H. Depth Conversion & Thickness Calculations

Owned by Harald Sund (Unlicensed)
Last updated: Oct 16, 2015 by Tore Sannes (Unlicensed)
4 min read

Introduction

In order to use the sediment formula criterion in UNCLOS Article 76, we need a data set containing the sediment thickness; a sediment profile. The seismic lines interpreted in the previous section is stored with milliseconds as a z coordinate, not meters. In order to compute a sediment thickness in meters, we need to do a depth conversion. In this section we will cover the basics for working with velocities, depth conversion and sediment thickness calculation.

Exercises

Velocities

Velocities are often provided as ASCII column files. The file should contain the shot point, time, and velocity. The file may also contain coordinates. In our training data set, there are no coordinates, therefore we need to assign navigation from a navigation data set. If the file contains velocities from more than one line, it should also have a column with the line name.

Exercise

Import a velocity data set

  1. In the folder 3. Sediment Data / Velocities right click the folder Stacking Velocities and click Import > Stacking Velocities (no navigation)
  2. Click the browse button and browse for the data set called ATL-LOS-00.vel (Data/Atlantis/Sediment data/ATL-LOS-00.vel). Click Open.
  3. Adjust the columns according to the screen shot below by clicking up or down in the spin box.
  4. Click Execute.
  5. You will be prompted with a dialog asking you to specify Datum and Coordinate System. Click All Undefined.
  6. The dataset has been imported. Click OK
  7. Click OK on the Stacking Velocity import menu.


Stacking Velocity import menu

 

The data sets have now been imported into your project. However they do not yet hold any coordinates. In order to visualize the stacking velocities with the seismic data, we need to assign navigation to the velocities. Geocap will use the shot point number in the velocity data set and match them with the shot points in a navigation file. Either a separate navigation file, or the actual seismic line can be used as source of navigation.

Exercise

Assign navigation to the velocity datasets

  1. In the folder 3. Sediment Data / Velocities / Stacking Velocities right click the velocity dataset ATL-LOS-00-1 and click Assign Navigation.
  2. Browse for the seismic line ATL-LOS-00-1 in the Seismic Data folder and click OK.
  3. In the folder 3. Sediment Data / Velocities / Stacking Velocities right click the velocity dataset ATL-LOS-00-2 and click Assign Navigation.
  4. Browse for the seismic line ATL-LOS-00-2 in the Seismic Data folder and click OK.

 

The velocity data sets are now in the right location and can be displayed together with the seismic data.

Exercise

Display the Stacking Velocities

  1. In the folder 3. Sediment Data / Velocities / Stacking Velocities right click the velocity dataset ATL-LOS-00-1 and click Map Data.
  2. In the folder 3. Sediment Data / Velocities / Stacking Velocities right click the velocity dataset ATL-LOS-00-2 and click Map Data.
  3. Display the two seismic lines in the folder 3. Sediment Data / Seismic Data / ATL-LOS-00.
  4. Investigate the velocity data together with the seismic data.

 

If you have imported stacking velocities or RMS velocities, we need to convert them to interval velocities using Dix formula before we can use them to generate velocity profiles. Dix's formula will use the stacking velocities as an approximation for RMS velocities and produce interval velocities.

Exercise

Convert stacking velocities to interval velocities

  1. In the folder 3. Sediment Data / Velocities right click the folder Stacking Velocities and click Compute Interval Velocities (Dix Formula)
  2. The result will be stored in the folder 3. Sediment Data / Velocities / Interval Velocities. Click OK.

 

Velocity Profiles

We now need to calculate the average velocity from the seabed down to the sediment base, along the seismic lines. To do this we have to generate velocity profiles.

Exercise

Generate velocity profile from seismic interpretation using interval velocity dataset

  1. In the folder 3. Sediment Data / Velocities right click the folder Velocity Profiles and click Generate Velocity Profile.
  2. Click the first browse icon and browse in the seismic data folder 3. Sediment Data / Seismic Data / ATL-LOS-00 and click OK.
  3. Click the second browse icon, browse in the folder Seismic Interpretations and click OK.
  4. Make sure Match points by: is set to Shot Point.
  5. Make sure Use interval velocities from: is set to Dataset in Project.
  6. Click the Interval Velocities browse icon, browse in the Interval Velocities folder and click OK.
  7. Leave the rest of the settings as default and click Execute.
  8. In the menu that pops up make sure that both lines are checked and click OK
  9. The result is stored in the folder 3. Sediment Data / Velocities / Velocity Profiles
  10. Click Cancel to close the menu.

If you do not have velocity data available it is possible to use a velocity look-up table or formula. See documentation for more information.

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Depth conversion and thickness calculation

The next step is to perform the depth conversion and calculate the sediment thickness. In Geocap this can be done in one operation by generating a sediment profile. The velocity profile holds all the data necessary to do this.

Exercise

Generate a sediment profile

  1. In the folder 3. Sediment Data / Velocities right click the folder Velocity Profiles and click Calculate Sediment Thickness.
  2. In the menu that pops up make sure that Calculate Sediment Thickness as been selected.
  3. Click Execute.
  4. The result will be stored in in 3. Sediment Data / Sediment profiles. Click OK.
  5. Click OK to close the menu
Exercise

Display the sediment profile

  1. In the folder 3. Sediment Data / Sediment profiles right click the sediment thickness profile ATL-LOS-00-1 and click Display profile as Cross Section
  2. In the folder 3. Sediment Data / Sediment profiles right click the sediment thickness profile ATL-LOS-00-2 and click Display profile as Cross Section
  3. Try displaying the atlantis seabed grid also and compare it with the sediment profiles.
Exercise

Inspect the table view of the sediment profile

  1. In the folder 3. Sediment Data / Sediment profiles right click the sediment thickness profile ATL-LOS-00-1 and click Table View
  2. Inspect the different columns. The X,Y and Z is the position and the seabed depth at that point. The Scalar value is the sediment thickness in meters. The Shot point column is the points corresponding shot point from the seismic line.
  3. Click Close to close the Table View