Friday, 31 July 2015

Can Envelope help us understand the subsurface?

The convolution model of the reflection seismic method states that the recorded signal is the convolution of Earth’s reflectivity series and a seismic wavelet (not to mention random and coherent noise). This can lead to misunderstandings when conclusions are drawn about the subsurface configuration of reflective interfaces.
This vertical seismic section is from a data set offshore New Zealand (blues are negative values), with the gamma ray log along the well. In the centre of the image we see a set of strong, continuous, parallel set of reflections. But do they really represent the actual geometry of the reflecting boundaries?

Quite frequently interpreters use a line interpretation to illustrate their concepts. GeoTeric offers the Bedform attribute, which is a simplified, skeletonised representation of the seismic data and can be considered as a sort of volumetric line interpretation. The next image shows the results of the Bedform calculations along the same section. Red colour indicates peaks; blue represents troughs; green and yellow highlight doublets. The image indicates approximately the same geometry as the seismic.

If the wavelet is not a spike, the convolution yields a train of positive and negative values representing a sharp interface. In addition, due to e.g. attenuation, the wavelet might change with depth (frequency content, phase, etc.). Therefore it can be a difficult call to pinpoint the location of a reflecting interface.
This is where Envelope (magnitude, instantaneous amplitude) can provide a bit of help. This phase-independent attribute is thought to have its peak values at the interface. The next image shows the Envelope of our section (orange and white represent high values):

This image reveals a completely different layer configuration, especially left of the well: instead of a parallel layering we see a set of dipping boundaries, indicating prograding units. This behaviour is also picked up by the High Definition Frequency Decomposition (but will be unidentifiable using standard frequency decomposition techniques), as shown in the next image (an HDFD RGB blend of 25 Hz, 41 Hz and 61 Hz):

Using the Differential attribute of GeoTeric, we can calculate the second vertical differential of the Envelope, and arrive at a sharper representation of all reflecting interfaces within the bandwidth of the seismic data set. 

Please note, that after this calculation a peak of the original Envelope attribute is replaced by a sharper trough and two small peaks on the side. Only the negative values are shown in the above image. Running Bedform on this attribute and showing only the Troughs, a line representation of the actual interfaces (layer boundaries) can be obtained.

This final volume can be used to aid interpretation, build a stratigraphic framework, extract attributes either along or in between the layer boundaries (with the help of the Cookie Cutter), or these interfaces can be embedded into other volumes and help establish a connection between the layer configuration and the attributes.

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