12.1 Seismic 2D Interpretation
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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 - > XI Seismic Interpretation.
The interpretation menu
Interpreting horizons
Use the Seismic Control tab when interpreting horizons. In the Data section you can browse in new 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.
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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 Y along p or <space> along the horizon. Click the Stop button to stop the digitizing.
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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 Y 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 Y p or <space> and the point the mouse where you want to end the automatic interpretation and press Y p or <space>. Geocap will then interpret this segment automatically.
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There are three options for erasing segments of an interpreted line:
- Between 2 picks - Use Y p or <space> to select which points to delete between
- Between beginning and pick - Use Y p or <space> to select end point.
- Between pick and end - Use Y 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 Attributes are calculated from a Seismic Line or a Seismic Brick Cube. Therefore they do not contain any information that is not present in the original seismic,
but they emphasise different aspects and therefore can make features that are hard to see in the original seismic clearly visible.
They can therefore be interpreted on and also used as input to the geobody process.
Seismic attributes are stored as children of the input seismic in the project tree. Seismic attributes for a Seismic Line are just datasets containing the attribute values. Seismic attributes for Seismic Brick Cubes on the other hand are complete Seismic Brick Cubes themselves.
In order to display a Seismic Attribute for a Seismic Line you must edit the Seismic Display command:
The Seismic Display command let you choose which Attribute to display (Envelope in this case). The original seismic is referred to as "Amplitudes".
Seismic Trace Attributes
Right-clicking on a Seismic Line or a Seismic Brick Cube and selecting "Seismic Trace Attributes..." pops up the following dialog:
The Seismic Trace Attributes dialog. 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.
The Seismic Trace Attribute value in a sample (green dot) is a function of the seismic amplitude values within a window (green arrows) around the sample on the same trace (black vertical line).
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.
Frequency Attributes
Low-frequency shadows beneath bright spots are often used as a substantiating hydrocarbon indicator. The explanation for this is the abnormal high attenuation in gas-filled reservoirs.
River channels often have little variation in frequency within the channel whereas the variation in frequency have larger variation.
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.
Relationships between the Instantanenous Frequency (center frequency), Dominat Frequency (RMS frequency) and the Instantaneous Bandwidth (spectral bandwidth). From "Instantaneous spectral bandwidth and dominant frequency with applications to seismic reflection data" by Arthur E. Barnes.
Other Trace Attributes
Iso-Frequency
The value of the amplitude spectrum at a specific frequency calculated by as Short Time Fourier Transform.
User-defined Function
Lets the user specify a mathematical function of the original seismic and the position of the sample in question.
For example you could use the function "abs(s)" to find the absolute value of the seismic.
From the Variables drop-down list you can choose from different variables such as I, inline trace position. Hoovering over a variable name brings up a description.
From the Functions drop-down list you can choose from many different mathematical functions such as abs(). Hoovering over a function name brings up a description.
Dip and Coherence
Right-clicking on a Seismic Line or a Seismic Brick Cube and selecting "Seismic Trace Attributes..." pops up the following dialog:
Dip and Coherence dialog
The Dip and Coherence process estimate the surface normal G=[Gx,Gy,Gt] at each sample in the seismic from a statistical analysis of the samples in a window around the sample in question.
If there happen to be a minima or maxima at the sample; this surface normal will point in the same direction as the normal to the interpreted horizon going trough the sample. If not one can still imagine an implicit surface; where the seismic amplitude is constant.
From the surface normal: the following are calculated:
InlDip=Gx/Gt
CrlDip=Gy/Gt
The InlDip in a point (black circle) is the ratio of the inline, x, component of the surface normal to the t component of the surface normal, G (black vector) and therefore a measure of how much the implicit surface (green line) is dipping relative to the horizontal (blue line).
Since the dip is calculated over a window the result is some form of average over the window and as a byproduct we therefore get Coherence which is a measure of how much the samples in the window are in accordance with this average, or how reliable the dip estimates are:
0 <= Coherence <=1, where 1: absolute certain.
But Coherence can also be viewed as a measure of "order" in the seismic. A non chaotic region of the seismic will have a high coherence, whereas a chaotic region, e.g. inside a salt, will have a low coherence.
A Seismic Line (left), its Dip (middle) and its Coherence (right).
Seismic Attribute Calculator
Right-clicking on a Seismic Line or a Seismic Brick Cube and selecting "Seismic Attribute Calculator..." pops up the following dialog:
The Seismic Attribute Calculator makes it possible to combine Seismic + up to 3 Seismic Attributes into a weighted combined Attribute.
From the Variables drop-down list you can choose from different variables such as I, inline trace position. Hoovering over a variable name brings up a description.
From the Functions drop-down list you can choose from many different mathematical functions such as abs(). Hoovering over a function name brings up a description.