azimvelan_notes

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The question "In the Azimvelan code, does the term Azim in velocity and
AVO mean fracture strike direction or the symmetry axis direction?" is
not a proper one to ask in the context of this code. "Azim" is the
azimuth of maximum of AVO gradient found by AzimVelAn. Whether it points
in the direction of fracture dip or strike depends upon the change
of elastic parameters and possibly anisotropies across a reflecting
interface. There is really no general answer this question even for the
simplest rotationally-invariant penny-shaped fractures.

Next, I do "-ampmin" to get the same output "Azim" for either peaks or
troughs. Of course, if you change my "-ampmin" to "ampmin," you flip
the "Azim" by 90 degrees. The answer to the question "If I have trough
and peak amplitude data, shall I use the program as it is?" is yes --
assuming that your version of the code coincides with mine.  :))  You
can eliminate the whole issue of peaks vs. troughs by computing the rms
or mean absolute amplitude in a given time gate. This is supposed to
produce the same value of "Azim".

The answer to your final question "do you have any AVO term that can be
related directly to the ellipticity" is no. The reason is that azimuthal
variation of AVO is not necessarily an ellipse. It can be a saddle
or a parabola. Then what "ellipticity" are we talking about? You can
certainly use "terms like (GRADMAX-GRADMIN) normalized by (abs( GRADMAX)
+ abs(GRADMIN))" to quantify the magnitude of azimuthal AVO variation
but this was not something I thought about when I wrote the code.

I hope my answers are helpful.

Best regards, Vladimir


-----Original Message----- From: John Stockwell
[mailto:[e-mail]] Sent: Wednesday, March 12, 2003 1:58 PM To:
Grechka, Vladimir V SIEP-EPT-AE Subject: RE: Azimvelan code (fwd)


Dear Vladimir,

Can you shed any light on this guy's question? Thanks in advance,
		John Stockwell

--- Date: Tue, 04 Mar 2003 13:26:59 +0300 From: "Marzoug, Ahmed
M." <[e-mail]> To: [e-mail] Subject: Azimvelan
code

Hello John,
    In the Azimvelan code, does the term Azim in velocity and AVO mean
fracture strike direction or the symmetry axis direction? In AVO, are
the trough and the peak amplitude data treated the same thing. I see
you that in your code, you use ampmin (the trough) and multiply it with
negative (which is equivalent to peak amplitude) and calculate the AVO
attributes. I see that using ampmin(trough) and -ampmin give Azim changed
by 90 degrees. If I have trough and peak amplitude data, shall I use the
program as it is?.  Also do you have any AVO term that can be related
directly to the ellipticity. I am using terms like (GRADMAX-GRADMIN)
normalized by (abs( GRADMAX) + abs(GRADMIN)).


Thanks, Ahmed Almarzoug GPT (Geophysical Technology Team)
Aramco,Dhahran,Saudi Arabia

From [e-mail] Mon Mar 17 11:38:06 2003 Date:
Sun, 16 Mar 2003 18:16:26 -0600 From: "Grechka, Vladimir V
SIEP-EPT-AE" <[e-mail]> To: Ahmad Al-Marzoug
<[e-mail]> Cc: [e-mail] Subject: RE: Azimvelan
code (fwd)


Dear Ahmad,

There is no general relation of the principal azimuths of AVO gradient to
the fracture direction. You might have multiple systems of fractures that
intersect each other (I bet you do :)) and produce a simple quadratic AVO
gradient. Just think about it. What would you relate your AVO azimuth
to? Such a relation makes sense in the only case of a single crack
system in an azimuthally isotropic material but azimuthal anisotropy of
underlying rock will mess up the AVO signature even then.

You are right, the azimuth of stacking velocity corresponds to velocity
maximum. It would be your fracture strike in the simplest case. Your
last question is supposed to be addressed to John.

Regards, Vladimir


-----Original Message----- From: Ahmad Al-Marzoug
[mailto:[e-mail]] Sent: Friday, March 14, 2003 10:51 PM
To: Grechka, Vladimir V SIEP-EPT-AE Cc: [e-mail] Subject: RE:
Azimvelan code (fwd)


Hello Dr. Vladimir,
   Thanks for your answers. In general, how do you relate the term Azim
in AVO to the fracture plane direction? It seems that there is some
sort of ambiguity regarding the determination of direction of fractures
from AVO. I suppose that the term Azim in velocity is the azimuth of of
maximum velocity (fracture stike direction). Can the code available on
CWP website on tomographic velocity stacking be also used for fracture
direction determination?

Thanks Ahmed

-----Original Message----- From: Grechka, Vladimir V SIEP-EPT-AE
[mailto:[e-mail]] Sent: Thursday, March 13, 2003 1:15
AM To: John Stockwell; [e-mail] Subject: RE: Azimvelan
code (fwd)


Dear John and Ahmed,

I looked at the code.

The question "In the Azimvelan code, does the term Azim in velocity and
AVO mean fracture strike direction or the symmetry axis direction?" is not
a proper one to ask in the context of this code. "Azim" is the azimuth
of maximum of AVO gradient found by AzimVelAn. Whether it points in the
direction of fracture dip or strike depends upon the change of elastic
parameters and possibly anisotropies across a reflecting interface.
There is really no general answer this question even for the simplest
rotationally-invariant penny-shaped fractures.

Next, I do "-ampmin" to get the same output "Azim" for either peaks or
troughs. Of course, if you change my "-ampmin" to "ampmin," you flip
the "Azim" by 90 degrees. The answer to the question "If I have trough
and peak amplitude data, shall I use the program as it is?" is yes --
assuming that your version of the code coincides with mine.  :))  You
can eliminate the whole issue of peaks vs. troughs by computing the rms
or mean absolute amplitude in a given time gate. This is supposed to
produce the same value of "Azim".

The answer to your final question "do you have any AVO term that can be
related directly to the ellipticity" is no. The reason is that azimuthal
variation of AVO is not necessarily an ellipse. It can be a saddle
or a parabola. Then what "ellipticity" are we talking about? You can
certainly use "terms like (GRADMAX-GRADMIN) normalized by (abs( GRADMAX)
+ abs(GRADMIN))" to quantify the magnitude of azimuthal AVO variation
but this was not something I thought about when I wrote the code.

I hope my answers are helpful.

Best regards, Vladimir


-----Original Message----- From: John Stockwell
[mailto:[e-mail]] Sent: Wednesday, March 12, 2003 1:58 PM To:
Grechka, Vladimir V SIEP-EPT-AE Subject: RE: Azimvelan code (fwd)


Dear Vladimir,

Can you shed any light on this guy's question? Thanks in advance,
		John Stockwell

--- Date: Tue, 04 Mar 2003 13:26:59 +0300 From: "Marzoug, Ahmed
M." <[e-mail]> To: [e-mail] Subject: Azimvelan
code

Hello John,
    In the Azimvelan code, does the term Azim in velocity and AVO mean
fracture strike direction or the symmetry axis direction? In AVO, are
the trough and the peak amplitude data treated the same thing. I see
you that in your code, you use ampmin (the trough) and multiply it with
negative (which is equivalent to peak amplitude) and calculate the AVO
attributes. I see that using ampmin(trough) and -ampmin give Azim changed
by 90 degrees. If I have trough and peak amplitude data, shall I use the
program as it is?.  Also do you have any AVO term that can be related
directly to the ellipticity. I am using terms like (GRADMAX-GRADMIN)
normalized by (abs( GRADMAX) + abs(GRADMIN)).


Thanks, Ahmed Almarzoug GPT (Geophysical Technology Team)
Aramco,Dhahran,Saudi Arabia