[Users] How-to: Display and store thickness maps

[Arnaud Huck]

Dear OpendTect user,
A common way at using inverted volumes is to extract thickness maps. 
 From an inverted impedance volume, and based on well analyses to define 
cut-offs, it is possible to separate sands from the background impedance 
data.

Let's assume such an analysis would have told us that (absolute) 
impedances lower than 9.000 (m/s)x(g/cc) are related to sands. In 
OpendTect, you can derive from such a cut-off thickness maps in less 
than five minutes with the help of three combined attributes:

1) At first, you need to enter you cut-off in a attribute of type 
"Mathematics". The formula would be the following (if-then-else statement):
x0 < 9000 ? 1 : 1e30
where x0 is your impedance volume.
This attribute is acting like a flag, giving us the sample locations 
where impedance is lower than our threshold, while the other samples are 
set to 1e30, which is our undefined value.
We can call this attribute "Blank outside".

2) Secondly, the thickness map itself is drawn by summing the number of 
samples set to 1 in our region of interest. This region is delimited by 
a time-gate, relative to the horizon on which we will apply this attribute.
For this purpose, we use an attribute of type "Volume statistics". The 
input data is the previous attribute "Blank outside", you can enter your 
desired time-gate. The stepout is 0 / 0 (trace-by-trace process), Output 
statistic is "Sum", and Steering is set to "None" as we will apply it on 
an horizon. Undefined values are not contributing to the sum.
We can call this attribute "Sum thickness", and returns a number of 
(flagged) samples

3) The third attribute aims at converting the result of the second 
attribute in meters or seconds.
If your impedance volume is in depth, you just need to multiply the 
previous attribute by the depth sampling rate.
In a more normal situation, your impedance cube is in time, and then we 
need to multiply the "Sum thickness" attribute by the (time) sampling 
rate, 4 msec. for instance, and by the average interval velocity within 
your formation. This is done with a second "Mathematics" attribute, with 
the kind of formula:
x0*0.004*2000.0
where x0 is the "Sum thickness attribute", 0.004 is the sampling rate in 
seconds, and 2000.0 is the interval velocity in m/s.
The thickness returned would then be in meter. We can call this 
attribute "Thickness in m." for instance.
Optionnaly, you can also read the velocity from another volume if available.

Finally, you can get your thickness map by applying the "Thickness in 
m." attribute allong your horizon. Please note that you can also use 
this approach with lithologies/facies cubes.

If you have any questions, do not hesitate to ask me.

Best regards,
Arnaud Huck
Geoscientist.

-- dGB Earth Sciences B.V.
-- Nijverheidstraat 11-2, 7511 JM Enschede, The Netherlands
-- mailto: arnaud.huck at dgb-group.com, http://www.dgb-group.com
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