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How to estimate Sonic velocity above first logged depth? 
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Joined: Wed Jul 18, 2012 3:20 pm
Posts: 2
Post How to estimate Sonic velocity above first logged depth?
All,

I have a problem with using a sonic velocity log when converting one of my wells. Whenever I use the log and invert it to get velocity I do get sensible values.

Then, when I use these velocities and try to convert them to average velocities I run in to trouble. I guess the problem is due to the fact the log is not available al the way to surface? I understand I need to suplement some sort of velocity for this interval, but how do I know what velocity to use? My log start at around 828m tvdss and the first interval velicity is about 2615 m/s. What's the best practice to calculate the velocity to use in the interval from 0 to 828m tvdss? Can I just interpolate to a certain value? My well is on land if that is relevant.

thanks! hope to get an answer here.

Rashied.


Fri Nov 28, 2014 4:29 pm

Joined: Wed Sep 01, 2010 10:06 pm
Posts: 107
Post Re: How to estimate Sonic velocity above first logged depth?
Rashied,

This is a common problem with sonic velocities. You will either have to estimate the replacement velocity (as you discussed), but I do not know the correct method for that. I can imagine you would be able to use a checkshot and calibrate the velocites that way. Alternatively you could probably make a synthetic and see if you can correlate any clear markers. This way you can apply a bulk shift to the Time Depth relationship (from Sonic), which will allow you to calculate the replacement velocity tus use in the interval above the logged interval.

Hope this helps.


Sat Nov 29, 2014 10:29 am

Joined: Sat Feb 15, 2014 7:33 am
Posts: 31
Post Re: How to estimate Sonic velocity above first logged depth?
As Petrel-User mentions, checkshots are the only reliable measure of the vertical seismic velocity from the borehole.

You only have accurate information about sub-surface velocities in the part of the well where you have both a sonic log and a checkshot survey. Outside of this, you are either missing the fine detail (checkshots only), or the information is not accurate (sonic only)

The sonic log has a number of drawbacks. Firstly its higher frequency - sonic our sound frequencies - than seismic vibrations, which means that the wavelength is smaller. This matter because the measurement will be more influenced by small-scale variations (and fractures) in the rock layer that seismic waves will not "see". The second issue is that the tool has a finite length; this means that it won't measure the response of thin beds (which are less than the tool length) properly. This smooths the response of the log, and of course its measuring transit times, not the actual velocity. If you smooth out the transit times, they will be slightly incorrect.


This is why we usually calibrate the sonic log with checkshot information (the "drift correction"), since as you progress deeper and deeper the cumulative effect of these errors can lead to significantly different travel-time measurements overall, and hence the a "drift" from the true velocity values.

In projecting back to the near surface you have a number of options, depending on he accuracy needed

- use the first checkshot to give a constant
The time and depth from the first checkshot gives you the average velocity to the surface. You could use this as an estimate, which of course would preserve the time-depth relationship accurately to the top layer. This replaces the entire 800m layer with a single velocity, so its likely not to correspond to any actual lithology.

- estimate a gradient
Depending on the degree of uplift, the top 800m of so of the section often show a strong compaction velocity in the seismic velocities. This means that a constant velocity is a poor estimate in many cases - which may or may not be important in your context.

Usually the velocity within a layer is expressed as V(z) = V(0) +Kz; where (z) is the depth, and K is the gradient. K is measured in "metres/second per metre". This can be used in two ways - Z can be relative to the top of layer (useful for compacted lithology) or to the actual surface. The gradient is the same in both cases.

A typical soft-sediment gradient for the near surface in the marine case is 0.6 (m/s per m)

You can estimate the gradient based on the information from other wells, measuring the velocity gradient from the calibrated sonic logs, or at a push from the seismic velocity information (converted to interval velocity and depth)

On land surveys you may also have an estimate for the velocity in the very near surface from the seismic data. They will have used a "replacement velocity" as part of the elevation and refraction statics solutions. This will normally be listed in the processing sequence and in the processing report. This can be used as a reliable V(0) at surface value as part of you modelling.

Hope this helps!


Sun Dec 21, 2014 9:11 pm
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