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In order to use the sediment formulae criterion in UNCLOS Article 76, we need a dataset containing the sediment thickness. This dataset can be based on seismic data. This tutorial has some exercises which cover the basics for working with seismic data. Aim of this tutorial

1. Import SEG-Y data
2. Interpret a seismic line

A SEG-Y file can only be imported into a "seismic lines" folder. If you are using Geocap's standard UNCLOS folder structure then such a folder is located in "3. Sediment Data/Seismic Lines" In this exercise we will also generate a sub-folder in order to organize our data.

• Locate the folder "3. Sediment Data/Seismic Lines" in your project
• Right click the folder and select add->new folder
• Right click the folder and select rename. Give the folder the name of the survey. In this case "ATL-LOS-00" (or give it another name if you are importing into the Atlantis project and the folder already exists)
• Right click the folder and select "Import -> Seismic 2D"
• The SEG-Y import command object will appear.
• Click the defaults button in order to reset the menu to the default options.
• Click the file browse button: and select the ATL-LOS-00-1.segy located in the Atlantis folder structure on disk (Atlantis/data/Atl-los-1-segy). Click Open. The file follows the SEG-Y standard and we do not have to change the default settings.
• It might be a good idea to check that the "Storage:" in the lower right corner is set to 8 bit.
• Click execute and the file is imported.
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• Click close in order to close the SEG-Y import command object.
  In order to calculate the sediment thickness based on a seismic line we have to interpret the seabed surface and the base of sediments. This is done by using the the Xi module.

First we need to display the seismic line we want to interpret.

• Right click the seismic line and select "Seismic display".
• Right click the seismic line and select "Scale window to data"
  Next we open the Xi dialog. In order to do this the Xi plugin has to be loaded
• Locate "Xi interpretation" under tools in the main menu. If it is not there, load the Xi plug-in by clicking "plugins->xi".
  After loading the plug-in, the menu entry should appear.
• Click "Tools -> Xi interpretation" in the main menu
  First we need to select where our interpretations should be stored:
• Click "browse" button to select the interpretation folder
• Select the seismic interpretation folder "3.Sediment Data / Seismic Interpretations" Next we need to specify the horizons we want to interpret.
• Click the new button in order to create a new IHorizon
  The IHorizon dialog will appear
• Type in "Seafloor" as the name of the new horizon, and click OK
• Click the same new button again, and call the next horizon "Sediment Base"
  Before we can start interpreting we need to specify which horizon it is that we want to interpret.

Click the browse button in the IHorizon button row. Select Seabed and click OK
Next we need to select which seismic line we want to interpret.

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Introduction

In order to accurately calculate the sediment thickness we have to interpret the seabed surface and the base of sediments on seismic lines. In Geocap these two layers are called horizons. The distance between the seabed and basement horizons is the sediment thickness. 

When collecting seismic data a seismic wave travels from its source to a given reflector and returns to a receiver at the sea surface. The elapsed time is called two way travel time. In most cases this will be the depth unit (in milliseconds) of your seismic. Since the seismic is in time (milliseconds) and we need to calculate the sediment thickness in meters we need to do a conversion. This is called a depth conversion. The formula itself is simple: Distance = Speed * Time. The difficult part of the formula is finding the correct speed/velocity model, since the velocity changes through the different layers in the subsurface. In Geocap the interpretation is normally done in the time domain. This gives us the flexibility to change the velocity model and just update the sediment thickness calculation accordingly. We will have a look at different velocity models in the next section.

Image Added
Illustration of a marine seismic survey using a towed streamer

In this section we will visualize seismic data, import seismic and interpret seismic.

Exercises

Display Seismic Lines and Interpretation

Interpret Seismic Lines

In the next exercises we are going to complete the interpretation of the Seabed and Sediment Base for the seismic line ATL-LOS-00-1. 

We are now going to interpret the Sediment Base but before we can do this we need to change the Ihorizon we are interpreting on.

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1. current active seismic color table – look in the lower right corner of Geocap
2. the color table specified in "Seismic display", if any
3. the color table dragged and dropped on top of the seismic line, if any

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Your interpretation should look something like this when you are finished:
Image Added 
Seabed and Sediment Base interpretations

Import Seismic Lines

There is one more seismic line in this area but it has not been imported yet. In the next exercise we will import this line and then continue to interpret it.

The format for seismic lines is SEG-Y. This format was developed by the Society of Exploration Geophysicists (SEG) in 1973. Since then format has evolved in to many different "flavors", which sometimes makes it difficult to import. When importing a SEG-Y file it is important to set the byte positions correctly. If you are lucky there might be some information about the byte positions in the EBCDIC header, and if you are even luckier the default settings in Geocap will be correct.  

This exercise will not go into details on seismic/SEG-Y import. It will cover the simple case for a plain SEG-Y file which follows the SEG-Y standard and has navigation stored in the trace header. If you have problems importing a SEG-Y file at a later stage, please contact support@geocap.no

Interpret Seismic Lines continued

We are now going to complete the interpretation of the Seabed and Sediment Base on the seismic line ATL-LOS-00-2. This seabed on this seismic line is very flat and has a strong continuous reflector. This lets us use the auto tracker to automatically interpret the seabed.

We are now going to interpret the Sediment Base on the seismic line ATL-LOS-00-2.

The seabed and sediement base interpretation for all lines should look something like this:
Image Added
Seabed and Sediment Base interpretations