mbprocess.
If the user uses the Save swath svp file option to save an SVP model
developed through the analysis of a particular swath data file,
MBvelocitytool also sets the associated mbprocess parameter file
so that mbprocess recalculates the bathymetry using the new SVP model.
The program mbset may be used to set the SVP file in the parameter
file for any swath data file. Users may also save SVP models without setting
any mbprocess parameters by using the Save editable profile
option.

Sometimes the bathymetry residuals show structure indicative of
persistent artifacts in the bathymetry (e.g. certain parts of
the swath may be persistently shallower or deeper than the
rest of the swath). In this situation, it is possible to export
the residuals and to then apply them in mbprocess
as static corrections to the bathymetry. This is accomplished by
using the Save residuals as offsets option under the File
menu.

If a user attempts to read in swath bathymetry that does not
contain the travel time and beam angle data required for bathymetry
recalculation, MBvelocitytool will estimate the travel times
and angles from the bathymetry by assuming a 1500 m/s half-space
(and then post a warning dialog). Although the user can proceed
to model bathymetry recalculation by modifying the active SVP just
as with proper data, the travel times and angles are not in general
correct and so the modeling and any results it gives are, well, bogus.
A more useful approach is to leave the SVP alone and simply export
the residuals to be applied as static corrections in mbprocess.
This approach allows users a practical means of correcting
older multibeam bathymetry that was originally calculated with
an incorrect SVP but which contains no travel time or angle data.

 
AUTHORSHIP

David W. Caress (caress@mbari.org)




  Monterey Bay Aquarium Research Institute



Dale N. Chayes (dale@ldeo.columbia.edu)




  Lamont-Doherty Earth Observatory

 
OPTIONS


-B


yr/mo/da/hr/mn/sc



Sets the starting time for data allowed in the input data; pings
with times before the starting time will be ignored.
Default: yr/mo/da/hr/mn/sc = 1962/2/21/10/30/0.
-E


yr/mo/da/hr/mn/sc



Sets the ending time for data allowed in the input data; pings
with times after the ending time will be ignored.
Default: yr/mo/da/hr/mn/sc = 2062/2/21/10/30/0.
-F


format



Sets the format at startup for the input and output swath sonar data using
MBIO integer format identifiers. This value can also be set
interactively when specifying the input file.
This program uses the MBIO library and will read any swath sonar
format supported by MBIO. A list of the swath sonar data formats
currently supported by MBIO and their identifier values
is given in the MBIO manual page.
Default: format = 11.
-H


This "help" flag cause the program to print out a description
of its operation and then exit immediately.
-I


file



Sets the data file from which the input swath data will be read at startup.
This value can also be set interactively. See below.
-S


svpfile



Sets the data file from which an input display sound velocity
profile will be read at startup. See below.
This value can also be set interactively.
-W


svpfile



Sets the data file from which an input editable sound velocity
profile will be read at startup.
This value can also be set interactively. See below.
-V


Normally, mbvelocitytool works "silently" without outputting
anything to the stderr stream.  If the
-V flag is given, then mbvelocitytool works in a "verbose" mode and
outputs the program version being used, all error status messages,
and a large amount of other information.


 
INTERACTIVE CONTROLS


File


This button brings up a menu containing the commands listed below:
File->Open Display Profile


Selecting this menu item brings up a popup window which allows the
user to select an input water velocity profile file. The profile
will be displayed in a depth vs velocity plot along with other
display profiles and the edit velocity profile, if one has been
opened.  Up to ten display velocity profiles may be loaded; the
color of the display profiles cycles through red, green, blue, and
black in the order that they are loaded.
File->Open Editable Profile


Selecting this menu item brings up a popup window which allows the
user to select an input water velocity profile file to be edited.
The profile
will be displayed in a depth vs velocity plot along with any
display profiles that have been opened.  This profile will be plotted
as black lines with small black squares at the velocity depth points.
The velocity depth points may be selected and dragged using the mouse
to change the editable velocity profile.
File->New Editable Profile


Selecting this menu item creates a default editable velocity profile
consisting of 14 velocity-depth points with a velocity of 1500 m/s.
Any previously defined editable velocity profile will disappear. The profile
will be displayed in a depth vs velocity plot along with any
display profiles that have been opened.  This profile will be plotted
as black lines with small black squares at the velocity depth points.
The velocity depth points may be selected and dragged using the left mouse button
to change the editable velocity profile. The center mouse button adds new
velocity-depth points, and the right mouse button removes velocity-depth
points.
File->Save Editable Profile


Selecting this menu item brings up a popup window which allows the
user to save the current editable velocity profile as a file.
File->Open Swath Sonar Data


Selecting this menu item brings up a popup window which allows the
user to select an input swath sonar data file to be processed
in conjunction with the current editable velocity profile.
The format id for the data file must be set appropriately
in the editable text widget labeled "MBIO Format ID:"; this
program uses the MBIO library and will read or write any swath sonar
format supported by MBIO. A list of the swath sonar data
formats currently supported by MBIO and their
identifier values is given in the MBIO manual page.
If the swath sonar data file is named using the MB-System
suffix convention (format 11 files end with ".mb11", format
41 files end with ".mb41", etc.), then the program will
automatically use the appropriate format id.
The swath sonar data file must be in one of the formats which store
travel time data for each beam. Up to 25000 data records
will be read into a buffer for processing.
If there is no current editable velocity profile, then a
default profile consisting of 14 velocity-depth points
with a velocity of 1500 m/s will be created. A display profile
generated by the mblevitus program will also be
automatically generated and loaded into mbvelocitytool;
this Levitus profile will serve as an average reference for
the water sound velocity structure at the location of the
swath data being analyzed. The travel time
data will then be processed and displayed in the manner described
below for the Reprocess button.
File->Save Swath SVP File


Selecting this menu item saves the current editable velocity
profile as a file. The SVP filename is that of the current
swath data input file with a ".svp" suffix added. MBvelocitytool
also sets the swath data file's mbprocess parameter file
to recalculate bathymetry using the new SVP file and using the same
angle mode setting applied in the interactive modeling (see the Angle
Mode dialog description below).
File->Save Residuals As Offsets


Selecting this menu item saves the current bathymetry residuals
as a "static bathymetry offset" file.
The filename is that of the current
swath data input file with a ".sbo" suffix added. MBvelocitytool
also sets the swath data file's mbprocess parameter file
to apply these residuals as static corrections to the bathymetry.
Plot Scaling


This button brings up a dialog containing the three controls
discussed immediately below.
Plot Scaling->Maximum Depth


This slider sets the maximum depth in meters of both the velocity profile
display and the raypath display. The minimum depths of these
displays are always zero. The acrosstrack distance axis of the raypath
display automatically adjusts so that the rays are plotted with
no vertical exageration.
Plot Scaling->Velocity Range


This slider sets the minimum and maximum velocity values of the velocity
profile display.  The minimum value is the velocity center minus the velocity range;
the maximum value is the velocity center plus the velocity range.
Plot Scaling->Velocity Center


This slider sets the center velocity value of the velocity
profile display.  The minimum value is the velocity center minus the velocity range;
the maximum value is the velocity center plus the velocity range.
Plot Scaling->Residual Range


This slider sets the minimum and maximum bathymetry residual values
of the bathymetry residual display.  The residual plot has a minimum
value of -range and a maximum value of +range.
Angle Mode


This button brings up a dialog containing three choices for how the starting
beam angles are corrected during raytracing. The selected angle mode is
used by mbvelocitytool in the interactive modeling, and is also passed
on to the mbprocess parameter file created or modified when an SVP
file is saved.
Angle Mode->Do Not Change Beam Angles


This button sets the angle mode so that the starting beam angles are not
corrected before raytracing for the difference between the surface sound
velocity (SSV) and the sound velocity of the starting depth in the SVP.
Angle Mode->Adjust Angles Using Snell's Law


This button sets the angle mode so that the starting beam angles are
corrected before raytracing, using Snell's Law, for the difference between the
surface sound velocity (SSV) and the sound velocity of the starting depth in
the SVP. This amounts to an assumption that the original SSV was correct
and that  the rays pass through an insignificantly thick layer in which the
sound speed equals the SSV before transitioning to the sound speed implied
by the SVP. This is the default setting for raytracing in mbvelocitytool
and in MB-System in general.
Angle Mode->Adjust Angles Using Sonar Geometry


This button sets the angle mode so that the starting beam angles are
corrected before raytracing, using Snell's Law and the sonar geometry, for
the difference between the surface sound velocity (SSV) and the sound velocity
of the starting depth in the SVP. This option is relevant to situations where
the SSV used by the sonar for beamforming was incorrect, and so the initial
angles themselves must be adjusted. This mode is indistinguishable from
that of "Adjust Angles Using Snell's Law" for multibeam sonars with flat,
horizontal receive arrays, but deviates strongly for sonars with tilted,
V-shaped, or curved receive arrays.
Reprocess


Selecting this button causes the swath sonar travel time data to be processed
into bathymetry using the current editable velocity profile. The processing
involves full raytracing through the water velocity model. The bathymetry of
each ping is fit with a line, and bathymetry residuals are calculated for each
good beam relative to the linear fit.  The average of the bathymetry residuals
is displayed in the upper right part of the window as a function of beam number,
along with "error bars" indicating the standard deviations of the residuals.
Raypaths are displayed without vertical exageration in the lower part of the
window; these represent the first raypath calculated for each beam while
processing the current data (most will be from the first ping). Anomalously
shallow bathymetry maps into negative residuals and deep bathymetry into positive
residuals; the residuals are displayed so that shallow is up and deep is down.
If the seafloor is reasonably smooth so that a linear fit is appropriate, then
the residuals will accurately reflect any problems with the water velocity profile.
If the water velocity profile is correct, then the residual plot will be roughly
flat.  If the water velocity profile is significantly in error, then the outer
beam depths may anomalously shallow (edge curl up) or deep (edge curl down).
In practice, the editable velocity profile is altered interactively until a
reasonably residual pattern is achieved; reference velocity profiles obtained from
CTD casts, XBT's, or data bases (see manual page for program mblevitus)
may be read in as display profiles to guide the editing process.
Quit


This button causes the program to exit gracefully, but without asking
any questions.  If you haven't already saved your edited velocity profile
it will be lost when you quit.


 
SEE ALSO

mbsystem(1), mbprocess(1), mbset, mbsvplist,
mbbath, mblevitus(1),
mbm_xbt(1), mbm_xbt(1)

 
BUGS

Users cannot remove display sound velocity profiles from the screen
once they are read in. The main window for mbvelocitytool
is too large for use on screens smaller than 1024 X 768 pixels.



 
Index

NAME

VERSION

SYNOPSIS

DESCRIPTION

AUTHORSHIP

OPTIONS

INTERACTIVE CONTROLS

SEE ALSO

BUGS

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