12.1 Seismic 2D Interpretation
Introduction
The seismic interpretation tool lets you interpret horizons, faults and observations. This can be done both manually and automatically. In your Geocap project you will typically have an interpretation folder with one or more Ihorizon datasets, one Faults folder and one Observations folder.
The structure of a horizon in Geocap is as follows:
- One horizon can contain interpretation from several seismic lines
- Each line can have many segments
- Each segment can have many picks
This means that each horizon will normally contain thousands of picks along different seismic lines. Each pick is stored with the following information:
- Time value
- Seismic line name
- Shot point
The way Geocap stores its horizon data is compatible with most other seismic interpretation systems.
The seismic interpretation tutorial is found in the Shelf section Seismic Interpretation.
In this section:
Importing interpretation from external files
To import an existing interpretation from file you can import it into an Interpretation folder or a standard Generic folder. A Generic folder is preferable if the interpretation is complete and the lines will be used in gridding and mapping.
If there is a need for reinterpret or edit the lines using the corresponding seismic sections create an Interpretation folder and then right-click on the Interpretation folder and select Import > Interpreted Horizons. The external file must be on ASCII format with one pick per line. Each pick must contain position and shot point in addition to the name of the corresponding seismic 2D line.
An Import dialog appears. You'll most likely need to Adjust header and change File column for X, Y, Time, SP, and Data separator. Take a moment to inspect the Data separator. This should be the name of the corresponding seismic 2D line. The remainder of the panel settings are usually kept as is.
Read more about how to import interpreted horizons in the import chapter Importing Interpretation to get the details.
Interpretation data structure
During interpretation (or import) a dedicated interpretation data structure is created. The base folder is an Interpretation folder. An Interpretation folder contains subfolders and IHorizons. An IHorizon (i.e. Interpreted Horizon) is the parent for any number of Pick Groups. Each Pick Group represents the interpretation of current horizon on a seismic line. The name of a Pick Group must and will be exactly the same as the corresponding seismic line.
Interpretation data structure
Interpretation interface
The seismic interpretation tool can be found under Tools > Seismic Interpretation.
The interpretation menu
Interpreting horizons
Use the Seismic Control tab when interpreting horizons. In the Data section you can browse for existing interpretations and horizons or you can create new ones. The Edit button lets you change the properties and graphical settings of interpretations and horizons. You can display horizons in the graphical window directly from the interpretation menu without using the regular commands.
Before you can start interpreting you will have to select the seismic line you want to interpret on. Do this by displaying one or several seismic lines from the project and then use the Select button in the interpretation menu and click on the one you want to interpret.
Manual interpretation
Select the Snap mode you want to use. Geocap can snap to either max (peak) or min (trough) or none of them. Using the snap mode will help you increase the precision when interpreting. When you have set the tracking parameters click the Start interpretation button and start digitizing with p or <space> along the horizon. Click the Stop button to stop the digitizing.
Automatic interpretation
Geocap can track a whole seismic line automatically. This saves a lot of time when dealing with strong continuous reflectors. Select a snap mode and set the track mode to Auto. The delta Z value will constrain the auto-tracker to a time interval it is allowed to "jump" between each shot point. Click the Start interpretation button, point your mouse on the line you want to interpret, press p or <space> and Geocap will interpret the whole line.
You can also partially auto-track a line using the Insert tracking mode. Click the Start interpretation button, point your mouse where you want to start the interpretation, press p or <space> and the point the mouse where you want to end the automatic interpretation and press p or <space>. Geocap will then interpret this segment automatically.
Edit or erasing interpretation
You can edit the interpreted line by clicking the Start interpretation button and simply re-digitize the section you want to edit. Geocap will automatically update the line when you click the Stop button.
There are three options for erasing segments of an interpreted line:
- Between 2 picks - Use p or <space> to select which points to delete between
- Between beginning and pick - Use p or <space> to select end point.
- Between pick and end - Use p or <space> to select start point.
Use the Erase button to start erasing.
Interpreting faults
Use the Faults tab when interpreting faults. You can browse for existing interpretation folders and faults or you can create new ones. You can display faults in the graphical window directly from the interpretation menu without using the regular commands.
Before you can start interpreting you will have to select the seismic line you want to interpret faults on. Do this by displaying one or several seismic lines from the project and then use the Select button in the interpretation menu and click on the line you want to interpret.
Manual interpretation
Click the Start interpretation button and start digitizing with p or <space> along the fault. Click the Stop button to stop the digitizing.
Interpreting observations
Use the Observation Centre tab when recording observations. You can browse for existing interpretation folders and observations or you can create new ones. You can display observations in the graphical window directly from the interpretation menu without using the regular commands.
Before you can start recording you will have to select the seismic line you want to record observations on. Do this by displaying one or several seismic lines from the project and then use the Select button in the interpretation menu and click on the line you want.
Manual recording
Click the Start interpretation button and record your observation by digitizing with p or <space>. Click the Stop button to stop the digitizing. The last two digitized points are stored.
12.2 Seismic Attributes
Seismic Trace Attributes
Right-clicking on a Seismic Line or a Seismic Brick Cube and selecting "Seismic Trace Attributes..." pops up the following dialog:
NB! Hovering over the name of an attribute in the drop-down list will display a short description of this attribute.
As the name implies these attributes are calculated from the values above and below each sample within a window in the same trace.
Complex Trace Attributes
A seismic trace (amplitude), x(t) is real, but can be viewed as the real part of an analytical trace: z(t) = x(t) + i*y(t), where i is the imaginary unit and y(t) is the Quadrature Amplitude.
The Quadrature Amplitude is calculated by taking the Hilbert transform of the Amplitude: y(t) = H{ x(t) }.
From this definition a whole range of attributes can be derived.
Quadrature Amplitude
The imaginary part of the complex seismic trace. Equals a 90 degrees phase shift of the real seismic trace.
Envelope
Also know as reflection strength, instantaneous energy and magnitude.
It is defined as the absolute value of the complex trace: Envelope = √(x^2+y^2).
Mainly useful for
- Bright spots
- Gas accumulation
- Sequence boundaries, major changes or depositional environments
- Thin-bed tuning effects
- Unconformities
- Major changes of lithology
- Local changes indicating faulting
Two Seismic Lines or Seismic Brick Cubes that are the same except for the phase will have the same envelope.
Instantaneous Phase
The Instaneous Phase is defined as: φ=arctan(y/x).
Reveals weak and strong events with equal strength, thereby improving reflector continuity and enhancing the visual appearance of edges.
Commonly displayed with a wrapped Color Table since the extreme values: -pi and pi refer to the same phase.
Cosine of Instantaneous Phase
Same characteristic as Instantaneous Phase but avoids the problem with the discontinuity at -pi,pi.
Commonly used for interpretation in areas with weak seismic.
Apparent Polarity
The sign (+/- 1) of the seismic trace at the peaks of the Envelope.
In a noisy seismic area the event continuity can be clearer on the apparent polarity than on the original seismic.
It is therefore sometimes used as input to auto-trackers.
Instantaneous Frequency
Rate of change of the Instantaneous Phase: ωC=dφ/dt.
Represents the mean of the instantaneous power spectrum and its unit is Hz.
Instantanous Bandwidth
Defined as the absolute value of the derivative of the Envelope divided by the Envelope: ωB=|dEnvelope/dt|/Envelope.
Represents the standard deviation of the instantaneous power spectrum about its mean and its unit is Hz.
Dominant Frequency
Defined as the Root Mean Square of the Instantaneous Frequency and the Instaneous Bandwidth: ωRMS=√(ωB^2+ωC^2).
Represents the Root Mean Square of the instantaneous power spectrum and its unit is Hz.
