2.3 Polarity of IWs in the Gulf of Cadiz

In the second case study, from the Gulf of Cadiz off Spain, you will observe how internal waves reverse polarity as they travel from deep to shallow water. The reasons for this fascinating behaviour are described in more detail in 'Inversion of polarity in SAR signatures of internal waves'.

Opening the ERS image as a flat file

The image you will be using is an ERS-2 SAR image dated 23 July 1998. This is not in a hierarchical format like the ASAR images you have just been studying, so it must be opened as a binary flat file of 1000 x 1024 pixels in size.

1. Open the file P025_871.8x8 ( .8x8 is not a file extension recognized by Bilko, so to see the file you need to select All files (*.*) as the file type.
2. The Open As flat file dialogue, which now appears, will require some input from you for Bilko to be able to open the file as a recognizable image. Here is what you need to know to fill in the dialogue:
   • There is no header,
   • the number of pixels per row is 1000,
   • the number of rows is 1024,
   • the numerical format is 8-bit unsigned integer,
   • the data have not been byteswapped, and
   • there is only one band.
3. Fill in the dialogue using this information (figure (8K) )
4. In the Extract dialogue accept the defaults to open the entire image at full resolution.
5. When the image opens it is like a mirror view, with East and West swapped round. To correct this, choose View from the menu bar and select Reflect > In X from the drop-down menus.

This image has already been filtered with a 8 x 8 mean filter to reduce speckle, so there is no need to apply any filters. You may, however, like to try applying a Gaussian stretch:

1. Select the whole image ( [CTRL+A] ) using box selection.
2. Open the Stretch menu on the menu bar and select Gaussian stretch

If you find you would rather leave the image as it was, select Clear from the 'Stretch' menu.

Internal waves of depression and elevation

In this region two types of internal wave SAR signatures can be observed: Depression and elevation internal solitary waves (see figure 1.3 (10K)).

• In depression waves the surface convergence zone precedes the divergence zone, so that we have a bright band first, followed by a dark band.

• In elevation waves the surface divergence zone precedes the convergence zone, so that we have a dark band first, followed by a bright band.

For more detailed information about these two types of internal wave SAR signatures, see 'Inversion of polarity in SAR signatures of internal waves'.

Knowing the propagation direction you can now use profiles across internal waves to determine what type of wave you are dealing with.

1. Draw a line using the 'Go To' dialogue ([CTRL+G]), with start position [940,255] and selection size DX = -10 and DY = 10 (figure (5K) ).
   Note: It may be difficult to see this short transect if you have a Gaussian stretch applied. If so, select 'Stretch' from the menu bar, followed by 'Clear' from the drop-down menu.
2. Open a new transect document based on this line (make sure the 'Apply stretches' box is unchecked), and save it as profile1_iw.tsc.

You will now select an internal wave profile from an area closer to the coast.

1. Draw a line using the 'Go To' dialouge ([CTRL+G]), with start position [801,13] and selection size DX = -10 and DY = 14
2. Open a new transect document based on this line and save it as profile2_iw.tsc.