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Importing interpretation from external files
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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.
NB! Hovering over the name of an attribute in the drop-down list will display a short description of this attribute
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.
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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.