Instruction Manual

Page 8

2.4 When and How Much To Process...

Below is a discussion on the different processing options for each data type.

SIDESCAN:

 Amount: Sidescan data generally requires multiple passes with the boxcar filter for

best results. Often sidescan recording systems have low frequency GPS signals (one

update per second) and multiple pings will have the same time stamp. One effect of

navigation processing is to evenly space the pings along the track so there is no

smearing of data where the beams are separated too far and no compression of data

where they are too close and overlap. To get overlapping beams evenly spaced, set

the

Number of Iterations =

3 to 5. Make sure to watch the beamlines to see if this

is enough, or if more iterations are needed.

Another problem with navigation that can strongly affect the quality of the sidescan

mosaic is line turns which causes overlapping beamlines on the inside of the curve

and beam separation on the outside. While this effect cannot be removed entirely

when there is a sharp curve in the survey track line, multiple passes with the boxcar

filter can minimize the effect as much as possible without overly straightening the

line and reducing the position accuracy of mosaic objects. The number of passes to

minimize this effect varies depending on the data so it is best to use beamlines to

monitor the processing results.

 Settings: Increasing the boxcar

Window Width

will increase the smoothing of the

line, reducing line curves making the line straighter. Generally I start with a boxcar

Window Width = 30

and then increase or decrease the size depending on the

processing results. For sidescan data, generally the

Navigation Source = Sensor

, and

the heading source is best set to CMG (course made good). Most towfish have very

poor heading sensors and CMG usually produces better results. The

Speed Filter

great for removing spikes in the data and needs to have the speed range set to

remove ping positions that require too fast a speed to get there and back to the

next ping location.

 When: In order to monitor the navigation processing results it is best to process

sidescan navigation prior to launching the Mosaic Wizard to create a mosaic.

However, once the best settings are known for a particular data set, processing the

navigation with the Mosaic Wizard will produce the same results.